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A giant Hubble mosaic of the Crab Nebula, a supernova remnant
Astronomy is the scientific study of celestial objects (such as stars, planets, comets, nebulae, star clusters and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.
Astronomy is one of the oldest sciences. Early cultures such as the Babylonians performed methodical observations of the night sky, and astronomical artifacts such as Stonehenge have been found from ancient times. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and even astrology, but professional astronomy is nowadays often considered to be synonymous with astrophysics.
During the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of celestial objects, which is then analyzed using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.
Amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.
Ancient astronomy is not to be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin and a part of their methods (namely, the use of ephemerides), they are distinct.[1]

Contents

Lexicology

The word astronomy (from the Greek words astron (ἄστρον), "star" and -nomy from nomos (νόμος), "law" or "culture") literally means "law of the stars" (or "culture of the stars" depending on the translation).

Use of terms "astronomy" and "astrophysics"

Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.[2][3][4] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties"[5] and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".[6] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.[7] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[2] Various departments that research this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,[3] and many professional astronomers actually have physics degrees.[4] One of the leading scientific journals in the field is named Astronomy and Astrophysics.

History

A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit.
In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.[8] A particularly important early development was the the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations.[9]
Before tools such as the telescope were invented early study of the stars had to be conducted from the only vantage points available, namely tall buildings and high ground using the naked eye. As civilizations developed, most notably in China, Egypt, Greece, India, the Islamic world, Maya, Mesopotamia, and Persia, astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the universe.
A number of notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1000 BC by Chinese astronomers. The Chaldeans discovered that lunar eclipses recurred in a repeating cycle known as a saros.[10] In the 2nd century BC, the size and distance of the Moon were estimated by Hipparchus[11] and later Arabic astronomers. The Andromeda Galaxy, the nearest galaxy to the Milky Way, was discovered in 964 by the Persian astronomer Azophi and first described in his Book of Fixed Stars.[12] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006.
The earliest known astronomical device is the Antikythera mechanism, an ancient Greek device for calculating the movements of planets, that dates from about 150–80 BC, and was the earliest ancestor of an astronomical analog computer. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.[13]
As recorded in the Bible, the Biblical Magi learned about the birth of Jesus by discovering the appearance of a new star.
During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe, at least until the 13th century. However, astronomy flourished in the Islamic world and other parts of the world. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragha, Samarkand and Istanbul observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[14][15] It is also believed that the ruins at Great Zimbabwe and Timbuktu[16] may have housed an astronomical observatory.[17] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[18][19][20]

Scientific revolution

Galileo's sketches and observations of the Moon revealed that the surface was mountainous.
During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo innovated by using telescopes to enhance his observations.[21]
Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.[22] It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.[21]
Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[23] The distance to a star was first announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.[24]
During the 18–19th centuries, attention to the three body problem by Euler, Clairaut, and D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrange and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.[25]
Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.[14]
The existence of the Earth's galaxy, the Milky Way, as a separate group of stars, was only proved in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the Universe, seen in the recession of most galaxies from us.[26] Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars, and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's law, and cosmological abundances of elements.

Observational astronomy

In astronomy, the main source of information about celestial bodies and other objects is the visible light or more generally electromagnetic radiation.[27] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.

Radio astronomy

Radio astronomy studies radiation with wavelengths greater than approximately one millimeter.[28] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[28]
Although some radio waves are produced by astronomical objects in the form of thermal emission, most of the radio emission that is observed from Earth is seen in the form of synchrotron radiation, which is produced when electrons oscillate around magnetic fields.[28] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.[7][28]
A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.[7][28]

Infrared astronomy

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets and circumstellar disks. Longer infrared wavelengths can also penetrate clouds of dust that block visible light, allowing observation of young stars in molecular clouds and the cores of galaxies.[29] Some molecules radiate strongly in the infrared. This can be used to study chemistry in space; more specifically it can detect water in comets.[30]

Optical astronomy

The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.
Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[31] Optical images were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using charge-coupled devices (CCDs). Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),[31] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy

Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm).[28] Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.[28] However, as ultraviolet light is easily absorbed by interstellar dust, an appropriate adjustment of ultraviolet measurements is necessary.[28]

X-ray astronomy

X-ray astronomy is the study of astronomical objects at X-ray wavelengths. Typically, objects emit X-ray radiation as synchrotron emission (produced by electrons oscillating around magnetic field lines), thermal emission from thin gases above 107 (10 million) kelvins, and thermal emission from thick gases above 107 Kelvin.[28] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be done from high-altitude balloons, rockets, or spacecraft. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.[28]

Gamma-ray astronomy

Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[28] The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.[32]
Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[28]

Fields not based on the electromagnetic spectrum

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.
In neutrino astronomy, astronomers use special underground facilities such as SAGE, GALLEX, and Kamioka II/III for detecting neutrinos. These neutrinos originate primarily from the Sun but also from supernovae.[28] Cosmic rays, which consist of very high energy particles that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of particles which can be detected by current observatories.[33] Additionally, some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.[28] Gravitational wave astronomy is an emerging new field of astronomy which aims to use gravitational wave detectors to collect observational data about compact objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO, but gravitational waves are extremely difficult to detect.[34]
Planetary astronomers have directly observed many of these phenomena through spacecraft and sample return missions. These observations include fly-by missions with remote sensors, landing vehicles that can perform experiments on the surface materials, impactors that allow remote sensing of buried material, and sample return missions that allow direct laboratory examination.

Astrometry and celestial mechanics

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation and in the making of calendars.
Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.[35]
The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.[36]
During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large extrasolar planets orbiting nearby stars.[37]

Theoretical astronomy

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Related topics
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Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.[38][39]
Theorists in astronomy endeavor to create theoretical models and figure out the observational consequences of those models. This helps observers look for data that can refute a model or help in choosing between several alternate or conflicting models.
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale structure of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.
Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.
A few examples of this process:
Physical process Experimental tool Theoretical model Explains/predicts
Gravitation Radio telescopes Self-gravitating system Emergence of a star system
Nuclear fusion Spectroscopy Stellar evolution How the stars shine and how metals formed
The Big Bang Hubble Space Telescope, COBE Expanding universe Age of the Universe
Quantum fluctuations Cosmic inflation Flatness problem
Gravitational collapse X-ray astronomy General relativity Black holes at the center of Andromeda galaxy
CNO cycle in stars
Dark matter and dark energy are the current leading topics in astronomy,[40] as their discovery and controversy originated during the study of the galaxies.

Specific subfields

Solar astronomy

An ultraviolet image of the Sun's active photosphere as viewed by the TRACE space telescope. NASA photo
At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.[41]
The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.[42] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[43]
The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated corona.
At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.[41]
A solar wind of plasma particles constantly streams outward from the Sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.[44]

Planetary science

This astronomical field examines the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.[45]
The black spot at the top is a dust devil climbing a crater wall on Mars. This moving, swirling column of Martian atmosphere (comparable to a terrestrial tornado) created the long, dark streak. NASA image.
The solar system is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus, and Neptune.[46] Beyond Neptune lies the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.
The planets were formed in the protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided, the leading hypothesis for how the Moon was formed.[47]
Once a planet reaches sufficient mass, the materials with different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer surface. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.[48]
A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.[49]

Stellar astronomy

The Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.
The study of stars and stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.[50]
Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[51]
Almost all elements heavier than hydrogen and helium were created inside the cores of stars.[50]
The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature, so that the star both expands in size, and increases in core density. The resulting red giant enjoys a brief life span, before the helium fuel is in turn consumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as they fuse increasingly heavier elements.[52]
The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae;[53] while smaller stars form planetary nebulae, and evolve into white dwarfs.[54] The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.[55] Close binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.[56] Planetary nebulae and supernovae are necessary for the distribution of metals to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.[57]

Galactic astronomy

Observed structure of the Milky Way's spiral arms
Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.
In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at the center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.[58][59]
Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[51]
As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.[60]
Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[61]

Extragalactic astronomy

This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lens effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.
The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies; their morphology and classification; and the examination of active galaxies, and the groups and clusters of galaxies. The latter is important for the understanding of the large-scale structure of the cosmos.
Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.[62]
As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust; few star-forming regions; and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may be formed through mergers of large galaxies.
A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and the Andromeda Galaxy are spiral galaxies.
Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.
An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas; and is powered by a compact region at the core, usually thought to be a super-massive black hole that is emitting radiation from in-falling material.
A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[63]
The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids in between.[64]

Cosmology

Cosmology (from the Greek κόσμος "world, universe" and λόγος "word, study") could be considered the study of the universe as a whole.
Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time, and thereafter expanded over the course of 13.7 Gyr to its present condition.[65] The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.[65]
In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early universe.[65] (See also nucleocosmochronology.)
When the first atoms formed, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a Dark Age due to the lack of stellar energy sources.[66]
A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early universe which tend to decay back to the lighter elements extending the cycle.[67]
Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.[68]
Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the mass of the universe. For this reason, much effort is expended in trying to understand the physics of these components.[69]

Interdisciplinary studies

Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, utilizing archaeological and anthropological evidence[70]. Astrobiology is the study of the advent and evolution of biological systems in the universe, with particular emphasis on the possibility of non-terrestrial life.
The study of chemicals found in space, including their formation, interaction and destruction, is called astrochemistry. These substances are usually found in molecular clouds, although they may also appear in low temperature stars, brown dwarfs and planets. Cosmochemistry is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry.

Amateur astronomy

Amateur astronomers can build their own equipment, and can hold star parties and gatherings, such as Stellafane.
Astronomy is one of the sciences to which amateurs can contribute the most.[71]
Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events which interest them.[72][73]
Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky, who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).[74][75]
Amateur astronomers continue to make scientific contributions to the field of astronomy. Indeed, it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.[76][77][78]

Major problems

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.
  • What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses – the initial mass function – apparently regardless of the initial conditions?[79] A deeper understanding of the formation of stars and planets is needed.
  • Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.[80][81] Is the Solar System normal or atypical?
  • What caused the Universe to form? Is the premise of the Fine-tuned universe hypothesis correct? If so, could this be the result of cosmological natural selection? What caused the cosmic inflation that produced our homogeneous universe? Why is there a baryon asymmetry?
  • What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet we are still uncertain about their true natures.[82] What will be the ultimate fate of the universe?[83]
  • How did the first galaxies form? How did supermassive black holes form?
  • What is creating the ultra-high-energy cosmic rays?

International Year of Astronomy 2009

During the 62nd General Assembly of the UN, 2009 was declared to be the International Year of Astronomy (IYA2009), with the resolution being made official on 20 December 2008. A global scheme laid out by the International Astronomical Union (IAU), it was also endorsed by UNESCO – the UN body responsible for Educational, Scientific and Cultural matters. IYA2009 was intended to be a global celebration of astronomy and its contributions to society and culture, stimulating worldwide interest not only in astronomy but science in general, with a particular slant towards young people.

See also

References

  1. ^ Unsöld, Albrecht; Baschek, Bodo; Brewer, W.D. (translator) (2001). The New Cosmos: An Introduction to Astronomy and Astrophysics. Berlin, New York: Springer. ISBN 3-540-67877-8. 
  2. ^ a b Scharringhausen, B.. "Curious About Astronomy: What is the difference between astronomy and astrophysics?". http://curious.astro.cornell.edu/question.php?number=30. Retrieved 2007-06-20. 
  3. ^ a b Odenwald, S.. "Archive of Astronomy Questions and Answers: What is the difference between astronomy and astrophysics?". http://www.astronomycafe.net/qadir/q449.html. Retrieved 2007-06-20. 
  4. ^ a b "Penn State Erie-School of Science-Astronomy and Astrophysics". http://www.erie.psu.edu/academic/science/degrees/astronomy/astrophysics.htm. Retrieved 2007-06-20. 
  5. ^ "Merriam-Webster Online". Results for "astronomy". http://www.m-w.com/dictionary/astronomy. Retrieved 2007-06-20. 
  6. ^ "Merriam-Webster Online". Results for "astrophysics". http://www.m-w.com/dictionary/astrophysics. Retrieved 2007-06-20. 
  7. ^ a b c Shu, F. H. (1982). The Physical Universe. Mill Valley, California: University Science Books. ISBN 0-935702-05-9. 
  8. ^ Forbes, 1909
  9. ^ Aaboe, A. (1974). "Scientific Astronomy in Antiquity". Philosophical Transactions of the Royal Society 276 (1257): 21-42. http://www.jstor.org/stable/74272. Retrieved 2010-03-09. 
  10. ^ "Eclipses and the Saros". NASA. http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros.html. Retrieved 2007-10-28. 
  11. ^ "Hipparchus of Rhodes". School of Mathematics and Statistics, University of St Andrews, Scotland. http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Hipparchus.html. Retrieved 2007-10-28. 
  12. ^ Kepple, George Robert; Glen W. Sanner (1998). The Night Sky Observer's Guide, Volume 1. Willmann-Bell, Inc. p. 18. ISBN 0-943396-58-1. 
  13. ^ In search of lost time, Jo Marchant, Nature 444, #7119 (November 30, 2006), pp. 534–538, doi:10.1038/444534a.
  14. ^ a b Berry, Arthur (1961). A Short History of Astronomy From Earliest Times Through the Nineteenth Century. New York: Dover Publications, Inc.. 
  15. ^ Hoskin, Michael, ed (1999). The Cambridge Concise History of Astronomy. Cambridge University Press. ISBN 0-521-57600-8. 
  16. ^ McKissack, Pat; McKissack, Frederick (1995). The royal kingdoms of Ghana, Mali, and Songhay: life in medieval Africa. H. Holt. ISBN 9780805042597. http://books.google.com/books?id=Pk-bZMS_KdUC&pg=PA103&lpg=PA103&dq=astronomy+in+medieval+Africa&source=bl&ots=aRLD55EWpi&sig=a6mM7gqHkev1GQPCFRY84gCNEcE&hl=en&ei=V4yQSbaQLZaitgea1aSRCw&sa=X&oi=book_result&resnum=10&ct=result. 
  17. ^ Clark, Stuart; Carrington, Damian (2002). "Eclipse brings claim of medieval African observatory". New Scientist. http://www.newscientist.com/article/dn3137-eclipse-brings-claim-of-medieval-african-observatory.html. Retrieved 2010-02-03. 
  18. ^ "Cosmic Africa explores Africa's astronomy". Science in Africa. http://www.scienceinafrica.co.za/2003/november/cosmic.htm. Retrieved 2002-02-03. 
  19. ^ Holbrook, Jarita C.; Medupe, R. Thebe; Urama, Johnson O. (2008). African Cultural Astronomy. Springer. ISBN 9781402066382. http://books.google.com/books?id=4DJpDW6IAukC&pg=PA180&lpg=PA180&dq=astronomy+in+medieval+Africa&source=web&ots=wcw8-0WyuY&sig=kwrd2tUrfvMHNLFMSvFkdb-w44Y&hl=en&ei=V4yQSbaQLZaitgea1aSRCw&sa=X&oi=book_result&resnum=14&ct=result. 
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  21. ^ a b Forbes, 1909, pp. 58–64
  22. ^ Forbes, 1909, pp. 49–58
  23. ^ Forbes, 1909, pp. 79–81
  24. ^ Forbes, 1909, pp. 147–150
  25. ^ Forbes, 1909, pp. 74–76
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  28. ^ a b c d e f g h i j k l m n Cox, A. N., ed (2000). Allen's Astrophysical Quantities. New York: Springer-Verlag. p. 124. ISBN 0-387-98746-0. http://books.google.com/books?id=w8PK2XFLLH8C&pg=PA124. 
  29. ^ Staff (2003-09-11). "Why infrared astronomy is a hot topic". ESA. http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html. Retrieved 2008-08-11. 
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  32. ^ Penston, Margaret J. (2002-08-14). "The electromagnetic spectrum". Particle Physics and Astronomy Research Council. http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=14F1&style=feature. Retrieved 2006-08-17. 
  33. ^ Gaisser, Thomas K. (1990). Cosmic Rays and Particle Physics. Cambridge University Press. pp. 1–2. ISBN 0521339316. 
  34. ^ Tammann, G. A.; Thielemann, F. K.; Trautmann, D. (2003). "Opening new windows in observing the Universe". Europhysics News. http://www.europhysicsnews.org/index.php?option=article&access=standard&Itemid=129&url=/articles/epn/abs/2003/02/epn03208/epn03208.html. Retrieved 2010-02-03. 
  35. ^ Calvert, James B. (2003-03-28). "Celestial Mechanics". University of Denver. http://www.du.edu/~jcalvert/phys/orbits.htm. Retrieved 2006-08-21. 
  36. ^ "Hall of Precision Astrometry". University of Virginia Department of Astronomy. http://www.astro.virginia.edu/~rjp0i/museum/engines.html. Retrieved 2006-08-10. 
  37. ^ Wolszczan, A.; Frail, D. A. (1992). "A planetary system around the millisecond pulsar PSR1257+12". Nature 355: 145–147. doi:10.1038/355145a0. 
  38. ^ Roth, H. (1932). "A Slowly Contracting or Expanding Fluid Sphere and its Stability". Physical Review 39: 525–529. 
  39. ^ Eddington, A.S. (1926). Internal Constitution of the Stars. Cambridge University Press. ISBN 9780521337083. http://books.google.com/books?id=hJW3JbhnFQMC&pg=PA182. 
  40. ^ "Dark matter". NASA. 2010. http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html. Retrieved 2009-11-02. "third paragraph, "There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is"" 
  41. ^ a b Johansson, Sverker (2003-07-27). "The Solar FAQ". Talk.Origins Archive. http://www.talkorigins.org/faqs/faq-solar.html. Retrieved 2006-08-11. 
  42. ^ Lerner, K. Lee; Lerner, Brenda Wilmoth. (2006). "Environmental issues : essential primary sources."". Thomson Gale. http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page. Retrieved 2006-09-11. 
  43. ^ Pogge, Richard W. (1997). "The Once & Future Sun" (lecture notes). New Vistas in Astronomy. http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html. Retrieved 2010-02-03. 
  44. ^ Stern, D. P.; Peredo, M. (2004-09-28). "The Exploration of the Earth's Magnetosphere". NASA. http://www-istp.gsfc.nasa.gov/Education/Intro.html. Retrieved 2006-08-22. 
  45. ^ Bell III, J. F.; Campbell, B. A.; Robinson, M. S. (2004). Remote Sensing for the Earth Sciences: Manual of Remote Sensing (3rd ed.). John Wiley & Sons. http://marswatch.tn.cornell.edu/rsm.html. Retrieved 2006-08-23. 
  46. ^ Grayzeck, E.; Williams, D. R. (2006-05-11). "Lunar and Planetary Science". NASA. http://nssdc.gsfc.nasa.gov/planetary/. Retrieved 2006-08-21. 
  47. ^ Montmerle, Thierry; Augereau, Jean-Charles; Chaussidon, Marc et al. (2006). "Solar System Formation and Early Evolution: the First 100 Million Years". Earth, Moon, and Planets (Spinger) 98: 39–95. doi:10.1007/s11038-006-9087-5. http://adsabs.harvard.edu/abs/2006EM%26P...98...39M. 
  48. ^ Montmerle, 2006, pp. 87–90
  49. ^ Beatty, J.K.; Petersen, C.C.; Chaikin, A., ed (1999). The New Solar System. Cambridge press. p. 70edition = 4th. ISBN 0-521-64587-5. http://books.google.com/books?id=iOezyHMVAMcC&pg=PA70. 
  50. ^ a b Harpaz, 1994, pp. 7–18
  51. ^ a b Smith, Michael David (2004). "Cloud formation, Evolution and Destruction". The Origin of Stars. Imperial College Press. pp. 53–86. ISBN 1860945015. http://books.google.com/books?id=UVgBoqZg8a4C&dq. 
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  54. ^ Harpaz, 1994, pp. 111–118
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  57. ^ Harpaz, 1994, pp. 245–256
  58. ^ Ott, Thomas (2006-08-24). "The Galactic Centre". Max-Planck-Institut für extraterrestrische Physik. http://www.mpe.mpg.de/ir/GC/index.php. Retrieved 2006-09-08. 
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  61. ^ Van den Bergh, Sidney (1999). "The Early History of Dark Matter". Publications of the Astronomy Society of the Pacific 111: 657–660. doi:10.1086/316369. http://www.journals.uchicago.edu/doi/full/10.1086/316369. 
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  66. ^ Hinshaw, Gary (2006-07-13). "Cosmology 101: The Study of the Universe". NASA WMAP. http://map.gsfc.nasa.gov/m_uni.html. Retrieved 2006-08-10. 
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  68. ^ "Galaxy Clusters and Large-Scale Structure". University of Cambridge. http://www.damtp.cam.ac.uk/user/gr/public/gal_lss.html. Retrieved 2006-09-08. 
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  72. ^ "The Americal Meteor Society". http://www.amsmeteors.org/. Retrieved 2006-08-24. 
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Bibliography

External links


Quotes

Up to date as of January 14, 2010

From Wikiquote

Astronomy is the science of celestial objects such as stars, planets, comets and galaxies.
Johannes Hevelius 1611-1687

Contents

Sourced

  • Although Uranus and Neptune are superficially twin planets, they are different enough to remind us - as do Venus and Earth - that we still have a lot to learn about the mix of natural laws and historical accidents that formed the planets and fashioned their destinies.
  • The wonder is, not that the field of stars is so vast, but that man has measured it.
  • Over the rim of waiting earth the moon lifted with majesty till it swung clear of the horizon and rode off, free of moorings...
  • A strange weasel-built creature with a curly tail.
    • Johannes Hevelius on his newly described constellation Lacerta the lizard in 1687 - reported in SkyNews The Canadian Magazine of Astronomy and Stargazing September/October 2002.
  • I open the scuttle at night and see the far sprinkled systems,
    And all I see multiplied as high as I can cyper edge but rim of the farthest systems.
    Wider and wider they spread, expanding, always expanding,
    Outward and outward, forever outward.
"...far sprinkled systems"

Attributed

  • Stars scribble in our eyes the frosty sagas, the gleaming cantos of unvanquished space.
  • Look at the stars! look, look up at the skies!
    O look at all the fire-folk sitting in the air!
    The bright boroughs, the circle citadels there!
  • Silently, one by one, in the infinite meadows of heaven, blossomed the lovely stars, the forget-me nots of the angels
  • The unquiet republic of the maze of planets, struggling fierce towards heaven's free wilderness.
  • Architects should be educated, skillful with the pencil, instructed in geometry, know much history, have followed the philosophers with attention, understand music, have some knowledge of medicine, know the opinions of the jurists, and be acquainted with astronomy and the theory of the heavens.
"...the lord-star Jupiter"
  • Up through the darkness,
    While ravening clouds, the burial clouds, in black masses spreading,
    Lower sullen and fast athwart and down the sky,
    Amid a transparent clear belt of ether yet left in the east,
    Ascends large and calm the lord-star Jupiter.

See also

External links

Wikipedia
Wikipedia has an article about:
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Look up astronomy in Wiktionary, the free dictionary

Study guide

Up to date as of January 14, 2010

From Wikiversity

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External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ASTRONOMY (from Gr. eturpov, a star, and vEu€ v, to classify or arrange). .The subject matter of astronomical science, considered in its widest range, comprehends all the matter of the universe which lies outside the limit of the earth's atmosphere.^ It is considered as a major part of the Earth sciences.

^ New Planet Galaxy Cluster Collision Proves Existence of Dark Matter First postulated in 1933, astronomers calculated the mysterious "dark matter" to comprise 90% of the universe.

^ By his persistent investigation of natural laws he laid foundations for modern experimental science, and by the construction of astronomical telescopes he greatly enlarged humanity's vision and conception of the universe.
  • Astronomy Jokes and Space Explorers - Jokes and Science 19 January 2010 9:52 UTC www.juliantrubin.com [Source type: General]

.The seeming anomaly of classifying as a single branch of science all that we know in a field so wide, while subdividing our knowledge of things on our own planet into an indefinite number of separate sciences, finds its explanation in the impossibility of subjecting the matter of the heavens to that experimental scrutiny which yields such rich results when applied to matter which we can handle at will.^ If we destroy our planet will science find a new one?
  • Astronomy: News & Videos about Astronomy - CNN.com 19 January 2010 9:52 UTC topics.cnn.com [Source type: News]

^ The moons of the other planets all have their own name, so what is our moon's own name?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I have my own opinion (always subject to change), but I sometimes think the "Is a Pluto a Planet" issue takes attention away from the other objects Out There.

.Astronomy is of necessity a science of observation in the pursuit of which experiment can directly play no part.^ Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science.
  • astronomy -- Britannica Online Encyclopedia 19 January 2010 9:52 UTC www.britannica.com [Source type: Academic]

^ For me, though, the science and mechanics are a big part of the experience, and I felt this was one of the most interesting chapters.
  • Astronomy | GeekDad | Wired.com 19 January 2010 9:52 UTC www.wired.com [Source type: General]

^ An introduction to astronomy and astrophysics for science majors and others with some background in physics, providing an observational and theoretical background for more advanced topics in astrophysics.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

.It is the most ancient of the sciences because, before the era of experiment, it was the branch of knowledge which could be most easily systematized, while the relations of its phenomena to day and night, times and seasons, made some knowledge of the subject a necessity of social life.^ Because the planet actually rotates on its side, it spends 42 years in darkness and 42 years in light – at least the poles do, but the equator actually experiences a short day and night cycle – before repeating the cycle.

^ The airglow varies with time of night, latitude, and season.

^ The latter of those two terms seems to describe most of what I personally encountered during the two night visit LASS made to the Ft Griffin Historical Site.

.In recent times it is among the more progressive of the sciences, because the new and improved methods of research now at command have found in its cultivation a field of practically unlimited extent, in which the lines of research may ultimately lead to a comprehension of the universe impossible of attainment before our time.^ Most of the time, astronomers derive information from an analysis of the light or the motions of celestial bodies, a process that, to the uninitiated, may seem more like sorcery than science.
  • A New Universe to Explore: Careers in Astronomy Brochure | American Astronomical Society 19 January 2010 9:52 UTC aas.org [Source type: FILTERED WITH BAYES]

^ To improve our understanding of the fundamental constituents of matter, and the nature of space-time itself, researchers work to isolate and observe rare events, predicted by a variety of new physics theories which go beyond our current understanding.
  • PLaNetS: Physics Lambda Network System 18 January 2010 2:02 UTC pcbunn.cithep.caltech.edu [Source type: FILTERED WITH BAYES]

^ Chiron and Ceres have no agreed characteristics as yet, though research suggests that Ceres is seriously malefic and Chiron may be ultimately benefic.
  • Astrology on the Web: The Planets 18 January 2010 2:02 UTC www.astrologycom.com [Source type: FILTERED WITH BAYES]

.The field we have defined is divisible into at least two parts, that of Astronomy proper, or " Astrometry," which treats of the motions, mutual relations and dimensions of the heavenly bodies; and that of Astrophysics, which treats of their physical constitution.^ A mutual physical force attracting two bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ It is divided into two main branches, distinguished as astrometry and astrophysics; the former concerned with determining the places of the investigation of the heavenly bodies, the latter, with the investigation of their chemical and physical nature.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

^ Kepler's laws opened the way for the development of celestial mechanics, i.e., the application of the laws of physics to the motions of heavenly bodies.
  • Astronomy Jokes and Space Explorers - Jokes and Science 19 January 2010 9:52 UTC www.juliantrubin.com [Source type: General]

.While it is true that the instruments and methods of research in these two branches are quite different in their details, there is so much in common in the fundamental principles which underlie their application, that it is unprofitable to consider them as completely distinct sciences.^ These two periods can be very different.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ Each of these theories predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to take observations that will allow the theories to be tested.
  • Mercury | planets 18 January 2010 2:02 UTC planets.net [Source type: FILTERED WITH BAYES]

^ Moveover, there are three larger intervals formed by taking pairs of these three fundamental day counts, two of which are clearly important in solar system design.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

.Speaking in the most comprehensive way, and making an exception of the ethereal medium (see Aether), which, being capable of experimental study, is not included in the subject of astronomy, we may say that the great masses of matter which make up the universe are of two kinds: - (t) incandescent bodies, made visible to us by their own light; (2) dark bodies, revolving round them or round each other.^ Dark matter makes up 90% of the universe.

^ Astronomy is the study of space and celestial bodies.

^ Asteroids Why Dark Matter Matters Dark matter is that invisible nothing that makes up a bulk of mass in our universe.

.These dark bodies are known to us in two ways: (a) by becoming visible through reflecting the light from incandescent bodies in their neighbourhood, (b) by their attraction upon such bodies.^ Join us on a special dramatized 26,000 light-year adventure to the Galaxy’s hulking heart of darkness.

^ Radar waves travel at the speed of light , which is accurately known, so the time it takes before reflected waves arrive yields the distance if it is multiplied by the speed of light .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Planetary dark ovals all pointing toward the center of Rigel are clearly visible in number approaching two dozen at least.
  • GIC - Planets by the score in halos of giant planets 18 January 2010 2:02 UTC www.cosmicastronomy.com [Source type: General]

The incandescent bodies are of two classes: stars and nebulae. .Among the stars our sun is to be included, as it has no properties which distinguish it from the great mass of stars except our proximity to it.^ The first discoveries Astronomers announced the discovery of the first planets around a star other than our sun in 1992.
  • NASA - Planets 18 January 2010 2:02 UTC www.nasa.gov [Source type: General]

^ Why is our sun sort of a yellowy white color while you’ve got red stars and blue stars?
  • Astronomy Cast - Ep. 133: Optical Astronomy 19 January 2010 9:52 UTC www.astronomycast.com [Source type: General]

^ And now astronomers have found evidence for an asteroid belt around another star similar to our Sun.
  • Planetary Science: News & Videos about Planetary Science - CNN.com 18 January 2010 2:02 UTC topics.cnn.com [Source type: News]
  • Planetary Science: News & Videos about Planetary Science - CNN.com 18 January 2010 2:02 UTC topics.edition.cnn.com [Source type: News]

.The stars are supposed to be generally spherical, like the sun, in form, and to have fairly well-defined boundaries; while the nebulae are generally irregular in outline and have no well-defined limits.^ Of course, no star is exactly like the Sun, and so many astronomers and scientists in related fields with a professional interest in the search for habitable planets among nearby stars have been examining some of the most critical issues for habitability.
  • Stars and Habitable Planets 18 January 2010 2:02 UTC solstation.com [Source type: General]

^ Old-hat theories of planet formation maintained that planets Jupiter’s size form farther from their star, leaving smaller Earth-like planets to reside peacefully in the warm embrace of the Sun.
  • Science Notes 2001: Tall Tales of Distant Planets 18 January 2010 2:02 UTC scicom.ucsc.edu [Source type: General]

^ Dying sun-like stars flung the Murchison grains into space more than 4.5.

.It is, however, probable that the one class runs into the other by imperceptible gradations.^ Because Uranus' axis of rotation is tilted into the ecliptic plane , One pole is heated while the other is put into the deep freeze for decades.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

^ A core-accreted planet, however, could be pushed and pulled into eccentric orbit if it were born in a system with other large planets, Lin says.
  • Science Notes 2001: Tall Tales of Distant Planets 18 January 2010 2:02 UTC scicom.ucsc.edu [Source type: General]

^ In between the two planets, however, the waves crashed into each other, quickly scattering the dust and gas.
  • Science Notes 2001: Tall Tales of Distant Planets 18 January 2010 2:02 UTC scicom.ucsc.edu [Source type: General]

.In the relation of the universe to us there is yet another separation of its bodies into two classes, one comprising the solar system, the other the remainder of the universe.^ Where else in the Solar System could there be life?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What is the densest body in our solar system?
  • Planets - UEN 18 January 2010 2:02 UTC www.uen.org [Source type: FILTERED WITH BAYES]

^ Is there other life in the Universe ?
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

.The former consists of the sun and the bodies which move round it.^ All the planets travel round the sun from west to east or counter clock-wise and most of the satellites move in the same direction round their primaries.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

^ Rule 3 means that some round celestial bodies that orbit directly around the Sun are yet not planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Considered as a part of the universe, our solar system is insignificant in extent, though, for obvious reasons, great in practical importance to us, and in the facility with which we may gain knowledge relating to it.^ What emerged was the Solar System of which the Earth is a part.
  • Planets in our Universe 18 January 2010 2:02 UTC www.annabelburton.com [Source type: FILTERED WITH BAYES]

^ What is the densest body in our solar system?
  • Planets - UEN 18 January 2010 2:02 UTC www.uen.org [Source type: FILTERED WITH BAYES]

^ That has given the researchers great confidence that our Solar System is not unique.
  • Space Today Online -- Faraway Planets Orbiting Distance Stars 18 January 2010 2:02 UTC www.spacetoday.org [Source type: General]

.Referring to special articles, Solar System, Star, Sun, MooN, &c.^ Here's an article about all the planets in the Solar System , and here's an article about the biggest planet in the Solar System .

^ Astronomers use the principles of physics and mathematics to learn about the fundamental nature of the universe, including the sun, moon, planets, stars, and galaxies.
  • Physicists and Astronomers 11 September 2009 3:15 UTC www.bls.gov [Source type: Academic]

^ That contrasts with Earth and other planets in our Solar System, which follow almost circular paths around the Sun.
  • Space Today Online -- Faraway Planets Orbiting Distance Stars 18 January 2010 2:02 UTC www.spacetoday.org [Source type: General]

for a description of the various parts of the universe, we confine ourselves, at present, to setting forth a few of the most general modern conceptions of the universe. .As to extent, it may be said, in a general way, that while no definite limits can be set to the possible extent of the universe, or the distance of its farthest bodies, it seems probable, for reasons which will be given under Star, that the system to which the stars that we see belong, is of finite extent.^ Stars and nebulae belong to the same system, if such the sidereal world may properly be called in the absence of any sufficient evidence of its being in a state of dynamical equilibrium.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

^ A project aiming to create an easier way to measure cosmic distances has instead turned up surprising evidence that our large and ancient universe may be even bigger and older than previously thought.
  • Astronomy: News & Videos about Astronomy - CNN.com 19 January 2010 9:52 UTC topics.cnn.com [Source type: News]

^ No human has yet traveled beyond our Moon to another planet, but in the coming decades, the first person may set foot on Mars.

.As the incandescent bodies of the universe are visible by their own light, the problem of ascertaining their existence and position is mainly one of seeing, and our facilities for attacking it have constantly increased with the improvement of our optical appliances.^ Space dust is everywhere in the cosmos, in our own neck of the universe and all the way back billions of light-years away in our infant universe.

^ Pamela: This is where I think we also have to think about the greatest problem with observing in optical light and that is our atmosphere.
  • Astronomy Cast - Ep. 133: Optical Astronomy 19 January 2010 9:52 UTC www.astronomycast.com [Source type: General]

^ In reality what you are seeing is not some sort of corporeal representation of your own ego or a mystical aura of self-realization, but simply a literal trick of lights and shadows.

But such is not the case with the dark bodies. .Such a body can be made known to us only when in the neighbourhood of an incandescent body; and even then, unless its mass or its dimensions are considerable, it will evade all the scrutiny of our science.^ I as well was able to see this marvelous composure of our fellow panets from 10,000 foot in the Colorado mountains were the sky and all around us is pitch black.
  • Monday Night: Planets Align in a Frown | Wired Science | Wired.com 18 January 2010 2:02 UTC www.wired.com [Source type: General]

^ Space, expanding: Does cosmic expansion move everything apart, even our the various parts of our bodies?
  • Astronomy mysteries solved - ask a question - get clear, accurate answers - by WonderQuest 19 January 2010 9:52 UTC www.wonderquest.com [Source type: General]

^ Given that the initial mass function predicts that such massive stars should be rare, this is not surprising, but identifying these stars is often made even more difficult due to the reddening caused by their nebulae .

.The question of the possible number and magnitude of such bodies is therefore one that does not admit of accurate investigation.^ Pluto, the other so-called planet, is one of several small bodies in the solar system which is not a "major planet" and does not appear to be part of the Lord's timepiece.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.We can do no more than balance vague estimates of probability.^ This data also indicated that the ring thickness was no more than 150m, with their temperature in sunlight at around -180 degC and as low as -200 degC in shadow.
  • Planets in our Universe 18 January 2010 2:02 UTC www.annabelburton.com [Source type: FILTERED WITH BAYES]

^ Probably, more than half of all stars are in multiple star systems.
  • Space Today Online -- Faraway Planets Orbiting Distance Stars 18 January 2010 2:02 UTC www.spacetoday.org [Source type: General]

^ It is estimated that there may altogether be no fewer than 40,000.

What we do know is that these bodies vary widely in size. .Those known to be revolving round certain of the stars are far larger in proportion to their central bodies than our planets are in respect to the sun; for were it otherwise we should never be able to detect their existence.^ If a planet is too far from its host star it will be too cold.

^ The planets and their stars are also much farther away than our sun.
  • NASA - Planets 18 January 2010 2:02 UTC www.nasa.gov [Source type: General]

^ A planet has far less mass than a star .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.At the other extreme we know that innumerable swarms of minute bodies, probably little more than particles, move round the sun in orbits of every degree of eccentricity, making themselves known to us only in the exceptional cases when they strike the earth's atmosphere.^ Scientists know of more than 100 stars other than the sun that have planets.
  • NASA - Planets 18 January 2010 2:02 UTC www.nasa.gov [Source type: General]

^ The plane of Earth's orbit about the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ It’s that the Earth orbits the sun in an elliptical, uneven, orbit.

.They then appear to us as " shooting stars " (see Meteor).^ Some of the stars we see today in the night sky are so far away that light takes a million years to reach us from them.

^ As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ They shut all the lights off in Reykjavik last Thursday so that residents might see the stars without light pollution .
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

.A general idea of the relation of the solar system to the universe may be gained by reflecting that the average distance between any two neighbouring stars is several thousand times the extent of the solar system.^ This is the average time between two successive Full Moons .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Stars and nebulae belong to the same system, if such the sidereal world may properly be called in the absence of any sufficient evidence of its being in a state of dynamical equilibrium.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

^ Diamonds Johann Bode Successfully Predicted the Structure of the Solar System in 1772 Since the beginning of time astronomers, mystics, magi, astrologers, and just about anyone who observed the stars, noticed...

.Between the orbit of Neptune and the nearest star known to us is an immense void in which no bodies are yet known to exist, except comets.^ A small rocky body that orbits a star.

^ Prior to August 8, 1600, the star was not known to exist, when suddenly, it appeared, flaring to 3rd magnitude.

^ A team of 41 astronomers from Australia, Japan, New Zealand, and the United States took advantage of a naturally occurring telescope to image a small planet orbiting a star somewhere between us and the Galactic Bulge (the dense concentration of stars that exists at the core of our Milky Way galaxy).
  • Life on Other Planets | Reasons To Believe 18 January 2010 2:02 UTC www.reasons.org [Source type: FILTERED WITH BAYES]

.But although these sometimes wander to distances considerably beyond the orbit of Neptune, it is probable that the extent of the void which separates our system from the nearest star is hundreds of times the distance of the farthest point to which a comet ever recedes.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The point in the orbit of one component of a binary system where it is farthest from the other.

^ The point at which a body in orbit around the Earth reaches its farthest distance from the Earth.

.We may conclude this brief characterization of astronomy with a statement and classification of the principal lines on which astronomical researches are now pursued.^ In addition, there is are large number of companies that, rather than conduct astronomy research, make use of the background and talents of the astronomer in related areas.
  • A New Universe to Explore: Careers in Astronomy Brochure | American Astronomical Society 19 January 2010 9:52 UTC aas.org [Source type: FILTERED WITH BAYES]

^ Astronomy Picture of the Day - Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

^ While the individual astronomer may devote some time to research of personal interest, the research area is more often defined by the employer than is the case with universities and colleges.
  • A New Universe to Explore: Careers in Astronomy Brochure | American Astronomical Society 19 January 2010 9:52 UTC aas.org [Source type: FILTERED WITH BAYES]

The most comprehensive problem before the investigator is that of the constitution of the universe. .It is known that, while infinite diversity is found among the bodies of the universe, there are also common characteristics throughout its whole extent.^ There can be a common period even if the orbital periods are not whole numbers .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In a certain sense we may say that the universe now presents itself to the thinking astronomer, not as a heterogeneous collection of bodies, but as a unified whole.^ Astronomers may have detected smoke signals generated by a group of supernovas that blew up when the universe was only about 1.2 billion years old.
  • Astronomy News - Topix 19 January 2010 9:52 UTC www.topix.com [Source type: General]
  • Astronomy News - Topix 19 January 2010 9:52 UTC www.topix.net [Source type: General]

^ A team led by an Indiana University astronomer has found a sample of massive galaxies with properties that suggest they may have formed relatively recently.

^ These sayings are all true when applied to Venus for she furnishes the unifying bond between all members of the human family in whatever relationship they may be placed.
  • The Strange House - Planets 18 January 2010 2:02 UTC www.strangehouse.com [Source type: Original source]

.The number of stars is so vast that statistical methods can be applied to many of the characters which they exhibit - their spectra, their apparent and absolute luminosity, and their arrangement in space.^ The chemistry of the stars is strictly analogous to that of the sun, although their spectra exhibit diversities symptomatic of a considerable variety in physical state.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

^ OK, there were many many more stars, but they were all in their right places, and nothing was there that I couldn’t have seen from home.

^ Many Greek myths were about famous heroes who if they could not attain immortality, were placed in their deaths among the stars.

.Thus has arisen in recent times what we may regard as a third branch of astronomical science, known as Stellar Statistics. The development of this branch has infused life and interest into what might a few years ago have been regarded as the most lifeless mass of figures possible, expressing merely the positions and motions of innumerable individual stars, as determined by generations of astronomical observers.^ The last time it happened was 8 years ago… .
  • Monday Night: Planets Align in a Frown | Wired Science | Wired.com 18 January 2010 2:02 UTC www.wired.com [Source type: General]

^ Most of the time, astronomers derive information from an analysis of the light or the motions of celestial bodies, a process that, to the uninitiated, may seem more like sorcery than science.
  • A New Universe to Explore: Careers in Astronomy Brochure | American Astronomical Society 19 January 2010 9:52 UTC aas.org [Source type: FILTERED WITH BAYES]

^ Their possible masses—5 to 15 times Jupiter’s—are low enough that the researchers called them “isolated giant planets” in a paper published on October 6, 2000, in the journal Science.
  • Science Notes 2001: Tall Tales of Distant Planets 18 January 2010 2:02 UTC scicom.ucsc.edu [Source type: General]

.The development of this new branch requires great additions to this mass, the product of perhaps centuries of work on the older lines of the science.^ Archive: Astronomy Picture of the Day Students for the Exploration and Development of Space Astronomy Digest is a Free Monthly On-line magazine with current Astronomy News, Product Reviews, and much more.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

^ The mnemonic could work for either the new 8 planet line-up, the 8 major + 3 dwarf planets, or the old 9 planet arrangement in protest of Pluto's demotion.
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

^ Others conduct applied research to build upon the discoveries made through basic research and work to develop new devices, products, and processes.
  • Physicists and Astronomers 11 September 2009 3:15 UTC www.bls.gov [Source type: Academic]

.To the statistician of the stars, catalogues of spectra, magnitude, position and proper motions are of the same importance that census tables are to the student of humanity.^ As seen from the planet , the stars are back in the same positions again after this much time .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The measurement of the speed with which the individual stars are moving towards or from our system is a work of such magnitude that what has yet been done is scarcely more than a beginning.^ As usual though, the Universe is stranger than we assume, and the planets orbiting other stars defy our expectations.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ Deep-sky objects Spot galaxies, nebulae, star clusters, and other objects outside of our solar system.
  • Forums - Astronomy.com Forums 19 January 2010 9:52 UTC cs.astronomy.com [Source type: General]

^ Nontraditional constellation outlines, drawn to resemble actual objects more than the usual ways that stars are connected.
  • Peterson Field Guide to the Stars and Planets 19 January 2010 9:52 UTC www.williams.edu [Source type: General]

.The discovery by improved optical means, and especially by photography, of new bodies of our system so small that they evaded all scrutiny in former times, is still going on, but does not at present promise any important generalization, unless we regard as such the conclusion that our solar system is a more complex organism than was formerly supposed.^ I make groundbreaking discoveries all the time.

^ More to Solar Cycle than Sunspots .

^ Volcanoes on Venus --Venus has more volcanoes than any other planet in our solar system!
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

.One characteristic of astronomy which tends to make its progress slow and continuous arises out of the general fact that, except in the case of motions to or from us, which can be determined by a single observation with the spectroscope, the motion of a heavenly body can be determined only by comparing its position at two different epochs.^ Advanced study of a topic in observational astronomy, culminating in a one- to two-week observing session at the observatory in Arizona.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Slight differences between the two above small dot views are enough to make each dot a little more understandable.
  • GIC - Planets by the score in halos of giant planets 18 January 2010 2:02 UTC www.cosmicastronomy.com [Source type: General]

^ Some nights I only see one or maybe two, and sometimes not a one.

.The interval required between these two epochs depends upon the speed of the motion.^ Moveover, there are three larger intervals formed by taking pairs of these three fundamental day counts, two of which are clearly important in solar system design.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ Planetary periods, and even some intervals between conjunctions, are multiples of two sacred time units.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ That interval between what are arguably the two most important dates in the history of mankind is 12,051 days.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

.In the case of the greater number of the fixed stars this is so slow that centuries may have to elapse before motion can be deduced.^ In particular, you may want to look at times when Mercury is conjunct the fixed star Spica, and extremely benefic and protective fixed star.
  • Planets | Gryphon Astrology Blog 18 January 2010 2:02 UTC gryphonastrology.com [Source type: General]

^ Because a star's position may change slightly (see proper motion and precession of the equinoxes ), such tables must be revised at regular intervals.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ As we have found that it is difficult for a number of young students to determine when Mercury goes before or after the Sun we may say in farther elucidation of that subject: .
  • The Strange House - Planets 18 January 2010 2:02 UTC www.strangehouse.com [Source type: Original source]

.Even in the case of the planets, the variations in the form and position of the orbits are so slow that long periods of observation are required for their correct determination.^ Lagrange points) in orbit of Jupiter where minor planets could stay almost indefinitely - the Trojan group of asteroids were later discovered at these positions .
  • Famous Astronomers and Astrophysicists 11 September 2009 3:15 UTC cnr2.kent.edu [Source type: FILTERED WITH BAYES]

^ Lin’s team thinks the two planets formed in more spacious orbits, farther from their star and from each other.
  • Science Notes 2001: Tall Tales of Distant Planets 18 January 2010 2:02 UTC scicom.ucsc.edu [Source type: General]

^ Long term observations also indicate a massive, unseen fourth component with a period of about 190 years.

.The process of development is also made slow and difficult by the great amount of labour involved in deriving the results of astronomical observations.^ Conjunction (0°) may be favourable, difficult, or both, depending to a great degree upon the nature of the planets involved.
  • Astrology on the Web: The Planets 18 January 2010 2:02 UTC www.astrologycom.com [Source type: FILTERED WITH BAYES]

^ If your VLBI involves radio telescopes every ten miles for one thousand miles, the resultant image is as if it were made by one telescope… one thousand miles across.

^ Others conduct applied research to build upon the discoveries made through basic research and work to develop new devices, products, and processes.
  • Physicists and Astronomers 11 September 2009 3:15 UTC www.bls.gov [Source type: Academic]

.When an astronomer has made an observation, it still has to be " reduced," and this commonly requires more labour than that involved in making it.^ Astronomers the world over will rejoice, but I will rejoice a bit more than average.

^ The astronomers made the discovery in less than three months.

^ But great discoveries require more than just a great mind.

.But even this labour may be small compared with that of the theoretical astronomer, who, in the future, is to use the result as the raw material of his work.^ The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of Condé Nast Digital.
  • Planets | Wired Science | Wired.com 18 January 2010 2:02 UTC www.wired.com [Source type: General]
  • Astronomy | GeekDad | Wired.com 19 January 2010 9:52 UTC www.wired.com [Source type: General]

^ Virtual astronomers use software such as SETI(at)home to search for new planets or even signs of extra terrestrial life.

^ Future astronomers may come to view the Book of Abraham as unlocking the key to the design of the solar system.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

.The computations required in such work are of extreme complexity, and the labour required is still further increased by the fact that cases are rather exceptional in which the results reached by one generation will not have to be revised and reconstructed by another; processes which may involve the repetition of the entire work.^ They are, naturally enough, more conducive to personal development, for they tend to generate complex results requiring considerable interpretation and self-analysis, while favourable aspects more often produce simple effects that are easy to understand.
  • Astrology on the Web: The Planets 18 January 2010 2:02 UTC www.astrologycom.com [Source type: FILTERED WITH BAYES]

^ This unit varies slightly from one gas to another, but in general it corresponds to 2.24 × 10 4 cm 3 .

^ At times, however, those who are deeply involved in research may work long or irregular hours.
  • Physicists and Astronomers 11 September 2009 3:15 UTC www.bls.gov [Source type: Academic]

.We may, in fact, regard the fabric of astronomical science as a building in the construction of which no stone can be added without a readjustment of some of the stones on which it has to rest.^ No medium transmits radiation without some energy loss.

^ Some archaeologists dispute this, however, claiming the stones were chosen for no other reason than their beauty and size.

^ For their newsletter, The Reflector, write The Astronomical League, Executive Secretary, c/o Science Service Building, 1719 N Street NW, Washington, D.C. 20030.
  • Peterson Field Guide to the Stars and Planets 19 January 2010 9:52 UTC www.williams.edu [Source type: General]

.Thus it comes about that the observer, the computer, and the mathematician have in astronomical science a practically unlimited field for the exercise of their powers.^ As science professions go, astronomy is a relatively small field, with about 6,000 professional astronomers in North America.
  • A New Universe to Explore: Careers in Astronomy Brochure | American Astronomical Society 19 January 2010 9:52 UTC aas.org [Source type: FILTERED WITH BAYES]

^ Combined teams of physics and computer science students from several institutions will be formed to pursue on the tutorial exercises.
  • PLaNetS: Physics Lambda Network System 18 January 2010 2:02 UTC pcbunn.cithep.caltech.edu [Source type: FILTERED WITH BAYES]

^ So it’s not that I am an astronomer and thus know the future, no, I think that the one qualification maybe I have for talking about the future really is my 11 month old daughter.

.In treating so comprehensive a subject we may naturally distinguish between what we know of the universe and the methods and processes by which that knowledge is acquired.^ People with an afflicted Neptune should particularly avoid dealings with large corporations or they may be subjected to swindles of the most fantastic nature.
  • The Strange House - Planets 18 January 2010 2:02 UTC www.strangehouse.com [Source type: Original source]

^ THE PLANET OF SORROW KEYNOTE: OBSTRUCTION A fruitful method of acquiring knowledge is by comparison of similars and contrasts of opposites; thus lights and sidelights are brought out, which otherwise may escape attention.
  • The Strange House - Planets 18 January 2010 2:02 UTC www.strangehouse.com [Source type: Original source]

^ The keynote of Venus is "love," "harmony," and "rhythm," and if we want to know her nature we may profitably read that chapter and substitute "Venus' for "Love."
  • The Strange House - Planets 18 January 2010 2:02 UTC www.strangehouse.com [Source type: Original source]

.The former may be termed general, and the latter practical, astronomy.^ Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

.When we descend more minutely into details we find these two branches of the subject to be connected by certain principles, the application of which relates to both subjects.^ This survey course discusses both the physical principles and practical applications of the more common modern methods of materials characterization.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ They are cleared for two more flights within a 45 minute window.
  • LiveScience Blog » Space & Astronomy 19 January 2010 9:52 UTC www.livescience.com [Source type: News]

^ Pamela: Both of these are ways of bringing things into focus that otherwise might be blurred out.
  • Astronomy Cast - Ep. 133: Optical Astronomy 19 January 2010 9:52 UTC www.astronomycast.com [Source type: General]

.Considering as general or descriptive astronomy a description of the universe as we now understand it, the other branches of the subject generally recognized are as follows: Geometrical or Spherical Astronomy, by the principles of which the positions and the motions of the heavenly bodies are defined.^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

^ Playfair’s judicious use of astronomy was countered by John Bentley with a Scriptural argument which we now must consider invalid.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Astronomy deals with the position, size, motion, composition, energy, and evolution of celestial objects.

.Theoretical Astronomy,which may be considered as an extension of geometrical astronomy and includes the determination of the positions and motions of the heavenly bodies by combining mathematical theory with observation.^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

^ Astronomy deals with the position, size, motion, composition, energy, and evolution of celestial objects.

^ Because a star's position may change slightly (see proper motion and precession of the equinoxes ), such tables must be revised at regular intervals.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Modern theoretical astronomy, taken in the most limited sense, is based upon Celestial Mechanics, the science by which, using purely deductive mechanical methods, the laws of motion of the heavenly bodies are derived by deductive methods from their mutual gravitation towards each other.^ Greek astronomy was embodied in Ptolerny's "Almagest" (the name is of mixed Greek and Arabic derivation), composed at Alexandria about the middle of the second century A.D. It was based upon the geocentric principle.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

^ Use of terms "astronomy" and "astrophysics" 2.1 Scientific revolution 3.1 Radio astronomy 3.2 Infrared astronomy 3.3 Optical astronomy 3.4 Ultraviolet astronomy 3.5 X-ray astronomy 3.6 Gamma-ray astronomy 3.7 Fields not based on the electromagnetic spectrum 3.8 Astrometry and celestial mechanics 4 Theoretical astronomy 5.1 Solar astronomy 5.2 Planetary science 5.3 Stellar astronomy 5.4 Galactic astronomy 5.5 Extragalactic astronomy 5.6 Cosmology 6 Interdisciplinary studies 7 Amateur astronomy 8 Major questions in astronomy 9 The International Year of Astronomy 2009 10 See also 11 References 12 External links .
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ A Sense of Time and Scale in the Universe Johannes Kepler: The Laws of Planetary Motion Students at Miami University (Ohio) made a Kepler Discovery Lab (using the Galilean satellites and a Celestron telescope) to demonstrate the applicability of Kepler's Laws.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

.Practical Astronomy, which comprises a description of the instruments used in astronomical observation, and of the principles and methods underlying their application.^ However, a detailed study of the original texts of the earliest works on astronomy and an appraisal of the observation methodology and attitude of the astronomers reveals their strong scientific basis.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ This survey course discusses both the physical principles and practical applications of the more common modern methods of materials characterization.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Refers to the calculation or description of the underlying mechanics of motion of an astronomical object.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Spherical or Geometrical Astronomy. In astronomy, as in analytical geometry, the position of a point is defined by stating its distance and its direction from a point of reference taken as known.^ Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.

^ The true anomaly of a star is the angular distance (as measured from the central body and in the direction of the star's motion) between periastron and the observed position of the star.

^ The distances of the heavenly bodies can only be determined (speaking generally) by measuring their parallaxes, in other words, their apparent changes of position when seen from different points of view.
  • CATHOLIC ENCYCLOPEDIA: Astronomy 19 January 2010 9:52 UTC www.newadvent.org [Source type: FILTERED WITH BAYES]

.The numerical quantities by which the distance and direction, and therefore the position, are defined, are termed co-ordinates of the point.^ Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.

^ This is a vector quantity having magnitude (which is always positive) and direction.

^ The true anomaly of a star is the angular distance (as measured from the central body and in the direction of the star's motion) between periastron and the observed position of the star.

.The latter are measured or defined with regard to a fixed system of lines and planes, which form the basis of the system.^ In astronomy, we define the plane of the solar system as the ecliptic -- the apparent path that the Sun makes on the sky as the Earth orbits it -- the ecliptic is nothing more than a projection of the Earth's orbit on the sky.
  • Planets Alignments: Fact or Fiction? 18 January 2010 2:02 UTC www.etsu.edu [Source type: FILTERED WITH BAYES]

.The following are the fundamental concepts of such a system.^ For such a small member of the solar system, about which relatively little is known, Pluto has an impressive following.
  • Planets | Wired Science | Wired.com 18 January 2010 2:02 UTC www.wired.com [Source type: General]

^ In the following we describe some of the features that are required to develop such a system .
  • PLaNetS: Physics Lambda Network System 18 January 2010 2:02 UTC pcbunn.cithep.caltech.edu [Source type: FILTERED WITH BAYES]

(a) An origin or point of reference. .The points most generally taken for this purpose in astronomical practice are the following :- (I) The position of a point of observation on the earth's surface.^ In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It's worth following the link to this article if for nothing else but the beautiful photo of the comet taken by amateur astronomer Jack Newton.

^ Tycho's supernova''; made the most precise observations of stellar and planetary positions then known .
  • Famous Astronomers and Astrophysicists 11 September 2009 3:15 UTC cnr2.kent.edu [Source type: FILTERED WITH BAYES]

We conceive its position to be that occupied by an observer. .The position of a heavenly body is then defined by its direction and distance from the supposed observer.^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The distances of the Moon and the Sun from the Earth was accurately measured as 108 times the diameters of these heavenly bodies.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

(2) The centre of the earth. .This point, though it can never be occupied by an observer, is used because the positions of the heavenly bodies in relation to it are more readily computed than they can be from a point on the earth's surface.^ Times more than Earth .

^ To do this, we will use a method called determining the “quality of the mind.” This is a much more nuanced examination than an astrological IQ test, because unlike such tests, this method shows the person’s facility in various thinking modes.
  • Planets | Gryphon Astrology Blog 18 January 2010 2:02 UTC gryphonastrology.com [Source type: General]

^ (Though maybe it's just that D&D admit their bugs more quickly than other people.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

(3) The centre of the sun.
(q.) .In addition to these three most usual points, we may, of course, take the centre of a planet or that of a star in order to define the position of bodies in their respective neighbourhoods.^ The telescope will spend the next 3.5 years taking a closer at the targeted stars, waiting for their light to dim periodically as orbiting planets pass in front of them.
  • Astronomy | Wired Science | Wired.com 19 January 2010 9:52 UTC www.wired.com [Source type: General]

^ As usual though, the Universe is stranger than we assume, and the planets orbiting other stars defy our expectations.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ Easy-to-use diagrams of the moon's phases and its daily changes in position against the starry background, plus diagrams of planetary conjunctions with bright stars, with the moon, and with other planets.
  • Peterson Field Guide to the Stars and Planets 19 January 2010 9:52 UTC www.williams.edu [Source type: General]

Co-ordinates referred to.a point of observation as the origin are termed " apparent," those referred to the centre of the earth are " geocentric," those referred to the centre of the sun, " heliocentric." (b) The next concept of the system is a fundamental plane, regarded as fixed, passing through the origin. In connexion with it is an axis perpendicular to it, also passing through the origin. We may consider the axis and the plane as a single concept, the axis determining the plane, or the plane the axis. .The fundamental concepts of this class most in use are: (I) When a point on the earth's surface is taken as the origin, the fundamental axis may be the direction of gravity at that point.^ Fourth he established that earth had a daily rotation, an yearly orbit , and and tilted its axis in two directions on an annual basis.

^ Lagrange points Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ During this phase, despite the absence of direct illumination by the Sun, the lunar surface is just visible in light reflected from the Earth.
  • Planets in our Universe 18 January 2010 2:02 UTC www.annabelburton.com [Source type: FILTERED WITH BAYES]

This direction defines the vertical line. .The fundamental plane which it determines is horizontal and is termed the plane of the horizon.^ This is the problem of determining the astrometric distortions in an image away from a basic tangent-plane projection of the sky; these distortions are a product of atmosphere and optics and are among the fundamental limitations of cameras.

Such a plane is realized in the surface of a liquid, a basin of quicksilver, for example.
.(2) When the centre of the earth is taken as origin, the most natural fundamental axis is that of the earth's rotation.^ Fourth he established that earth had a daily rotation, an yearly orbit , and and tilted its axis in two directions on an annual basis.

^ Mercury rotates very slowly on its axis, completing a single day every 59 Earth days.

^ There is a considerable magnetic field around the planet which is offset from the centre, and is inclined at 60 deg to the rotational axis.
  • Planets in our Universe 18 January 2010 2:02 UTC www.annabelburton.com [Source type: FILTERED WITH BAYES]

.This axis cuts the earth's surface at the North and South Poles.^ The fact that perfectly polarity opposite flares issue straight from a north and south pole of Alpha Pegasus is significant.
  • GIC - Planets by the score in halos of giant planets 18 January 2010 2:02 UTC www.cosmicastronomy.com [Source type: General]

^ You can see the vents of water ice blasting out of the surface of Enceladus, near its south pole.

^ But if you had the patience to watch, you should see the clouds move from the south pole to the north pole over the next 15 years before coming back 15 years later.

The fundamental plane perpendicular to it is the plane of the equator. .This plane intersects the earth's surface in the terrestrial equator.^ For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Coordinates referred to this system are termed equatorial.^ The vernal equinox is a reference point in the equatorial coordinate system .
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.A system of equatorial co-ordinates may also be used when the origin is on the earth's surface.^ Orbits Viewed from Earth's surface, the planets of the solar system and the stars appear to move around Earth.
  • NASA - Planets 18 January 2010 2:02 UTC www.nasa.gov [Source type: General]

^ Titan is the only place in the solar system other than the earth that appears to have large quantities of liquid sitting on the surface.

^ Some say it may represent what the earth's atmosphere was like when the solar system first formed.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

.The fundamental axis, instead of being the earth's axis itself, is then a line parallel to it, and the fundamental plane is the plane passing through the point, and parallel to the plane of the equator.^ The smallest planets currently being found are being found through microlensing events, where the planet passes in front of a background star and its gravitational pull affects the light from that background object.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ A fundamental particle produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The gylphs associated with each day are numbered from that point, being called G1 through G9 (for "Glyph 1").
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

.(3) In the system of heliocentric co-ordinates, the plane in which the earth moves round the sun, which is the plane of the ecliptic, is taken as the fundamental one.^ The plane of Earth's orbit about the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Because Uranus' axis of rotation is tilted into the ecliptic plane , One pole is heated while the other is put into the deep freeze for decades.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

^ Some Definitions to get started ecliptic क्तांतीव्रुत्त (Kranthivruth) (ēklĬptĬk, Ĭ-) , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The axis of the ecliptic is a line perpendicular to this plane.^ Because Uranus' axis of rotation is tilted into the ecliptic plane , One pole is heated while the other is put into the deep freeze for decades.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

^ The line that marks the intersection between the two inclined planes is called the line-of-nodes and the projected position of this line on the sky called the nodes of the planet's orbit (which of course must lie on the ecliptic).
  • Planets Alignments: Fact or Fiction? 18 January 2010 2:02 UTC www.etsu.edu [Source type: FILTERED WITH BAYES]

.(c) The third concept necessary to complete the system is a fixed line passing through the origin, and lying in the fundamental plane.^ A fundamental particle produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Dark lines superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source.

^ Kirchhoff's third law Continuous radiation viewed through a low-density gas will produce an absorption-line spectrum.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

This line defines an initial direction from which other directions are counted.
The geometrical concepts just defined are shown in fig. .1. Here 0 is the origin, whatever point it may be; OZ is the fundamental axis passing through it.^ [A84] (b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.

^ The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The cone of rays through a lens from an off-axis object does not focus at a point.

.In order to represent in the figure the 2 position of the f u ndamental plane, we conceive a circle to be drawn round 0, lying in that plane.^ Some Definitions to get started ecliptic क्तांतीव्रुत्त (Kranthivruth) (ēklĬptĬk, Ĭ-) , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

This circle, projected in Q perspective as an ellipse, is shown in X the figure. OX is the fixed initial ,% line by which e directions are to be defined.
.Now let P be any point in space, say the centre of a heavenly body.^ In maybe 21 years or so – let’s say June of 2027 -- she could be sitting down right there where you are now.

.Conceive a perpendicular PQ to be dropped from this point on the fundamental plane, meeting the latter in the point Q; PQ will then be parallel to OZ. The co-ordinates of P will then be the following three quantities: - (I) The length of the line OP, or the distance of the body from the origin, which distance is called the radius vector of the body.^ If the object (or a point on it) moves from point P 1 to point P 2 in a plane perpendicular to the axis, is the angle P 1 OP 2 , where O is the point at which the perpendicular plane meets the axis.

^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.

.(2) The angle XOQ which the projection of the radius vector upon the fundamental plane makes with the initial line OX. This angle is called the Longitude, Right Ascension or Azimuth of the body, in the various systems of co-ordinates.^ It's been shown, initially by Victor Zebehay, when you get multiple objects gravitationally interacting, you can have a three-body problem where one of the objects gets radically flung out of the system.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ The line that marks the intersection between the two inclined planes is called the line-of-nodes and the projected position of this line on the sky called the nodes of the planet's orbit (which of course must lie on the ecliptic).
  • Planets Alignments: Fact or Fiction? 18 January 2010 2:02 UTC www.etsu.edu [Source type: FILTERED WITH BAYES]

^ Pluto, the other so-called planet, is one of several small bodies in the solar system which is not a "major planet" and does not appear to be part of the Lord's timepiece.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

.We may term it in a general way the longitudinal co-ordinate.^ Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ Large gas planets may continue to settle, shrink ("collapse") very slowly, for billions of years and generate some heat in that way.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.(3) The angle QOP, which the radius vector makes with the fundamental plane.^ A narrowed laboratory frame of reference makes it impossible to tell the radius vector of the orbital axis, but, I happen to know (as would anyone who cut the zooms).
  • GIC - Planets by the score in halos of giant planets 18 January 2010 2:02 UTC www.cosmicastronomy.com [Source type: General]

This we may call the latitudinal co-ordinate. .Instead of it is frequently used the complementary angle ZOP, known as the polar distance of the body.^ Because the vernal equinox is not always visible in the night sky (especially in the spring), whereas the sigma point is always visible, the hour angle is used in actually locating a body in the sky.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The distance to the Sun remained essentially unknown (except that it was known to be "large") until 1761 when a transit of Venus could be used to estimate it.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Since ZOQ is a right angle, it fellows that the sum of the polar distance and the latitudinal coordinates is always 90°.^ The angular distance from the sigma point to a star's hour circle is called its hour angle ; it is equal to the star's right ascension minus the local sidereal time.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Either may be used for astronomical purposes.^ Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

.It is readily seen that the position of a heavenly body is completely defined when these co-ordinates are given.^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The distances of the Moon and the Sun from the Earth was accurately measured as 108 times the diameters of these heavenly bodies.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.One of the systems of co-ordinates is familiar to every one, and may be used as a general illustration of the method.^ An angular value used to describe the position of one member of a binary system with respect to the other.

^ Mercury completes one full orbit around the Sun every 88 days, giving it the shortest year in the Solar System.

^ The term “Very Long Baseline Interferometry” (VLBI) isn’t one with which many people are familiar, but I bet that although you may not know the name you are familiar with the concept.

.It is our system of defining the position of a point on the earth's surface by its latitude and longitude.^ Scientists have found an Earth-like planet orbiting one of the closest stars to our solar system .
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

^ Observing the planets People have known the inner six planets of our solar system for thousands of years because they are visible from Earth without a telescope.
  • NASA - Planets 18 January 2010 2:02 UTC www.nasa.gov [Source type: General]

^ Orbits Viewed from Earth's surface, the planets of the solar system and the stars appear to move around Earth.
  • NASA - Planets 18 January 2010 2:02 UTC www.nasa.gov [Source type: General]

Regarding 0 (fig. .I) as the centre of the earth, and P as a point on the earth's surface, a city for example, it will be seen that OZ being the earth's axis, the circle MN will be the equator.^ At what speed does a point at the equator rotate around the rotation axis of the Earth?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It represents the entire sky; all celestial objects other than the earth are imagined as being located on its inside surface.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The initial line OX then passes through the foot of the perpendicular dropped from Greenwich upon the plane of the equator, and meets the surface at N. The angle QOP is the latitude of the place and the angle NOQ its longitude.^ The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic ; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The obliquity of the ecliptic is the inclination of the plane of the ecliptic to the plane of the celestial equator, an angle of about 23 1/2 .
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

The longitudes and latitudes thus defined are geocentric, and the latitude is slightly different from that in ordinary use for geographic purposes. The difference arises from the oblateness of the earth, and need not be considered here.
The conception of the co-ordinates we have defined is facilitated by introducing that of the celestial sphere. .This conception is embodied in our idea of the vault of heaven, or of the sky.^ Astrology is filled with these kinds of ideas, but people also start looking at the skies when special calendar dates are reached in our calendar or in someone else's calendar.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Taking as origin the position of an observer, the direction of a heavenly body is defined by the point in which he sees it in the sky; that is to say, on the celestial sphere.^ The celestial sphere is an imaginary sphere with the observer at its center.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The direction in the sky to which the telescope is pointed.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

.Imagining, as we may well do, that the radius of this sphere is infinite - then every direction, whatever the origin, may be represented by a point on its surface.^ It represents the entire sky; all celestial objects other than the earth are imagined as being located on its inside surface.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Indeed, both the solar and the lunar Zodiac may well originate in India.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The supplementary SI unit of solid angle defined as the solid central angle of a sphere that encloses a surface on the sphere equal to the square of the sphere's radius.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

Take for example the vertical line which is embodied in the direction of the plumb line. .This line, extended upwards, meets the celestial sphere in the zenith.^ If the earth's axis is extended, the points where it intersects the celestial sphere are called the celestial poles; the north celestial pole is directly above the earth's North Pole, and the south celestial pole directly above the earth's South Pole.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The earth's axis, continued indefinitely upwards, meets the sphere in a point called the Celestial Pole.^ If the earth's axis is extended, the points where it intersects the celestial sphere are called the celestial poles; the north celestial pole is directly above the earth's North Pole, and the south celestial pole directly above the earth's South Pole.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Some Definitions to get started ecliptic क्तांतीव्रुत्त (Kranthivruth) (ēklĬptĬk, Ĭ-) , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ If the object (or a point on it) moves from point P 1 to point P 2 in a plane perpendicular to the axis, is the angle P 1 OP 2 , where O is the point at which the perpendicular plane meets the axis.

.This point in our middle latitudes is between the zenith and the north horizon, near a certain star of the second magnitude familiarly known as the Pole Star.^ The latitude can be determined, for example, from the height of the Pole Star above the horizon .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Prior to August 8, 1600, the star was not known to exist, when suddenly, it appeared, flaring to 3rd magnitude.

^ Both are near the middle of the star diskface, slightly to the left and below the midaxial horizon.
  • GIC - Planets by the score in halos of giant planets 18 January 2010 2:02 UTC www.cosmicastronomy.com [Source type: General]

.As the earth revolves from west to east the celestial sphere appears to us to revolve in the opposite direction, turning on the line j oining the Celestial Poles as on a pivot.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Some Definitions to get started ecliptic क्तांतीव्रुत्त (Kranthivruth) (ēklĬptĬk, Ĭ-) , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The columns "East" and "West" show whether the Sun rises or sets in about that direction.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.As we conceive of the sky, it does not consist of an entire sphere but only as a hemisphere bounded by the horizon.^ Sometimes the thing that you want to look at in the sky is only viewable from the southern hemisphere, so down to Chile you go.

^ It does not matter where those stars are in the sky, as long as they are above the horizon , so people noticed this retrograde motion already thousands of years ago.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.But we have no difficulty in extending the conception below the horizon, so that the earth with everything upon it is in the centre of a complete sphere.^ The earliest concept of a heliocentric model of the solar system , in which the Sun that is at the centre of the solar system and the Earth that is orbiting it, is found in several Vedic Sanskrit texts written in ancient India .
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ BC ) recognized that the Earth was round and believed that the Sun was " the centre of the spheres " as described in the Vedas at the time.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The two parts of this sphere are the visible hemisphere, which is above the horizon, and the invisible, which is below it.^ He wrote the Siddhanta-Shiromani which consists of two parts: Goladhyaya (sphere) and Grahaganita (mathematics of the planets).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Then the plumb line not only defines the zenith as already shown, but in a downward direction it defines the nadir, which is the point of the sphere directly below our feet.^ Our paper came out only this week, but, since their paper was already published, one of the referees asked us to compare and comment on their paper.

^ I won’t give away the ending (perhaps you know it already), but instead simply point you in the right direction.

^ If the earth's axis is extended, the points where it intersects the celestial sphere are called the celestial poles; the north celestial pole is directly above the earth's North Pole, and the south celestial pole directly above the earth's South Pole.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.On the side of this sphere opposite to the North Celestial is the South Pole, invisible in the Northern Terrestrial Hemisphere but visible in the Southern one.^ The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern.

^ It divides the celestial sphere into the northern and southern skies.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

The relation of geocentric to apparent co-ordinates depends upon the latitude of the observer. .The changes which the aspect of the heaven undergoes, as we travel North and South, are so well known that they need not be described in detail here; but a general statement of them will give a luminous idea of the geometrical co-ordinates we have described.^ The Dan&Dave add-ons These add-ons are well-known and popular; they are also severely crippled in the shareware versions, bulky, and (according to some) prone to bugs.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ Planets rotate around their axis, which generates Coriolis forces that make air traveling north or south deviate from its straight course.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The Sun is not considered a variable star , but it does undergo periodic changes in activity known as the sunspot cycle .
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

.Imagine an observer starting from the North Pole to travel towards the equator, carrying his zenith with him.^ The equator is hotter than the poles , so, on average, hotter air travels from the equatorial region to the poles, and colder air travels from the poles to the equatorial region, so there is a north-south component to the prevailing winds.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Why does weather usually travel to the west along the equator but to the east between the equator and the poles?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.When at the pole his zenith coincides with the celestial pole, and as the earth revolves on its axis, the heavenly bodies perform their apparent diurnal revolutions in horizontal circles round the zenith.^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Some Definitions to get started ecliptic क्तांतीव्रुत्त (Kranthivruth) (ēklĬptĬk, Ĭ-) , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ To designate the position of a star, the astronomer considers an imaginary great circle passing through the celestial poles and through the star in question.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.As he travels South, his zenith moves along the celestial sphere, and the circles of diurnal rotation become oblique to the horizon.^ [A84] (b) The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith.

^ Planets rotate around their axis, which generates Coriolis forces that make air traveling north or south deviate from its straight course.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ [A84] (b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.

The obliquity continually increases until the observer reaches the equator. .His zenith is then in the equator and the celestial poles are in the North and South horizon respectively.^ The columns "North" and "South" show whether the planet turns clockwise (cw) or counterclockwise (ccw) if you look at the planet from above that pole.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The great circle on the celestial sphere halfway between the celestial poles is called the celestial equator; it can be thought of as the earth's equator projected onto the celestial sphere.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ If the earth's axis is extended, the points where it intersects the celestial sphere are called the celestial poles; the north celestial pole is directly above the earth's North Pole, and the south celestial pole directly above the earth's South Pole.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The circles in which the heavenly bodies appear to revolve are then vertical.^ He explicitly mentions that the luminous heavenly bodies, despite being stationary appear to move from east to west - " Achalani bhani samapashchimagani", he said.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Continuing his journey towards the south, the north celestial pole sinks below the horizon; the south celestial pole rises above it; or to speak more exactly, the zenith of the observer approaches that pole.^ It happens in the summertime, when the south pole spends something like 10 years in continuous sunlight.

^ But if you had the patience to watch, you should see the clouds move from the south pole to the north pole over the next 15 years before coming back 15 years later.

^ The columns "North" and "South" show whether the planet turns clockwise (cw) or counterclockwise (ccw) if you look at the planet from above that pole.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The circles of diurnal revolution again become oblique. .Finally, at the south pole the circles of diurnal revolution are again apparently horizontal, but are described in a direction apparently (but not really) the reverse of that near the north pole.^ This huge feature is circling the north pole and has six distinctive sides.

^ The equator is hotter than the poles , so, on average, hotter air travels from the equatorial region to the poles, and colder air travels from the poles to the equatorial region, so there is a north-south component to the prevailing winds.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The fact that perfectly polarity opposite flares issue straight from a north and south pole of Alpha Pegasus is significant.
  • GIC - Planets by the score in halos of giant planets 18 January 2010 2:02 UTC www.cosmicastronomy.com [Source type: General]

The reader who will trace out these successive concepts and study the results of his changing positions will readily acquire the notions which it is our subject to define.
.We have next to point out the relation of the co-ordinates we have described to the annual motion of the earth around the sun.^ The most widespread opinion in ancient writings about this subject until the 16th century was that all of the planets and the Sun and Moon orbit around the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Wall chart, linen-backed, with rods showing the motions of the planets around the sun.
  • Astronomy,Antiquarian Books, Rare Books,antique maps,antique globes,historical prints, travel guides, atlases, gazetteers 19 January 2010 9:52 UTC www.murrayhudson.com [Source type: General]

^ The planets and the Earth all orbit around the Sun , each at its own speed, so the distance of a planet from the Earth is not always the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In consequence of this motion the sun appears to us to describe annually a great circle, called the ecliptic, round the celestial sphere, among the stars, with a nearly uniform motion, of somewhat less than 1 0 in a day.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The results of this article show that the periods of the planets all are nearly whole numbers of either of two different celestial measures: the sacred round of 260 days or the "merc," defined herein to be 117 days.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ Some Definitions to get started ecliptic क्तांतीव्रुत्त (Kranthivruth) (ēklĬptĬk, Ĭ-) , the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Were the stars visible in the daytime in the immediate neighbourhood of the sun, this motion could be traced from day to day.^ If you could stand on the surface of Mercury, you would see the Sun rise and not set for another 176 Earth days.

^ The apparent motion of the Sun , Moon , planets , and stars in the sky can be explained in two ways: (1) the Sun , Moon , planets , and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ "It is a green beauty that could become visible to the naked eye any day now," says Ye."

.The ecliptic intersects the celestial equator at two opposite points, the equinoxes, at an angle of 23° 27'. The vernal equinox is taken as the initial point on the sphere from which co-ordinates are measured in the equatorial and ecliptic systems.^ The obliquity of the ecliptic is the inclination of the plane of the ecliptic to the plane of the celestial equator, an angle of about 23 1/2 .
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ An important reference point on the celestial equator is the vernal equinox , the point at which the sun crosses the celestial equator in March.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

Referring to fig. .I, the initial line OX is defined as directed toward the vernal equinox, at which point it intersects the celestial sphere.^ (Vasanth Sampat) Vernal equinox (ēkwĬnŏks) , either of two points on the celestial sphere where the ecliptic and the celestial equator intersect.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Because the vernal equinox is not always visible in the night sky (especially in the spring), whereas the sigma point is always visible, the hour angle is used in actually locating a body in the sky.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The following is an enumeration of the co-ordinates which we have described in the three systems: Apparent System.^ However, if the three objects form a right angle, in other words one Triton follows behind the other, then apparently Neptune cancels their rotation with repect to each other.

^ All three of the intervals of the vientena, trecena and novena are apparently fundamental to the design of the solar system.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ In the following we describe some of the features that are required to develop such a system .
  • PLaNetS: Physics Lambda Network System 18 January 2010 2:02 UTC pcbunn.cithep.caltech.edu [Source type: FILTERED WITH BAYES]

Latitudinal Co-ordinate; Altitude or Zenith Distance. Longitudinal „ Azimuth.
Equatorial System.
Latitudinal Co-ordinate; Declination or Polar Distance. Longitudinal „ Right Ascension.
Ecliptic System.
Latitudinal Co-ordinate; Latitude or Ecliptic Polar Distance. Longitudinal „ Longitude.
Table of contents

Relation of the Diurnal Motion to Spherical Co-ordinates

.The vertical line at any place being the fundamental axis of the apparent system of co-ordinates, this system rotates with the earth, and so seems to us as fixed.^ He gave a remarkably accurate measure of the period of one rotation of the earth with reference to the fixed stars in the sky, as 23hours, 56minutes, 4.1seconds.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Mercury rotates very slowly on its axis, completing a single day every 59 Earth days.

^ In 1543, Nicolaus Copernicus (1473 - 1543) published a book in which he swept away Ptolemy's ideas and said that the Earth rotates around its axis, the stars are fixed, and the Earth orbits around the Sun as well.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The other two systems, including the vernal equinox, are fixed on the celestial sphere, and so seem to us to perform a diurnal revolution from east towards west.^ It features one of the last two telescopes belonging to the astronomer, as well as his notes, paintings, and other instruments, including the cylindrical sundial and Michelangelo's compass .
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

^ The vernal equinox, also known as “the first point of Aries,” is the point at which the sun appears to cross the celestial equator from south to north.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Regarding the period of the revolution as 24 hours, the apparent motion goes on at the rate of 15° per hour.^ The rate is about 60 meteors per hour.

^ The Earth's circumference at the equator is about 40000 km or 24900 miles and the Earth rotates once per 24 hours , so the rotation speed at the equator is 40000/24 = 1670 km/h or 24900/24 = 1038 mph.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Here we have to make a distinction of fundamental importance between the diurnal motions of the sun and of the stars.^ As seen from the planet , the Sun returns to (about) the same place between the stars after this much time .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This makes the star (which is a lot like our Sun) fill basically 70 degrees of the sky.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ A superior planet (one that is further away from the Sun than the Earth is) usually moves towards the east between the stars , but around its opposition it temporarily moves to the west and then makes a kind of loop between the stars , before continuing with its usual eastward motion.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Owing to the unceasing apparent motion of the sun toward the east, the interval between two passages of the same star over the meridian is nearly four minutes less than the interval between consecutive passages of the sun.^ Interval between two successive culminations of the Sun - i.e., the period from apparent noon to apparent noon.

^ The interval (27.555 days) between two successive perigee passages of the Moon.

^ The difficulty is to measure the gravity, because here on Earth the gravity of the Earth is very much greater than the gravity between any two things that we can handle in a laboratory.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The latter is the measure of the day as used in civil life. .In astronomical practice is introduced a day, termed " sidereal," determined, not by the diurnal revolution of the sun, but of the stars.^ This is also called the synodical period of revolution or the planet day or sol (which means " Sun " in Latin).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The astronomical unit is defined as the length of the radius of the unperturbed circular orbit of a body of negligible mass moving around the Sun with a sidereal angular velocity of 0.017202098950 radian per day of 86,400 ephemeris seconds.

^ Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.The year, which comprises 365.25 solar days, contains 366.25 sidereal days.^ A sidereal day is equal to 365/366 days, or about 4 minutes shorter than a solar day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Forty years of 365 days = 25 Venus cycles of 584 days and also completes 126 Mercury cycles.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ The sidereal orbital period (planet year ) is given in Earth years of 365.25 days (i.e., Julian years ) and in planet days (sols).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The latter are divided into sidereal hours, minutes and seconds as the solar day is. The conception of a revolution through 360° in 24 hours is applicable to each case.^ One assumes that the Mayan long count prophetic year of 360-days begins on day 1 of the novena because 9 divides evenly into 360.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ We also know that the Lord divides periods into 12 parts, such as 12 hours in a day (John 11:9) or 12 months in a year.
  • Planets Testify 18 January 2010 2:02 UTC www.johnpratt.com [Source type: FILTERED WITH BAYES]

^ This leads to the need to support Terabyte-scale transactions, where the data is transferred in minutes rather than many hours, so that many transactions per day can be completed.
  • PLaNetS: Physics Lambda Network System 18 January 2010 2:02 UTC pcbunn.cithep.caltech.edu [Source type: FILTERED WITH BAYES]

.The sun apparently moves at the rate of 15° in a solar hour; the stars at the rate of 15° in a sidereal hour.^ It is a first strong and direct indication of the presence of phenomena of convection, transport of heat by the moving matter, in a star other than the Sun.

^ The angular distance from the sigma point to a star's hour circle is called its hour angle ; it is equal to the star's right ascension minus the local sidereal time.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ These include the solar system, planets, comets, meteorites, asteroids, stars, the sun, galaxies and of course our closest neighbor in space, the moon.

.The latter motion leads to the use, in astronomical practice, of time instead of angle, as the unit in which the right ascensions are to be expressed.^ Right Ascension A coordinate which, along with declination , may be used to locate any position in the sky.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The right ascensions and declinations of many stars are listed in various reference tables published for astronomers and navigators.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The angular distance from the sigma point to a star's hour circle is called its hour angle ; it is equal to the star's right ascension minus the local sidereal time.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

Considering the position of the vernal equinox, and also of a star on the celestial sphere, it will be seen that the interval between the transits of these two points across the meridian may be used to measure the right ascension of a star, since the latter amounts to FIG. I.
15° for every sidereal hour of this interval. .For example, if the right ascension of a star is exactly 15°, it will pass the meridian one sidereal hour after the vernal equinox.^ A photograph of the newest possible moon , one that's only about 15 hours old.
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

^ When one star passes in front of the other, the light of the system dims.

For the relations thus arising, and their practical applications, see Time, Measurement Of.
.Theoretical Astronomy. Theoretical Astronomy is that branch of the science which, making use of the results of astronomical observations as they are supplied by the practical astronomer, investigates the motions of the heavenly bodies.^ In neutrino astronomy , astronomers use special underground facilities such as SAGE , GALLEX , and Kamioka II/III for detecting neutrinos .
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm).
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ Astronomers Jyotisha, the Vedic Astrology of India Vedic Astrology is still commonly used in India to help make important decisions.

.In its most important features it is an offshoot of celestial mechanics, between which and theoretical astronomy no sharp dividing line can be drawn.^ Theorists in astronomy endeavor to create theoretical models and between several alternate or conflicting models.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ High energy x-rays, often from about 10 keV to nearly 1000 keV. The dividing line between hard and soft x-rays is not well defined and can depend on the context.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Theoretical straight line through a celestial body, around which it rotates.

.While it is true that the one is concerned altogether with general theories, it is also true that these theories require developments and modifications to apply them to the numberless problems of astronomy, which we may place in either class.^ One of the best places to keep up with current events in astronomy is NASA’s web site.

^ Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ One solution to this problem is that greenhouse gases may come from elsewhere, too, for example from volcanoes.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Among the problems of theoretical astronomy we may assign the first place to the determination of orbits, which is auxiliary to the prediction of the apparent motions of a planet, satellite or star.^ As usual though, the Universe is stranger than we assume, and the planets orbiting other stars defy our expectations.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ They know this first through measuring the wobble caused to stars by planets and other objects orbiting them.

^ An object in hydrostatic equilibrium orbiting a star is a planet.

.The computations involved in the process, while simple in some cases, are extremely complex in others.^ Some people prefer the 'pure' gaming experience, because there are fewer variables involved; while others prefer to have as many addons as possible in the game.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ Fraser: In some cases it is coming from just pure temperature and in other cases there are other things that can excite it.
  • Astronomy Cast - Ep. 133: Optical Astronomy 19 January 2010 9:52 UTC www.astronomycast.com [Source type: General]

.The orbit of a newly-discovered planet or comet may be computed from three complete observations by well-known methods in a single day.^ These planets have orbits of just nine days, 32 days, 197 days.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ By Jason Kottke • May 14, 2007 • astronomy physics science space Scientists have found an Earth-like planet orbiting .
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

^ This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit).
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.From the resulting elements of the orbit the positions of the body from day to day may be computed and tabulated in an ephemeris for the use of observers.^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

^ On the airplane to Abu Dhabi, I checked my calculations for transformations between position and velocity three-vectors and the standard orbital elements for Keplerian orbits.

^ The astronomical unit is defined as the length of the radius of the unperturbed circular orbit of a body of negligible mass moving around the Sun with a sidereal angular velocity of 0.017202098950 radian per day of 86,400 ephemeris seconds.

.But when definitive results as to the orbits are required, it is necessary to compute the perturbations produced by such of the major planets as have affected the motions of the body.^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Many planetarium programs enable you to tie the viewing direction to a particular celestial body such as a planet or a star .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

With this complicated process is associated that of combining numerous observations with a view of obtaining the best definitive result. .Speaking in a general way, we may say that computations pertaining to the orbital revolutions of double stars, as well as the bodies of our solar system, are to a greater or less extent of the classes we have described.^ Lowell NASA Satellite to Travel to and Study Asteroids An upcoming voyage beyond the reaches of Mars may tell astronomers just what happened in the earliest days of our solar system.

^ This can give new insight into how busy our solar system is as far as impact capable objects, as well as reveal the true extent of how other planets, like Saturn, serve as object scrubbers in our solar system.

^ Our planet is also the densest one in the Solar System due to its core, which is comprised mostly of iron.

.The principal modification is that,up to the present time, stellar astronomy has not advanced so far that a computation of the perturbations in each case of a system of stars is either necessary or possible, except in exceptional cases.^ However, all of these extrasolar planets exhibit characteristics that would eliminate the possibility of another planet residing in the same planetary system that could possibly support advanced life for a brief time or even primitive life for a long time.
  • Life on Other Planets | Reasons To Believe 18 January 2010 2:02 UTC www.reasons.org [Source type: FILTERED WITH BAYES]

^ So it's possible that when you have multiple stars forming, and planets forming around these multiple stars, that some of the planets can get ejected from the system and end up roaming the galaxy completely on their own.
  • Astronomy Cast - Ep. 3: Hot Jupiters and Pulsar Planets 18 January 2010 2:02 UTC www.astronomycast.com [Source type: General]

^ BTW, when you're dealing with stars, "on the brink" could refer to a period of time up to 100,000 years from now.
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

.Celestial Mechanics. Celestial Mechanics is, strictly speaking, that branch of applied mathematics which, by deductive processes, derives the laws of motion of the heavenly bodies from their gravitation towards each other, or from the mutual action of the parts which form them.^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.The science had its origin in the demonstration by Sir Isaac Newton that Kepler's three laws of planetary motion, and the law of gravitation, in the case of two bodies, could be mutually derived from each other.^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ A Sense of Time and Scale in the Universe Johannes Kepler: The Laws of Planetary Motion Students at Miami University (Ohio) made a Kepler Discovery Lab (using the Galilean satellites and a Celestron telescope) to demonstrate the applicability of Kepler's Laws.
  • Astronomy Resources: Links, Telescopes, Movies, Deep Space, Instructional Materials 19 January 2010 9:52 UTC jrscience.wcp.muohio.edu [Source type: FILTERED WITH BAYES]

^ Distant bodies in a planetary system are like some distant in-laws.
  • Life on Other Planets | Reasons To Believe 18 January 2010 2:02 UTC www.reasons.org [Source type: FILTERED WITH BAYES]

.A body can move round the sun in an elliptic orbit having the sun in its focus, and describing equal areas in equal times, only under the influence of a force directed towards the sun, and varying inversely as the square of the distance from it.^ It’s that the Earth orbits the sun in an elliptical, uneven, orbit.

^ If the orbit is elliptical the radius will vary.
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^ If P moon is the sidereal period of such a moon (in its orbit around Jupiter), and P J is the sidereal period of Jupiter (in its orbit around the Sun), then the synodical period P syn of the moon is equal to .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Conversely, assuming this law of attraction, it can be shown that the planets will move according to Kepler's laws.^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Kepler's third law The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.
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^ How can you use Kepler's Law to find the distance to a planet?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Thus celestial mechanics may be said to have begun with Newton's Principia. The development of the science by the successors of Newton, especially Laplace and Lagrange, may be classed among the most striking achievements of the human intellect. .The precision with which the path of an eclipse is laid down years in advance cannot but imbue the minds of men with a high sense of the perfection reached by astronomical theories; and the discovery, by purely mathematical processes, of the changes which the orbits and motions of the planets are to undergo through future ages is more impressive the more fully one apprehends the nature of the problem.^ They know this first through measuring the wobble caused to stars by planets and other objects orbiting them.

^ The solar system makes more sense now, and, in three years most people have come to terms with the new solar system.

^ The distances between the planets change all the time , because each planet goes along its own orbit at its own speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The purpose of the present article is to convey a general idea of the methods by which the results of celestial mechanics are reached, without entering into those technical details which can be followed only by a trained mathematician. .It must be admitted that any intelligent comprehension of the subject requires at least a grasp of the fundamental conceptions of analytical geometry and the infinitesimal calculus, such as only one with some training in these subjects can be expected to have.^ The Dan&Dave add-ons These add-ons are well-known and popular; they are also severely crippled in the shareware versions, bulky, and (according to some) prone to bugs.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ And now I must in the end admit that one of those is actually true.

^ If such a feat is not an indication of literacy and of written records, at the least it supposes a mnemotechnical device capable of preserving information orally, and the one that was available then was verse.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.This being assumed, the hope of the writer is that the exposition will afford the student an insight into the theory which may facilitate his orientation, and convey to the general reader with a certain amount of mathematical training a clear idea of the methods by which conclusions relating to it are drawn.^ A single fact can branch off into a veritable catacomb cluster of other facts, ideas, notions, and related information.

.The non-mathematical reader may possibly be able to gain some general idea, though vague, of the significance of the subject.^ Somehow, John assumed that the religious depth of his text would gain from including some allusions to mathematics.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Large gas planets may continue to settle, shrink ("collapse") very slowly, for billions of years and generate some heat in that way.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The fundamental hypothesis of the science assumes a system of bodies in motion, of which the sun and planets may be taken as examples, and of which each separate body is attracted toward all the others according to the law of Newton.^ It's got all the planets of the solar system on it, plus the Sun.
  • astronomy (kottke.org) 19 January 2010 9:52 UTC www.kottke.org [Source type: General]

^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.The motion of each body is then expressed in the first place by Newton's three laws of motion (see Motion, Laws Of, and Mechanics).^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

The first step in the process shows in a striking way the perfection of the analytic method. The conception of force is, so to speak, eliminated from the conditions of the problem, which is reduced to one of pure kinematics. .At the outset, the position of each body, considered as a material particle, is defined by reference to a system of co-ordinate axes, and not by any verbal description.^ A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.
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Differential equations which express the changes of the co-ordinates are then constructed. .The process of discovering the laws of motion of the particle then consists in the integration of these equations.^ A hypothetical process of nucleosynthesis (now considered obsolete terminology), which consisted of redistributing -particles in the region from 20 Ne to 56 Fe (and perhaps slightly higher).

.Such equations can be formed for a system of any number of bodies, but the process of integration in a rigorous form is possible only to a limited extent or in special cases.^ A process such as that in which a single ionization leads to a large number of ions.

^ [C95] (b) The absolute magnitude ( g ) of a Solar-System body such as an asteroid is defined as the brightness at zero phase angle when the object is 1 AU from the Sun and 1 AU from the observer.

^ And if such memory was possible, the existence of a system of time-reckoning going back that far is not impossible either.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

The problems to be treated are of two classes. .In one, the bodies are regarded as material particles, no account being taken of their dimensions.^ Why is Haumea long in one dimension and flattened in two, instead of being long in two and flattened in one?

^ Photons are generally regarded as particles with zero mass and no electric charge.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.The earth, for example, may be regarded as a particle attracted by another more massive particle, the sun.^ In fact, it's 95 times more massive than Earth.

^ Of course Saturn is more massive than Earth.

^ The Sun , for example, has about 1000 times more mass than all planets of the Solar System put together.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In the other class of problems, the relative motion of the different parts of the separate bodies is considered; for example, the rotation of the earth on its axis, and the consequences of the fact that those parts of a body which are nearer to another body are more strongly attracted by it.^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ The relative amount of a given element among others; for example, the abundance of oxygen in the Earth's crust is approximately 50% by weight.

^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Beginning with the first branch of the subject, the fundamental ideas which it is our purpose to convey are embodied in the simple case of only two bodies, which we may call the sun and a planet.^ The first person to propose that all planets orbit around the Sun was Nicholas Copernicus , whose book describing this idea was printed just before his death in 1543 [ Dreyer, Chapter XIII ] [ Pannekoek, Chapter 18 ] [ Crowe, Chapter 6 ].
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This is the case, for example, with the so-called Trojan and Greek asteroids that go around the Sun in the same orbit as Jupiter .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This is also called the synodical period of revolution or the planet day or sol (which means " Sun " in Latin).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In this case the two bodies really revolve round their common centre of gravity; but a very slight modification of the equations of motion reduces them to the relative motion of the planet round the sun, regarding the moving centre of the latter as the origin of co-ordinates.^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ If you take ever more accurate approximations for the orbital periods of the planets, then the common period after which the planets return to the same relative positions gets on the whole longer and longer.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The motion of this centre, which arises from the attraction of the planet on the sun, need not be considered.^ Wall chart, linen-backed, with rods showing the motions of the planets around the sun.
  • Astronomy,Antiquarian Books, Rare Books,antique maps,antique globes,historical prints, travel guides, atlases, gazetteers 19 January 2010 9:52 UTC www.murrayhudson.com [Source type: General]

^ The apparent motion of the Sun , Moon , planets , and stars in the sky can be explained in two ways: (1) the Sun , Moon , planets , and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In the actual problems of celestial mechanics three co-ordinates necessarily enter, leading to three differential equations and six equations of solution.^ Topics include the Fourier integral, finite and infinite dimensional vector spaces, boundary value problems, eigenfunction expansions, Green’s functions, transform techniques for partial differential equations, and series solution of ordinary differential equations.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Detailed solutions of the Schrödinger equation for a variety of systems including bound states and scattering states in one and three dimensions.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Lagrange developed the calculus of variations, established the theory of differential equations, and provided many new solutions and theorems in number theory.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.But the general principles of the problem are completely exemplified with only two bodies, in which case the motion takes place in a fixed plane.^ During the nineteenth century, attention to the three body problem by Euler , Clairaut , and D'Alembert led to more accurate predictions about the motions of the Moon and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

^ The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place.
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^ Last week I wrote about the International Astronomical Union (#IAU) General Assembly taking place in Rio de Janeiro, to which I was headed.

.By taking this plane, which is that of the orbit in which the planet performs its revolution, as the plane of xy, we have only two co-ordinates to consider.^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic ; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.
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^ The planet Mercury is the closest planet to orbit the Sun, with an average distance from the Sun of only 58 million km.

.Let us use the following notation: x, y, the co-ordinates of the planet relative to the sun as the origin.^ [F88] (b) Divination using the positions of the planets, the Sun and the Moon as seen against the stars in the constellations of the zodiac - a "science" almost as old as homo sapiens.

^ The sizes, mass, and density of the Sun and the planets are listed in the following table.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Even though most of the planets seem to follow the Law of Titius-Bode quite well, there is probably quite a lot of chance to the distances between the planets and the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.M, m, the masses of the attracting bodies, sun and planet.^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The Sun , for example, has about 1000 times more mass than all planets of the Solar System put together.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The sizes, mass, and density of the Sun and the planets are listed in the following table.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.r, the distance apart of the two bodies, or the radius vector of m relative to M. This last quantity is analytically defined by the equation r 2 = x 2 - + y2. t, the time, reckoned from any epoch we choose.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The distances of the Moon and the Sun from the Earth was accurately measured as 108 times the diameters of these heavenly bodies.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The differential equations which completely determine the changes in the co-ordinates x and y, or the motion of m relative to M, are: - d2x (M+m)x di' r3 (1) d2y _ (M+m)y dt 2 r3 These formulae are worthy of special attention.^ In scientific study of motion in two or three dimensions acceleration means rate of change of velocity; a = dv / dt .

^ For example, in radioastronomy, spectral line graphs are used to determine the kinematics or relative motions of material at the center of a galaxy or surrounding a star as it is born.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.They are the expression in the language of mathematics of Newton's first two laws of motion.^ At first glance the two papers look more or less like they say there are clouds in the same spots.

^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Their statement in this language may be regarded as perfect, because it completely and unambiguously expresses the naked phenomena of the motion.^ Because a star's position may change slightly (see proper motion and precession of the equinoxes ), such tables must be revised at regular intervals.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

The equations do this without expressing any conception, such as that of force, not associated with the actual phenomena. .Moreover, as a third advantage, these expressions are entirely free from those difficulties and ambiguities which are met with in every attempt to express the laws of motion in ordinary language.^ Newton's third law of motion In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction noise The random fluctuations that are always associated with a measurement that is repeated many times over.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.They afford yet another great advantage in that the derivation of the results requires only the analytic operations of the infinitesimal calculus.^ They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Three astronomers have discovered yet another of the many delicate balances operating in our solar system, balances that protect life on this planet.
  • Life on Other Planets | Reasons To Believe 18 January 2010 2:02 UTC www.reasons.org [Source type: FILTERED WITH BAYES]

^ The sun is only average size for a star, yet it’s size is another terrific source of astronomy fun facts.

The power and spirit of the analytic method will be appreciated by showing how it expresses the relations of motion as they were conceived geometrically by Newton and Kepler. .It is quite evident that Kepler's laws do not in themselves enable us to determine the actual motion of the planets.^ Determination of the astronomical distance scale, Hubble’s law, and measurements of the space distribution and peculiar motions of galaxies.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Uranus turned out to fit the Titius-Bode Law quite well (for n = 6), so people started looking for the mysterious n = 3 planet more carefully.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Even though most of the planets seem to follow the Law of Titius-Bode quite well, there is probably quite a lot of chance to the distances between the planets and the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

We must have, in addition, in the case of each special planet, certain specific facts, viz. the axes and eccentricity of the ellipse, and the position of the plane in which it lies. .Besides these, we must have given the position of the planet in the orbit at some specified moment.^ This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit).
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^ If space were full of some gas, then the friction of the gas on the planet would slow the planet down and then it would not stay in its orbit but gradually get closer to the Sun and finally fall into the Sun.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Rule 3 means that some round celestial bodies that orbit directly around the Sun are yet not planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Having these data, the position of the planet at any other time may be geometrically constructed by Kepler's laws.^ In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Because VGA Planets stores much of its information in external files, it's possible to replace these with new data.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ These substances are usually found in molecular clouds , although they may also appear in low temperature stars, brown dwarfs and planets.
  • Top20Astronomy.com - Online Directory for Astronomy Education. 19 January 2010 9:52 UTC www.top20astronomy.com [Source type: Academic]

.The third law enables us to compute the time taken by the radius vector to sweep over the entire area of the orbit, which is identical with the time of revolution.^ Kepler's third law The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.
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^ He has computed the number of revolutions likely to be made by the planets in one mahayuga , the traditional duration of which time period is considered as being 43,20,000 years.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Kepler's second law A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.
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.The problem of constructing successive radii vectores, the angles of which are measured off from the radius vector of the body at the original given position, is then a geometric one, known as Kepler's problem.^ The angular momentum of a system about a specified origin is the sum over all the particles in the system (or an integral over the different elements of the system if it is continuous) of the vector products of the radius vector joining each particle to the origin and the momentum of the particle.

^ A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area.
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^ Next the observer measures along the star's hour circle the angle between the celestial equator and the position of the star.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.In the analytic process these specific data, called elements of the distance.^ These types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.If M is the central mass, n the angular velocity, and a the distance, the balance of the two forces is expressed by the equation an' =.1111a2, whence a 3 n 2 = M, a constant.^ Since the mass is constant, the velocity changes.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ You can render the influence of the gravity of the Earth unimportant by measuring the gravity horizontally between two objects, and you can measure that gravity by balancing it with a force that you know.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ To calculate the gravitational constant, we need to know the gravity between two objects, the masses of the objects, and the distance between the objects.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The periodic time varying inversely as n, this equation expresses Kepler's third law.^ Kepler's third law The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The sky can be shown for any month, period of the month (early or late,) time and at various magnifications.

^ If we combine the Law of Titius-Bode and Kepler 's Harmonic Law, then we get that the period P n of a planet , measured in years , is about equal to .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.This reasoning tacitly supposes the orbit to be a circle of radius a, and the mass of the planet to be negligible.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Johannes Kepler the careful observations by Tycho Brahe .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The orbits of the planets have been squashed by only a tiny bit, so if you draw them as circles, then that is usually good enough.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The rigorous relation is expressed by a slight modification of the law. .Putting M and m for the respective masses of the sun and planet, a for the semi-major axis of the orbit, and n for the mean.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The major axis of an elliptical orbit.

^ All planets orbit around the Sun and turn around their own axis.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

angular motion in unit of time, the relation then is a 3 n 2 =M - Fm.
What is noteworthy in this theorem is that this relation depends only on the sum of the masses. .It follows, therefore, that were any portion of the mass of the sun taken from it, and added to the planet, the relation would be unchanged.^ The sizes, mass, and density of the Sun and the planets are listed in the following table.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The Sun , for example, has about 1000 times more mass than all planets of the Solar System put together.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Kepler's third law therefore expresses the fact that the mass of the sun is the same for all the planets, and deviates from the truth only to the extent that the masses of the latter differ from each other by quantities which are only a small fraction of the mass of the sun.^ Do all planets travel around the Sun in the same direction?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ [CD99] (d) A particle of the same mass and spin, but opposite charge (and other properties) to its corresponding particle.

^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.==Problem of Three Bodies== As soon as the general law of gravitation was fully apprehended, it became evident that, owing to the attraction of each planet upon all the others, the actual motion of the planets must deviate from their motion in an ellipse according to Kepler's laws.^ The orbits of all planets are ellipses.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.In the Principia Newton made several investigations to determine the effects of these actions; but the geometrical method which he employed could lead only to rude approximations.^ Splitting of spectral lines into several components, in contrast to the normal Zeeman effect which results in only two distinct components.

^ When these stars explode, they spread the newly made elements through the Universe , and only after this could earth-like planets be made.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This Principle of Least Action can be used instead of Newton's Laws to determine the motion of a system.

.When the subject was taken up by the continental mathematicians, using the analytical method, the question naturally arose whether the motions of three bodies under their mutual attraction could not be determined with a degree of rigour approximating to that with which Newton had solved the problem of two bodies.^ AW197 uses the art of the stratagem in problem-solving, like the strategic therapy used by Prot with Howie in the movie "K-pax" (2001).

^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ This remains true whether one uses the Tropical (abstract, solstice/ equinox-based) or the Sidereal (visible, constellation-based) Zodiac, a question which is not really relevant here.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

Thus arose the celebrated " problem of three bodies." .Investigation soon showed that certain integrals expressing relations between the motions not only of three but of any number of bodies could be found.^ Thus, the Hindus’ most sacred number 108 is, with an inaccuracy of only 1%, the distance earth-sun expressed in solar diameters (i.e.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ He also made important contributions to mechanics, stating that in a collision between bodies, neither loses nor gains ``motion'' (his term for momentum).
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ He questioned a number of things, including whether the fractal dimension could be less than three but only very slightly, and what would that mean?

.These were: First, the law of the conservation of the centre of gravity.^ First law of black hole dynamics: For interactions between black holes and normal matter, the conservation laws of mass-energy, electric charge, linear momentum, and angular momentum , hold.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

This. expresses the general fact that whatever be the number of the bodies. which act upon each other, their motions are so related that the centre of gravity of the entire system moves in a straight line with a constant velocity. This is expressed in three equations, one for each of the three rectangular co-ordinates.
Secondly, the law of conservation of areas. .This is an extension of Kepler's second law.^ Kepler's second law A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Kepler's Second Law .

.Taking as the radius vector of each body the line from the body to the common centre of gravity of all, the sum of the products formed by multiplying each area described, by the mass of the body, remains a constant.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Because of gravity , all mass tries to get as close together as possible.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.In the language of theoretical mechanics, the moment of momentum of the entire system is a constant quantity.^ In quantum mechanics, angular momentum is quantized, i.e., is measured in indivisible units equivalent to Planck's constant divided by 2 pi.

^ A quantity related to the momentum and position of a body or system of particles.

^ In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.This law is also expressed in three equations, one for each of the three planes on which the areas are projected.^ Detailed solutions of the Schrödinger equation for a variety of systems including bound states and scattering states in one and three dimensions.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

.Thirdly, the entire vis viva of the system or, as it is now called, the energy, which is obtained by multiplying the mass of each body into half the square of its velocity, is equal to the sum of the quotients formed by dividing the product of every pair of the masses, taken two and two, by their distance apart, with the addition of a constant depending on the original conditions of the system.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The energy released is equal to the sum of the rest energies of the particles and their kinetic energies.

^ At velocities approaching that of light the mass of the particles increases dramatically, adding greatly to the energy released on impact.

In the language of algebra putting m l, m2, m 3, &c. for the masses of the bodies, r1.2 r1.3 r2.3, &c. for their mutual distances apart; vi, v .2, v 3, &c., for the velocities with which they are moving at any moment; these quantities will continually satisfy the equation orbit, appear as arbitrary constants, introduced by the process of integration.^ "If we assume that the result proved for a polygonal line is also valid for a continuously curved line, we arrive at this result: If one of two synchronous clocks at A is moved in a closed curve with constant velocity uk.sci.astronomy Google Group .

^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Then it's orbiting faster than it's rotating for about 8 days and so the Sun appears to move backwards.

.In a case like the present one, where there are two differential equations of the second order, there will be four such constants.^ At first glance the two papers look more or less like they say there are clouds in the same spots.

^ The two best-known ones are NAVGAP and Robo; there are also many others, including my own GHost.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ One such change is a variation with a period of a year, but there are others.

.The result of the integration is that the co-ordinates x and y and their derivatives as to the time, which express the position, direction of motion and speed of the planet at any moment, are found as functions of the four constants and of the time.^ The distances between the planets change all the time , because each planet goes along its own orbit at its own speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ He was also the first to discover that the light from the Moon and the planets were reflected from the Sun, and that the planets follow an elliptical orbit around the Sun, and thus propunded an eccentric elliptical model of the planets, on which he accurately calculated many astronomical constants, such as the times of the solar and lunar eclipses , and the instantaneous motion of the Moon (expressed as a differential equation ).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ If tying the viewing direction to a planet or star is not possible with your planetarium program, then you can change the time in steps of 23 hours and 56 minutes .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Putting a, b, c, d, for the constants, the general form of the solution will be x = fl (a,b,c,d,t) y = f2(a,b,c,d,t) From these may be derived by differentiation as to t the velocities dt =f '1(a,b,c,d,t) = x'  ?^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

=f'2(a,b,c,d,t) =y ' (3)
The symbols x' and y' are used for brevity to mean the velocities expressed by the differential coefficients. .The arbitrary constants, a, b, c and d, are the elements of the orbit, or any quantities from which these elements can be obtained.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

We note that, in the actual process of integration, no geometric construction need enter.
Let us next consider the problem in another form. .Conceive that instead of the orbit of the planet, there is given a position P (fig.^ Because there was now a larger object than Pluto found orbiting the Sun, astronomers needed to decide whether this would be come the tenth planet.

^ I don't think there is anything special about the positions of the planets in December 2012.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Could there be a second Earth-like planet in the same orbit as the Earth but hidden exactly on the other side of the Sun?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

2), through which the planet passed at an assigned moment, with a given velocity, and in a given P direction, represented by the arrowhead. .Logically these data completely determine the orbit in which the planet shall move, because there is only one such orbit passing through P, a; planet moving in which would have the given speed.^ They know this first through measuring the wobble caused to stars by planets and other objects orbiting them.

^ The distances between the planets change all the time , because each planet goes along its own orbit at its own speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If a planet were suddenly transported to another part of the Solar System, or even removed completely, then that would only have great effect in the part of space where the planet dominated before, and in the part of space where the planet ends up.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

It follows that the elements of the orbit admit of determination when the co-ordi nates of the planet at an assigned moment FIG. .2 and their derivatives as to time are given.
.Analytically the elements are determined from these data by solving the four equations just given, regarding a, b, c and d as unknown quantities, and x, y, x', y' and t as given quantities.^ If you know four of these five quantities, then you can calculate the missing fifth one.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The solution of these equations would lead to expressions of the form a= 41(x,y,x',y',t) b= (x,y,x',y',t) (4) &c. &c.
one for each of the elements.
.The general equations expressing the motion of a planet considered as a material particle round a centre of attraction lead to theorems the more interesting of which will now be enunciated.^ A hypothetical process of nucleosynthesis (now considered obsolete terminology), which consisted of redistributing -particles in the region from 20 Ne to 56 Fe (and perhaps slightly higher).

^ You’ve already had to throw away the correct relative spacings between planets to make the placemat more interesting.

^ Our Solar System contains 8 planets (now that Pluto isn't a planet any more).

.(I) The motion of such a planet may take place not only in an ellipse but in any curve of the second order; an ellipse, hyperbola, or parabola, the latter being the bounding curve between the other two.^ But the mission may take place anytime between 2018 to 2030.

^ A change taking place in a system that has perfect thermal insulation, so that heat cannot enter or leave the system and energy can only be transferred by work.

^ Because a star's position may change slightly (see proper motion and precession of the equinoxes ), such tables must be revised at regular intervals.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.A body moving in a parabola or hyperbola would recede indefinitely from its centre of motion and never return to it.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.The ellipse is therefore the only closed orbit.^ They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It orbits so close to the Sun that it only takes 88 days to go around the Sun.

(2) The motion takes place in accord with Kepler's laws, enunciated elsewhere.
.(3) Whewell's theorem: if a point R be taken at a distance from the sun equal to the major axis of the orbit of a planet and, therefore, at double the mean distance of the planet, the speed of the latter at any point is equal to the speed which a body would acquire by falling from the point R to the actual position of the planet.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ It says that the distance a n of each planet from the Sun is equal to about .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ All planets orbit around the Sun and turn around their own axis.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The speed of the latter may, therefore, be expressed as a function of its radius vector at the moment and of the major axis of its orbit without introducing any other elements into the expression.^ The major axis of an elliptical orbit.

^ By how far the Sun is from the center of the orbit, compared with the size of the semimajor axis (something like the radius) of the orbit, is called the eccentricity .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The angular momentum of a system about a specified origin is the sum over all the particles in the system (or an integral over the different elements of the system if it is continuous) of the vector products of the radius vector joining each particle to the origin and the momentum of the particle.

.Another corollary is that in the case of a body moving in a parabolic orbit the velocity at any moment is that which would be acquired by the body in falling from an infinite distance to the place it occupies at the moment.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The movement of one celestial body which is in orbit around another.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ If space were full of some gas, then the friction of the gas on the planet would slow the planet down and then it would not stay in its orbit but gradually get closer to the Sun and finally fall into the Sun.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.(4) If a number of bodies are projected from any point in space with the same velocity, but in various directions, and subjected only to the attraction of the sun, they will all return to the point of projection at the same moment, although the orbits in which they move may be ever so different.^ Do all planets travel around the Sun in the same direction?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ If all goes well, they’ll spend nearly two weeks confined to a tiny container holding the only patch of livable space for 400 miles in any direction, before they drop back to earth in a flaming descent that transforms into a supersonic glissade to the ground.

.(5) At each distance from the sun there is a certain velocity which a body would have if it moved in a circular orbit at that distance.^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Just for comparison, Earth orbits at an average distance of 150 million km from the Sun.

.If projected with this velocity in any direction the point of projection will be at the end of the minor axis of the orbit, because this is the only point of an ellipse of which the distance from the focus is equal to the semi-major axis of the curve, and therefore the only point at which the distance of the body from the sun is equal to its mean distance.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The major axis of an elliptical orbit.

^ The point in a planetary orbit that is at the greatest distance from the Sun.

.(6) The relation between the periodic time of a planet and its mean distance, approximately expressed by Kepler's third law, follows very simply from the laws of centrifugal force.^ What is the distance between all planets?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ How can you use Kepler's Law to find the distance to a planet?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Mean distance between the Earth and the Sun: 149,598,500km.

.It is an elementary principle of mechanics that this force varies directly as the product of the distance of the moving body from the centre of motion into the square of its angular velocity.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
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^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

When bodies revolve at different distances around a centre, their velocities must be such that the centrifugal force of each shall be balanced by the attraction of the central mass, and therefore vary inversely as the square of the v(m i v -+m 2 vz+
.) = 'n ' m 2 - - 'nl'n3 'n2m3+ ... +a constant.
r .1.2 r1.3 r2.3 The theorems of motion just cited are expressed by seven integrals, or equations expressing a law that certain functions of the variables and of the time remain constant.^ Continuation of Physics 100 with emphasis on variational calculus, integral equations, and asymptotic and perturbation methods for integrals and differential equations.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ He was also the first to discover that the light from the Moon and the planets were reflected from the Sun, and that the planets follow an elliptical orbit around the Sun, and thus propunded an eccentric elliptical model of the planets, on which he accurately calculated many astronomical constants, such as the times of the solar and lunar eclipses , and the instantaneous motion of the Moon (expressed as a differential equation ).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.It is remarkable that although the seven integrals were found almost from the beginning of the investigation, no others have since been added; and indeed it has recently been shown that no others exist that can be expressed in an algebraic form.^ Scientists have used new techniques to discover that the protosun did indeed emit ultraviolet and other particles in an early form of the solar wind.

^ This year is especially good since the moon is almost new, no moon light will interfere the show.

^ Astronomers often express units for other objects in terms of solar units, since it makes the resulting numbers smaller and easier to deal with.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

In the case of three bodies these do not suffice completely to define the motion. In this case, the problem can be attacked only by methods of approximation, devised so as to meet the special conditions of each case. .The special conditions which obtain in the solar system are such as to make the necessary approximation theoretically possible however complex the process may be.^ Plasma processes in the solar system.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here.

^ The solar system makes more sense now, and, in three years most people have come to terms with the new solar system.

.These conditions are: - (I) The smallness of the masses of the planets in comparison with that of the sun, in consequence of which the orbit of each planet deviates but slightly from an ellipse during any one revolution; (2) the fact that the orbits of the planets are nearly circular, and the planes of their orbits but slightly inclined to each other.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The plane of Earth's orbit about the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ One of these is whether the fact that… .

.The result of these conditions is that all the quantities required admit of development in series proceeding according to the powers of the eccentricities and inclinations of the orbits, and the ratio of the masses of the several planets to the mass of the sun.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The point in its orbit where a planet is closest to the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

Perturbations of the Planets. - .Kepler's laws do not completely express the motion of a planet around a central body, except when no force but the mutual attraction of the two bodies comes into play.^ A mutual physical force attracting two bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ There is no centrifugal force to counter the two Tritons' gravitational pull and they collide.

^ If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.When one or more other bodies form a part of the system, their action produces deviations from the elliptic motion, which are called perturbations. The problem of determining the perturbations of the (2) Q heavenly bodies is perhaps the most complicated with which the mathematical astronomer has to grapple; and the forms under which it has to be studied are so numerous that they cannot be easily arranged under any one head.^ A serious criticism against ancient Indian astronomers is that they were not scientific observers but only mathematical manipulators.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Part time (one credit) thesis research under the guidance of a staff member.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

.But there is one conception of perturbations of such generality and elegance that it forms the common base of all those methods of determining these deviations which have high scientific interest.^ Galileo complied, continuing his study of falling objects, comets, and methods to determine longitude at sea based on the phases of Jupiter’s moons.

^ General There are four very basic tactical concepts that you should be aware of, that go far beyond VGA Planets...
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ There has been much talk recently about all of this, and even some interesting experiments done by scientific journals.

.This conception is embodied in the method of " variation of elements," originally due to J. L. Lagrange.^ Lagrange also invented the method of solving differential equations known as variation of parameters.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

The simplest method of presenting it starts with the second view of the elliptic motion already set forth.
.We have shown that, when the position of a planet and the direction and speed of its motion at a certain instant are given, the elements of the orbit can be determined.^ The distances between the planets change all the time , because each planet goes along its own orbit at its own speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.

^ On the airplane to Abu Dhabi, I checked my calculations for transformations between position and velocity three-vectors and the standard orbital elements for Keplerian orbits.

.We have supposed this to be done at a certain point P of the orbit, the direction and speed being expressed by the variables x, y, x' and y'. Now, consider the values of these same variables expressing the position of the planet at a second point Q, and the speed with which it passes that point.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The point in its orbit where a planet is closest to the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The distances between the planets change all the time , because each planet goes along its own orbit at its own speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.With this position and speed the elements of the orbit can again be determined.^ On the airplane to Abu Dhabi, I checked my calculations for transformations between position and velocity three-vectors and the standard orbital elements for Keplerian orbits.

.Since the orbit is unchanged so long as no disturbing force acts, it follows that the elements determined by means of the two sets of values of the variables are in this case the same.^ Each electron in the atom has four quantum numbers and, according to the Pauli exclusion principle, no two electrons can have the same set of quantum numbers.

^ The maximum value of a varying quantity from its mean or base value.

^ They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations.

.In a word, although the position and speed of the planet and the direction of its motion are constantly changing, the values of the elements determined from these variables remain constant.^ Because a star's position may change slightly (see proper motion and precession of the equinoxes ), such tables must be revised at regular intervals.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ A simple harmonic motion of frequency f can be represented by a point moving in a circular path at constant speed.

^ This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit).
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.This fact is fully expressed by the equations (4) where we have constants on one side of the equation equal to functions of the variables on the other.^ Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ One such change is a variation with a period of a year, but there are others.

.Functions of the variables possessing this property of remaining constant are termed integrals. Now let the planet be subjected to any force additional to that of the sun's attraction, - say to the attraction of another planet.^ In maybe 21 years or so – let’s say June of 2027 -- she could be sitting down right there where you are now.

^ It seems likely that the rotation of Mercury was slowed down (in part) because of tidal forces from the Sun , which is close to that planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This understanding is demonstrated in another Sloka which says that when one sun sinks below the horizon, a thousand suns take its place.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.To fix the ideas let us suppose that the additional attraction is only an impulse received at the moment of passing the point P. The first effect will evidently be to change either the velocity or the direction in which the planet is moving at the moment, or both.^ It is a first strong and direct indication of the presence of phenomena of convection, transport of heat by the moving matter, in a star other than the Sun.

^ If tying the viewing direction to a planet or star is not possible with your planetarium program, then you can change the time in steps of 23 hours and 56 minutes .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ On the first point: Even if a planet starts out with the same temperature everywhere, it will soon be colder at the surface than in the center.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

If, with the changed velocity we again compute the elements they will be different from the former elements. But, if the impulse is not repeated, these new elements will again remain invariable. If repeated, the second impulse will again change the elements, and so on indefinitely. .It follows that, if we go on computing the elements a, b, c, d from the actual values of x, y, x' and y', at each moment when the planet is subject to the attraction of another body, they will no longer be invariable, but will slowly vary from day to day and year to year.^ On Mercury, a day lasts longer than a year , and in fact exactly twice as long.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Something like that could happen also near the Earth or near another planet , so perhaps the Earth will someday (very many years from now) have some pretty rings like Saturn.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ However, conjunctions of the planets have no influence on Earth and are not important, except that they provide opportunities to take nice pictures of them.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.These ever varying elements represent an ever varying elliptic orbit, - not an orbit which the planet actually describes through its whole course, but an ideal one in which it is moving at each instant, and which continually adjusts itself to the actual motion of the planet at the instant.^ If the orbit is elliptical the radius will vary.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Venus ("c"on the Planet Phenomena Pages ) sufficiently close to a passage of Venus through a node of its orbit ("x" on the Planet Phenomena Pages).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.This is called the osculating orbit: The essential principle of Lagrange's elegant method consists in determining the variations of this osculating ellipse, the co-ordinates and velocities of the planet being ignored in the determination.^ Lagrange also invented the method of solving differential equations known as variation of parameters.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ We determined their sizes and gave up on any of the things in the Kuiper belt being planets (I lost my bet, too).

^ Also called minor planet [H76] (c) Also called planetoids or minor planets, the asteroids are tiny planets most of which orbit the Sun between Mars and Jupiter.

.This may be done because, since the elements and co-ordinates completely determine each other, we may concentrate our attention on either, ignoring the other.^ Each chemical element has a particular type of atom, which may join with like atoms to form molecules of the element, or with atoms of other elements to form molecules of a compound.

.The reason for taking the elements as the variables is that they vary very slowly, a property which facilitates their determination, since the variations may be treated as small quantities, of which the squares and products may be neglected in a first solution.^ The number of neutrons in the nucleus may vary, forming different isotopes of an element.

^ They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations.

In a second solution the squares and products may be taken account of, and so on as far as necessary.
If the problem is viewed from a synthetic point of view, the stages of its solution are as follows. .We first conceive of the planets as moving in invariable elliptic orbits, and thus obtain approximate expressions for their positions at any moment.^ He was also the first to discover that the light from the Moon and the planets were reflected from the Sun, and that the planets follow an elliptical orbit around the Sun, and thus propunded an eccentric elliptical model of the planets, on which he accurately calculated many astronomical constants, such as the times of the solar and lunar eclipses , and the instantaneous motion of the Moon (expressed as a differential equation ).
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Johannes Kepler the careful observations by Tycho Brahe .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.With these expressions we express their mutual action, or their pull upon each other at any and every moment.^ Every once in a while, though, something happens that pulls the moon landings out of the abstract haze of history and makes me remember: these things were real!

This pull determines the variations of the ideal elements. .Knowing these variations it becomes possible to represent by integration the value of the elements as algebraic expressions containing the time, and the elements with which we started.^ As we look as these processes weaving into one another, we see Saturn as the image of the Ego individuality, placed in space but becoming another opportunity in time.
  • planets 18 January 2010 2:02 UTC www.oregonbd.org [Source type: FILTERED WITH BAYES]

.But the variations thus determined will not be rigorously exact, because the pull from which they arise has been determined on the supposition that the planets are moving in unvarying orbits, whereas the actual pull depends on the actual position of the planets.^ They know this first through measuring the wobble caused to stars by planets and other objects orbiting them.

^ Because the vernal equinox is not always visible in the night sky (especially in the spring), whereas the sigma point is always visible, the hour angle is used in actually locating a body in the sky.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Another approximation is, therefore, to be made, when necessary, by correcting the expression of the pull through taking account of the variations of the elements already determined, which will give a yet nearer approximation to the truth.^ You should therefore take these reported sizes with a grain of salt, except if they have been determined in an independent manner (for example, through a measurement of the temperature of the object).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In theory these successive approximations may be carried as far as we please, but in practice the labour of executing each approximation is so great that we are obliged to stop when the solution is so near the truth that the outstanding error is less than that of the best observations.^ We meet here a preparation in which the planets from beyond the Sun are active and they require less potentising than the near planets.
  • planets 18 January 2010 2:02 UTC www.oregonbd.org [Source type: FILTERED WITH BAYES]

^ After all, the planet Neptune was also discovered from its gravitational tugging on the other planets, even though those planets remain so far from Neptune that the gravitational acceleration that Neptune causes in these other planets is less than the gravitational acceleration that the anti-Earth would cause in the terrestrial planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A planet has far less mass than a star .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Even this degree of precision may be impracticable in the more complex cases.^ Would showing it more clearly involve complex mathematics (I'm still in the first year of my degree, so I might not be ready for that level of maths yet!

The results which are required to compare with observations are not merely the elements, but the co-ordinates. .When the varying elements are known these are computed by the equations (2) because, from the nature of the algebraic relations, the slowly varying elements are continuously determined by the equations (4), which express the same relations between the elements and the variables as do the equations (2) and (3).^ It is now known to be slowly and irregularly variable.

^ Continuation of Physics 100 with emphasis on variational calculus, integral equations, and asymptotic and perturbation methods for integrals and differential equations.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ Because the outer electrons form the chemical bonds between atoms, the chemical properties of an element depend on the electronic structure of the atom, and therefore also on the number of protons.

This method is, therefore, in form at least, completely rigorous. There are some cases in which it may be applied unchanged. But commonly it proves to be extremely long and cumbrous, and modifications have to be resorted to. Of these modifications the most valuable is one conceived by P. A. Hansen.
.A certain mean elliptic orbit, as near as possible to the actual varying orbit of the planet, is taken.^ If the orbit is elliptical the radius will vary.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Rule 3 means that some round celestial bodies that orbit directly around the Sun are yet not planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.In this orbit a certain fictitious planet is supposed to move according to the law of elliptic motion.^ According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion .
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Comparing the longitudes of the actual and the fictitious planet the former will sometimes be ahead of the latter and sometimes behind it. .But in every case, if at a certain time t, the actual planet has a certain longitude, it is certain that at a very short interval dt before or after t, the fictitious planet will have this same longitude.^ As seen from the planet , the Sun returns to (about) the same place between the stars after this much time .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ You'll learn more about Planets this way than by playing winning games in the same way every time.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ More than two planets never return to exactly the same relative positions that they had before.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.What Hansen's method does is to determine a correction dt such that, being applied to the actual time t, the longitude of the fictitious planet computed for the time t+dt, will give the longitude of the true planet at the time t. By a number of ingenious devices Hansen developed methods by which dt could be determined.^ You can keep the planets in the right order, but give up on showing their true distances from each other.

^ If planet A is y times further from the Sun than planet B is, then planet A takes y √ y longer to orbit the Sun than planet B does.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ [Modern astronomy] gives the longitude of that star 13’ from the vernal equinox, at the time of the Calyougham, agreeing, within 53’, with the determination of the Indian astronomy.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.The computations are, as a general rule, simpler, and the algebraic expressions less complex, than when the computations of the longitude itself are calculated.^ All things that have mass generate gravity , but things with more mass generate stronger gravity than things with less mass, and gravity decreases with increasing distance.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Although the longitude of the fictitious planet at the fictitious time is then equal to that of the true planet at the true time, their radii vectores will not be strictly equal.^ Kepler's second law A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Hansen, therefore, shows how the radius vector is corrected so as to give that of the true planet.^ The dashed lines show how far the planet can get above or below the ecliptic : If the dashed curve is somewhere above the solid orbit (close to the beginning of this page), then the planet is at that location that far above the ecliptic , and if the dashed curve is below the solid orbit, then the planet is at that location that far below the ecliptic.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ You can keep the planets in the right order, but give up on showing their true distances from each other.

.In all that precedes we have considered only two variables as determining the position of the planet, the latter being supposed to move in a plane.^ This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit).
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ We determined their sizes and gave up on any of the things in the Kuiper belt being planets (I lost my bet, too).

^ He has computed the number of revolutions likely to be made by the planets in one mahayuga , the traditional duration of which time period is considered as being 43,20,000 years.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

.Although this is true when there are any number of bodies moving in the same plane, the fact is that the planets move in slightly different planes.^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ This holds also for Pluto , which from its discovery in 1930 until 2006 was called a planet, and for 1 Ceres (the largest of the asteroids) and for various celestial bodies that are about as large as Pluto and that move through the same region of space as Pluto .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ There is also a Spanish-language Planets mailing list: to subscribe, mail majordomo@bbs.mundivia.es with body text "subscribe vgap".
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

.Hence the position of the plane of the orbit of each planet is continually changing in consequence of their mutual action.^ The distances between the planets change all the time , because each planet goes along its own orbit at its own speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The orbit of the Earth changes slowly under the influence of the gravity of the other planets, and if, for example, Mars is closest to the Earth so it can change Earth's orbit the most, then Mars is furthest from the anti-Earth and so changes its orbit the least.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The problem of determining the changes is, however, simpler than others in perturbations. The method is again that of the variation of elements. .The position and velocity being given in all three co-ordinates, a certain osculating plane is determined for each instant in which the planet is moving at that instant.^ On the airplane to Abu Dhabi, I checked my calculations for transformations between position and velocity three-vectors and the standard orbital elements for Keplerian orbits.

^ The Sun attracts a planet just as hard as the planet attracts the Sun, but the Sun is very much more massive than the planets so it is much harder to move, and that's why the planets have wide orbits while the Sun hardly moves at all.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Spirit was made for only three months, but it has surpassed all expectations, even while being stuck in the martian soil.

.This plane remains invariable so long as no third body acts; when it does act the position of the plane changes very slowly, continually rotating round the radius vector of the planet as an instantaneous axis of rotation.^ Do the planets rotate clockwise or counterclockwise around their axis?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If the planet does rotate around its own axis, then the ideal shape is a sphere that is slightly flattened, so that the diameter from pole to pole is a bit less than the diameter at the equator .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Secular and Periodic Variations

.When, following the preceding method, the variations of the elements are expressed in terms of the time, they are found to be of two classes, periodic and secular.^ Time-dependent and time-independent perturbation theory, and the variational method.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations.

.The first depend on the mean longitudes of the planets, and always tend back to their original values when the planets return to their original positions in their orbits.^ Dos Planets First, take a backup of the original files.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ This process is now called 1st order Fermi acceleration, because the mean energy gain is dependent on the shock velocity only to the first power.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

The others are, at least through long periods of time, continually progressive.
A luminous idea of the nature of these two classes of variation may be gained by conceiving of the ,motion of a ship, floating on an ocean affected by a long ground swell. In consequence of the swell, the ship is continually pitching in a somewhat irregular way, the oscillations up and down being sometimes great and sometimes small. An observer on board of her would notice no motion except this. But, suppose the tide to be rising. .Then, by continued observation, extended over an hour or more, it will be found that, in the general average, the ship is gradually rising, so that two different kinds of motion are superimposed on each other.^ In TimHost, planets have an ordering value of 1000, but a bit more thought is needed when two ships meet.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ An introduction to astronomy and astrophysics for science majors and others with some background in physics, providing an observational and theoretical background for more advanced topics in astrophysics.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ A given element may have two or more isotopes, which differ in the number of neutrons in the nucleus.

The effect of the rising tide is in the nature of a secular variation, while the pitching is periodic.
But the analogy does not end here. .If the progressive rise of the ship be watched for six hours or more, it will be found gradually to cease and reverse its direction.^ This means that for centuries before and for some more centuries after that time, the sea level was progressively rising.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

That is to say, making abstraction of the pitching, the ship is slowly rising and falling in a total period of nearly twelve hours, while superimposed upon this slow motion is a more rapid motion due to the waves. It is thus with the motions of the planets going through their revolutions. .Each orbit continually changes its form and position, sometimes in one direction and sometimes in another.^ The movement of one celestial body which is in orbit around another.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.But when these changes are averaged through years and centuries it is found that the average orbit has a secular variation which, for a number of centuries, may appear as a very slow progressive change in one direction only.^ They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ When these stars explode, they spread the newly made elements through the Universe , and only after this could earth-like planets be made.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Those shadows may be very small, but each tiny heap of ground may have one, and all in all this can give a brightness difference.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.But when this change is more fully investigated, it is found to be really periodic, so that after thousands, tens of thousands, or hundreds of thousands of years, its direction will be reversed and so on continually, like the rising and falling tide.^ The city in which their poets’ academy or Sangam (recorded in the early Christian era, but claimed to be ten thousand years old) was established, was said to have been moved thrice because of the rising waters.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ If, for example, the anti-Earth were one kilometer (one part in a hundred fifty million) closer to the Sun than the Earth, then the orbital period (the year ) of the anti-Earth would be 0.3 seconds less than that of Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ During that time, I’ve looked at M42 more than a thousand times through more than a hundred different scopes with some of the world’s top observers.

The orbits thus present themselves to us in the words of a distinguished writer as " Great clocks of eternity which beat ages as ours beat seconds." The periodic variations can be represented algebraically as the resultant of a series of harmonic motions in the following way: Let L be an angle which is increasing uniformly with the time, and let n be its rate of increase. We put Lo for its value at the moment from which the time is reckoned. The general expression for the angle will then be L =nt+Lo.
.Such an angle continually goes through the round of 360° in a definite period.^ He even quoted good proof of that, such as the fact that the shadow of the Earth that the Moon passes through during a lunar eclipse is always round.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.For example, if the daily motion is 5°, and we take the day as the unit of time, the round will be completed in 72 days, and the angle will continually go through the value which it had 72 days before.^ Radar waves travel at the speed of light , which is accurately known, so the time it takes before reflected waves arrive yields the distance if it is multiplied by the speed of light .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ (Pulsatance) Symbol: The number of complete rotations per unit time.

^ That's almost two hundred times longer than before when we rounded to the nearest full year .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Let us now consider an equation of the form U =a sin (nt+Lo).
.The value of U will continually oscillate between the extreme values +a and - a, going through a series of changes in the same period in which the angle nt+Lo goes through a revolution.^ The distance between adjacent peaks in a series of periodic waves.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ If you go through a sharp curve on your bicycle, then you lean into the curve, too, for the very same reason: otherwise, you'll fall over.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The mean anomaly is the angular distance (measured in the same manner) between periastron and a fictitious body in the direction of the star, which is moving in a circular orbit with a period equal to that of the star.

.In this case the variation will be simply periodic, The value of any element of the planet's motion will generally be represented by the sum of an infinite series of such periodic quantities, having different periods.^ If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The direction of motion of the air and the weather varies with time , with latitude on the planet , and also with height.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The earth-like planets are made of rock and metal, which contains elements such as oxygen, silicon, iron, and nickel.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

For example U =a sin (nt+Lo) +b sin (mt+L1) +c sin (kt+L2) &c.
.In this case the motion of U, while still periodic, is seemingly irregular, being much like that of a pitching ship, which has no one unvarying period.^ [I've been watching the moon, which made me remember a much earlier column that almost no one read.

^ Rather than being far away but spending much of its time closer like, say, a comet would, it was far away and spent almost all of its time even further away.

^ A system whose vibration, while still periodic, cannot be described in terms of simple harmonic motions (i.e.

.In the problems of celestial mechanics the angles within the parentheses are represented by sums or differences of multiples of the mean longitudes of the planets as they move round their orbits.^ In August 2006, the IAU invented a definition of planets that means that Pluto and similar celestial objects are not planets, but dwarf planets .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ When things move across a rotating planet , then they notice the rotation in the form of Coriolis forces.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If 1 be the mean longitude of the planet whose motion we are considering, and 1' that of the attracting planet affecting it, the periodic inequalities of the elements as well as of the co-ordinates of the attracted planet, may be represented by an infinite series of terms like the following: a sin (l' - l)+b sin (2l' - l)+c sin (l'- 2l)+&c.^ This is also called the synodical period of revolution or the planet day or sol (which means " Sun " in Latin).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A system whose vibration, while still periodic, cannot be described in terms of simple harmonic motions (i.e.

Here the coefficients of 1 and l' may separately take all integral values, though as a general rule the coefficients a, b, c, &c. diminish rapidly when these coefficients become large, so that only small values have to be considered.
The most interesting kind of periodic inequalities are those known as " terms of long period." .A general idea both of their nature and of their cause will be gained by taking as a special case one celebrated in the history of the subject - the great inequality between Jupiter and Saturn.^ The effect occurs in both special and general relativity , and is quite pronounced for speeds approaching the speed of light , and in regions of high gravity.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf , neutron star , or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found.

.We begin by showing what the actual fact is in the case of these two planets.^ The dashed lines show how far the planet can get above or below the ecliptic : If the dashed curve is somewhere above the solid orbit (close to the beginning of this page), then the planet is at that location that far above the ecliptic , and if the dashed curve is below the solid orbit, then the planet is at that location that far below the ecliptic.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ For years I’ve been getting these emails, asking if Eris, the biggest of the dwarf planets, and something that actually does exists, is somehow related to Nibiru, a made-up planet allegedly known to the Sumerians that, in fact, does not actually exist.

^ These two celestial objects are probably made of ice and dust and rock, just like comets and the dwarf planet Pluto .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Let S 2 "' fig. .3 represent the two orbits, the sun being at C. We know that the period of Jupiter is nearly twelve years, and that of Saturn a little less than thirty years.^ If P moon is the sidereal period of such a moon (in its orbit around Jupiter), and P J is the sidereal period of Jupiter (in its orbit around the Sun), then the synodical period P syn of the moon is equal to .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The orbits of Mars and the Earth are not perfect circles but rather like circles that are a little squashed, and the orbits are also shifted a little so that the Sun is not quite in the center of the orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This is the case, for example, with the so-called Trojan and Greek asteroids that go around the Sun in the same orbit as Jupiter .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.It will be seen that these numbers are nearly in the ratio of 2 to 5. It follows that L the motions of the mean longitudes are nearly in the same proportion re versed.^ The ratio of the number of atoms of an isotope to the number of atoms of another isotope of the same element in a sample.

.The annual motion of Jupiter is nearly 30°, that of Saturn a little more than 12°.^ For example, Jupiter is about 5.2 times further from the Sun than the Earth is, so Jupiter takes about 5.2√5.2 = 12 times as long to orbit the Sun as the Earth takes, so Jupiter takes about 12 years to do that.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Venus is often brighter than Jupiter, so it can be a bit closer to the Sun than Jupiter and yet be visible, but the difference is not so great that Venus is yet visible more often than Jupiter is.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Relativity More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than Newtonian mechanics.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Let us now consider the effect of this relation upon the configurations and relations of the two planets.^ The host can configure whether or not to let you see all planets within range, or have a chance of detecting them depending on the ship/planet doing the scanning.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

.Let the line CJ represent the common direction of the two planets from the sun when they are in conjunction, and let us follow the motions until they again come into conjunction.^ If you look at the average heat ( infrared radiation) coming from the planets , then Mercury is the hottest, mostly because it is closest to the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The most widespread opinion in ancient writings about this subject until the 16th century was that all of the planets and the Sun and Moon orbit around the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.This will occur along a line CR 1, making an angle of nearly 240° with CJ. At this point Saturn will have moved 240° and Jupiter an entire revolution+240°, making 600°.^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If the object (or a point on it) moves from point P 1 to point P 2 in a plane perpendicular to the axis, is the angle P 1 OP 2 , where O is the point at which the perpendicular plane meets the axis.

^ The foot of a perpendicular from the point to a diameter of the circle moves backward and forward along the diameter with simple harmonic motion.

.These two motions, it will be seen, are in the proportion 5: 2. The next conjunction will take place along CS I, and the third after the initial one will again take place near the original position JQ, Jupiter having made five revolutions and Saturn two.^ The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ Third, the principle of concentration of force is the one that will make, or break, your Plan to Take Over the Cluster (henceforth the PTOC)...
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ In addition, the Sun must then be in the same direction as Venus, as seen from Earth , so Venus must then be near an inferior conjunction with the Sun, and the Earth must then be near the same node of the orbit of Venus as Venus itself.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The result of these repetitions is that, during a number of revolutions, the special mutual actions of the two planets at these three points of their orbits repeat themselves, while the actions corresponding to the three intermediate arcs are wanting.^ The point in its orbit where a planet is farthest from the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The point in its orbit where a planet is closest to the Sun.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ The point in an orbit when two objects are closest together.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Thus it happens that if the mutual actions are balanced through a period of a few revolutions only there is a small residuum of forces corresponding to the three regions in question, which repeats itself in the same way, and which, if it continued indefinitely, would entirely change the forms of the two orbits.^ In scientific study of motion in two or three dimensions acceleration means rate of change of velocity; a = dv / dt .

^ They have orbits that differ in size by only a few miles, so they regularly get close to one another, and then they swap orbits.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Only near the equator does the temperature sometimes get above freezing, but the atmosphere is so dry there than any open water would evaporate very quickly.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

But the actual mean motions deviate slightly from the ratio 2: 5, and we have next to show how this deviation results in an ultimate balancing of the forces. .The annual mean motions, with the corresponding combinations, are as follows Jupiter: - n = 30° 349043 Saturn: - n' = 12.221133 2n=60 69809 5n'=61.10567 5n'-2n= o 40758 If we make a more accurate computation of the conjunctions from these data, we shall find that, in the general mean, the consecutive conjunctions take place when each planet has moved through an entire number of revolutions +242.7°.^ If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you take ever more accurate approximations for the orbital periods of the planets, then the common period after which the planets return to the same relative positions gets on the whole longer and longer.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ However, conjunctions of the planets have no influence on Earth and are not important, except that they provide opportunities to take nice pictures of them.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.It follows that the third conjunction instead of occurring exactly along the line CQ 1 occurs along CQ2, making an angle of nearly 8° with CQ I. The successive conjunctions following will be along CR 2, CS2, CQ3, &c., the law of progression being obvious.^ The natural order of things is not that they slow down and stand still, but (according to the First Law of Newton ) that they move along a straight line at constant speed.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The balancing of the series of forces will not be complete until the respective triplets of conjunctions have filled up the entire space between them.^ You can render the influence of the gravity of the Earth unimportant by measuring the gravity horizontally between two objects, and you can measure that gravity by balancing it with a force that you know.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If the thread is wound up sufficiently, then its opposing force exactly balances the force of gravity between the balls.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

This will occur when the angle whose annual motion is 5n'-2n has gone through 360°. .From the preceding value of 5n'-2n we see that this will require a little more than 883 years.^ But great discoveries require more than just a great mind.

^ It’s not that I don’t see them all the time when I am looking at the sky, but I never think of them as anything more than spots of light moving across the heavens.

^ They had formatted a calendar which lasted for more than 3000 years.

.The result of the continued action of the two planets upon each other is that during half of this period the motion of one planet is constantly retarded and of the other constantly accelerated, while during the other half the effects are reversed.^ "If we assume that the result proved for a polygonal line is also valid for a continuously curved line, we arrive at this result: If one of two synchronous clocks at A is moved in a closed curve with constant velocity uk.sci.astronomy Google Group .

^ What are the results of the impact of a meteorite on Earth and the other planets?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.There is thus in the case of each planet an oscillation of the mean longitude which increases it and then diminishes it to its original value at the end of the period of 883 years.^ This period is also called the planet year .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ This is also called the synodical period of revolution or the planet day or sol (which means " Sun " in Latin).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The sidereal orbital period (planet year ) is given in Earth years of 365.25 days (i.e., Julian years ) and in planet days (sols).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The longitudes, latitudes and radii vectores of a planet, being algebraically expressed as the sum of an infinite periodic series of the kind we have been describing, it follows that the problem of finding their co-ordinates at any moment is solved by computing these expressions.^ Topics include the Fourier integral, finite and infinite dimensional vector spaces, boundary value problems, eigenfunction expansions, Green’s functions, transform techniques for partial differential equations, and series solution of ordinary differential equations.
  • Physics and Astronomy 19 January 2010 9:52 UTC www.dartmouth.edu [Source type: Academic]

^ He has computed the number of revolutions likely to be made by the planets in one mahayuga , the traditional duration of which time period is considered as being 43,20,000 years.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

This is facilitated by the construction of tables by means of which the co-ordinates can be computed at any time. .Such tables are used in the offices of the national Ephemerides to construct ephemerides of the several planets, showing their exact positions in the sky from day to day.^ I am not sure that all of the pictures that I used show the natural colors of the planets .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Atomic Host Ahost is an addon to the VGA Planets host that, through the use of friendly codes, puts in several more planet and ship abilities.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ If you want to know the positions of the planets in the sky with great precision, then you can have them calculated by JPL or by a planetarium program, or you can look them up in an astronomical almanac (such as the "Astronomical Ephemerides" or [in Dutch] the Sterrengids ), or you can calculate them yourself using formulas from an appropriate book (such as the book "Astronomical Algorithms" by Jean Meeus).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

We pass now to the second branch of celestial mechanics, viz. that in which the planets are no longer considered as particles, but as rotating bodies of which the dimensions are to be taken into account. .Such a body, in free space, not acted on by any force except the attraction of its several parts, will go on rotating for ever in an invariable direction.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ It seems likely that the rotation of Mercury was slowed down (in part) because of tidal forces from the Sun , which is close to that planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.But, in consequence of the centrifugal force generated by the rotation, it assumes a spheroidal form, the equatorial regions bulging out.^ These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.Such a form we all know to be that of the earth and of the planets rotating on their axes.^ What does the rotation of a planet such as the Earth do to its atmosphere?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ When things move across a rotating planet , then they notice the rotation in the form of Coriolis forces.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The most widespread opinion in ancient writings about this subject until the 16th century was that all of the planets and the Sun and Moon orbit around the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Let us study the effect of this deviation from the spherical form upon the attraction exercised by a distant body.
.We begin with the special case of the earth as acted upon by the sun and moon.^ He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ The most widespread opinion in ancient writings about this subject until the 16th century was that all of the planets and the Sun and Moon orbit around the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The distances of the Moon and the Sun from the Earth was accurately measured as 108 times the diameters of these heavenly bodies.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

Let fig. .4 represent a section of the earth through its axis AB, ECQ being a diameter of the equator.^ It represents the entire sky; all celestial objects other than the earth are imagined as being located on its inside surface.
  • Ancient Indian  Astronomy 19 January 2010 9:52 UTC www.indicethos.org [Source type: FILTERED WITH BAYES]

^ If the planet does rotate around its own axis, then the ideal shape is a sphere that is slightly flattened, so that the diameter from pole to pole is a bit less than the diameter at the equator .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The polar diameter of the Earth (from one pole through the center to the other pole) is 12713.51 km or 7899.83 mi.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Let the dotted lines show the direction of the distant attracting body. .The point E, being more distant than C, will be attracted with less force, while Q will be attracted with a greater force than will the centre C. Were the force equal on every point of the earth it would have no influence on its rotation, but would simply draw its whole mass toward the attracting body.^ You can render the influence of the gravity of the Earth unimportant by measuring the gravity horizontally between two objects, and you can measure that gravity by balancing it with a force that you know.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The difficulty is to measure the gravity, because here on Earth the gravity of the Earth is very much greater than the gravity between any two things that we can handle in a laboratory.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Newton's third law of motion In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction noise The random fluctuations that are always associated with a measurement that is repeated many times over.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

.It is therefore only the difference of the forces on different parts of the earth that affects the rotation.^ It seems likely that the rotation of Mercury was slowed down (in part) because of tidal forces from the Sun , which is close to that planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The force of gravity keeps the atmosphere and other things as close as possible to the ground, and friction with the Earth causes the atmosphere to rotate with the Earth on average, so the atmosphere on average stands still relative to the ground.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ A difference of 2400 km in the distance of the Earth from the Sun corresponds to a difference of about one part in thirty thousand in the amount of sunlight per unit area that reaches the Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Let us, therefore, divide the attracting forces at each point into two parts, one the average force, which we may call F, and which for our purpose may be regarded as equal to the force acting at C; the others the residual forces which we must superimpose upon the average force F in order that the combination may be equal to the actual force.^ The ability of the eye to see separately two points close to each other.

^ Lagrange points Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance.
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^ It may also be the passage of all or part of one body through the shadow of another (e.g.
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

It is clear that at Q this residual force as represented by the arrow will be in the same direction as the actual force. .But at E, since the actual force is less than F, the residual force must tend to diminish F, and must, therefore, act toward the right, as shown by the arrow.^ At less than this distance the tidal forces of the larger object would break up the smaller object.
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.These residual forces tend to make the whole earth turn round the centre C in a clockwise direction.^ BC ) recognized that the Earth was round and believed that the Sun was " the centre of the spheres " as described in the Vedas at the time.
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.If nothing modified this tendency the result would be to bring the points E and Q into the dotted lines of the attraction.^ Splitting of spectral lines into several components, in contrast to the normal Zeeman effect which results in only two distinct components.

In other words the equator would be drawn into coincidence with the ecliptic. .Here, however, the same action comes into play, which keeps a rotating top from falling over.^ There is no such storm, and things that you throw into the air just come down again, so the Earth does not rotate.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If you go through a sharp curve on your bicycle, then you lean into the curve, too, for the very same reason: otherwise, you'll fall over.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

(See Gyroscope and Mechanics.) .For the same reason as in the case of the gyroscope the actual motion of the earth's axis is at right angles to the line joining the earth and the attracting centre, and without going into the details of the mathematical processes involved, we may say that the ultimate mean effect will be to cause the pole P of the earth to move at right angles to the circle joining it to the pole of the ecliptic.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
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^ An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer.
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^ Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.
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Were the position of the latter invariable, the celestial pole would move round it in a circle. .Actually the curve in which it moves is nearly a circle; but the distance varies slightly owing to the minute secular variation in the position of the ecliptic, caused by the action of the planets.^ This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit).
  • Imagine the Universe! Dictionary 19 January 2010 9:52 UTC imagine.gsfc.nasa.gov [Source type: FILTERED WITH BAYES]

^ They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations.

^ And then, precisely, on schedule, it silently and majestically moved from the southwest horizon to nearly overhead to the northern horizon over the course of about 4 minutes.

.This motion of the celestial pole results in a corresponding revolution of the equinox around the celestial sphere.^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

.The rate of motion is slightly variable from century to century owing to the secular motion of the plane of the ecliptic.^ The rate of diurnal motion undergoes seasonal variation because of the obliquity of the ecliptic and because of the eccentricity of the Earth's orbit.

.Its period, with the present rate of motion, would be about 26,000 years, but the actual period is slightly indeterminate from the cause just mentioned.^ Join us on a special dramatized 26,000 light-year adventure to the Galaxy’s hulking heart of darkness.

^ A Titanian mountain would have to be about ~15,000 feet high before the air would be cold enough to condense.

^ At this point we have observed Titan well for about 7 years, from the winter southern solstice until the northern spring equinox, which actually just occurred last week, the terrestrial equivalent of late December to late March.

.The residual force just described is not limited to the case of an ellipsoidal body.^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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.It will be seen that the reasoning applies to the case of any one body or system of bodies, the dimensions of which are not regarded as infinitely small compared with the distance of the attracting body.^ An ephemeris of a Solar-System body in which the tabulated positions are essentially comparable to catalog mean places of stars at a standard epoch.

^ [C95] (b) A small planet-like body of the Solar System, < e > ~ 0.15, < i > ~ 9 ° .7.

.In all such cases the residual forces virtually tend to draw those portions of the body nearest the attracting centre toward the latter, and those opposite the attracting centre away from it.^ A mutual physical force attracting two bodies.
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^ Newton's law of universal gravitation (Sir I. Newton) Two bodies attract each other with equal and opposite forces; the magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centers of mass of the two bodies.
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^ Those people – all of you out there – are like my grandparents pushing their children from opposite ends of the Mississippi River in the same direction towards their own goals.

.Thus we have a tide-producing force tending to deform the body, the action of which is of the same nature as the force producing precession.^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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^ Newton's second law of motion For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.
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.It is of interest to note that, very approximately, this deforming force varies inversely as the cube of the distance of the attracting body.^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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^ Asymptotic freedom is the principle which says that these forces become weaker for very close encounters between quarks, so that the quarks become `free' of the forces at very short distances.

^ A mutual physical force attracting two bodies.
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.The action of the sun upon the satellites of the several planets and the effects of this action are of the same general nature.^ Could there be a second Earth-like planet in the same orbit as the Earth but hidden exactly on the other side of the Sun?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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^ Do all planets travel around the Sun in the same direction?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

For the same reason that the residual forces virtually act in opposite directions upon the nearer and more distant portions of a planet FIG. 3.
FIG. 4.
they will virtually act in the case of a satellite. .When the latter is between its primary and the sun, the attraction of the latter tends to draw the satellite away from the primary.^ A far-away planet travels slower than a closer-by planet because the force of gravity between a planet and the Sun gets weaker when the planet and the Sun are further apart.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.When the satellite is in the opposite direction from the sun, the same action tends to draw the primary away from the satellite.^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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^ This implies that the satellite always keeps the same hemisphere facing its primary (e.g.
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^ When you are looking at the shadow of your own head, you are looking, by necessity, directly in the opposite direction of the sun.

.In both cases, relative to the primary, the action is the same.^ That period is called the synodical period of both planets , and after that period both planets are again in the same relative position.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ In the first case, you'll see the planet trace a loop between the stars, and in the second case, you'll see all stars trace the same loop relative to the planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.When the satellite is in quadrature the convergence of the lines of attraction toward the centre of the sun tends to bring the two bodies together.^ Roche limit The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary.
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.When the orbit of the satellite is inclined to that of the primary planet round the sun, the action brings about a change in the plane of the orbit represented by a rotation round an axis perpendicular to the plane of the orbit of the primary.^ The point in its orbit where a planet is farthest from the Sun.
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^ The plane of Earth's orbit about the Sun.
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^ All planets orbit around the Sun and turn around their own axis.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If we conceive a pole to each of these orbits, determined by the points in which lines perpendicular to their planes intersect the celestial sphere, the pole of the satellite orbit will revolve around the pole of the planetary orbit precisely as the pole of the earth does around the pole of the ecliptic, the inclination of the two orbits remaining unchanged.^ The plane of Earth's orbit about the Sun.
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^ The point at which a body in orbit around the Earth reaches its farthest distance from the Earth.

^ At what speed does a point at the equator rotate around the rotation axis of the Earth?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.If a planet rotates on its axis so rapidly as to have a considerable ellipticity, and if it has satellites revolving very near the plane of the equator, the combined actions of the sun and of the equatorial protuberances may be such that the whole system will rotate almost as if the planes of revolution of the satellites were solidly fixed to the plane of the equator.^ Something like that could happen also near the Earth or near another planet , so perhaps the Earth will someday (very many years from now) have some pretty rings like Saturn.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ In 1543, Nicolaus Copernicus (1473 - 1543) published a book in which he swept away Ptolemy's ideas and said that the Earth rotates around its axis, the stars are fixed, and the Earth orbits around the Sun as well.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Only near the equator does the temperature sometimes get above freezing, but the atmosphere is so dry there than any open water would evaporate very quickly.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

This is the case with the seven inner satellites of Saturn. .The orbits of these bodies have a large inclination, nearly 27°, to the plane of the planet's orbit.^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic ; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.
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^ Rule 3 means that some round celestial bodies that orbit directly around the Sun are yet not planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The action of the sun alone would completely throw them out of these planes as each satellite orbit would rotate independently; but the effect of the mutual action is to keep all of the planes in close coincidence with the plane of the planet's equator.^ The plane of Earth's orbit about the Sun.
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^ If a planet were suddenly transported to another part of the Solar System, or even removed completely, then that would only have great effect in the part of space where the planet dominated before, and in the part of space where the planet ends up.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Literature

.The modern methods of celestial mechanics may be considered to begin with Joseph Louis Lagrange, whose theory of the variation of elements is developed in his Mecanique analytique. The practical methods of computing perturbations of the planets and satellites were first exhaustively developed by Pierre Simon Laplace in his Mecanique celeste. The only attempt since the publication of this great work to develop the various theories involved on a uniform plan and mould them into a consistent whole is that of de Pontecoulant in Theorie analytique du systeme du monde (1829-46, Paris).^ Modern methods involve photoelectric filtering and the UBV system.

^ His work Mecanique Analytique (Analytical Mechanics; 1788) was a mathematical masterpiece.
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^ Lagrange also invented the method of solving differential equations known as variation of parameters.
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An approximation to such an attempt is that of F. F. Tisserand in his Traite de mecanique celeste (¢ vols., Paris). This work contains a clear and excellent resume of the methods which have been devised by the leading investigators from the time of Lagrange until the present, and thus forms the most encyclopaedic treatise to which the student can refer.
.Works less comprehensive than this are necessarily confined to the elements of the subject, to the development of fundamental principles and general methods, or to details of special branches.^ A gas planet also contains other elements, but those make up only a tiny proportion (less than 1 percent).
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ All things that have mass generate gravity , but things with more mass generate stronger gravity than things with less mass, and gravity decreases with increasing distance.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Einstein, Albert (1879 - 1955) German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics.
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An elementary treatise on the subject is F. R. Moulton's Introduction to Celestial Mechanics (London, 1902). .Other works with the same general object are H. A. Resal, Mecanique celeste; and O. F. Dziobek, Theorie der Planetenbewegungen. The most complete and systematic development of the general principles of the subject, from the point of view of the modern mathematician, is found in J. H. Poincare, Les Methodes nouvelles de la mecanique celeste (3 vols., Paris, 1899, 1892, 1893).^ Some methods are suited for measuring the distances to nearby stars, while other methods work at greater distances.

^ The same synodical period also holds if the observer moves to one of the other two objects.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Einstein, Albert (1879 - 1955) German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics.
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Of another work of Poincare, Legons de mecanique celeste, the first volume appeared in 1905.
.Practical Astronomy. Practical Astronomy, taken in its widest sense, treats of the instruments by which our knowledge of the heavenly bodies is acquired, the principles underlying their use, and the methods by which these principles are practically applied.^ In astronomy, these are often used for liquid nitrogen (at 77K), but can also be used for solid neon (17K) or liquid helium (4.2K).
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.Our knowledge of these bodies is of necessity derived through the medium of the light which they emit; and it is the development and applications of the laws of light which have made possible the additions to our stock of such knowledge since the middle of the 19th century.^ Eddington limit (Sir A. Eddington) The theoretical limit at which the photon pressure would exceed the gravitational attraction of a light-emitting body.
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^ For a blackbody at a temperature T , the intensity of radiation emitted I at a particular energy E is given by Plank's law: I(E,T) = 2 E 3 [h 2 c 2 (e E/kT - 1)] -1 where h is Planck's constant , k is Boltzmann's constant , and c is the the speed of light .
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.At the base of every system of astronomical observation is the law that, in the voids of space, a ray of light moves in a right line.^ The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving.
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.The fundamental problem of practical astronomy is that of determining by measurement the co-ordinates of the heavenly bodies as already defined.^ A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.
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Of the three co-ordinates,the radius vector does not admit of direct measurement, and must be inferred by a combination of indirect measurements and physical theories. .The other two coordinates, which define the direction of a body, admit of direct measurement on principles applied in the construction and use of astronomical instruments.^ Lagrange points Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance.
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^ A description of a force, such as Newton's law of gravity, in which two separated bodies are said to directly exert forces on each other.

^ A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.
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.In the first system of co-ordinates already described the fundamental axis is the vertical line or direction of gravity at the point of observation.^ Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.
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.This is not the direction of gravity proper, or of the earth's attraction, but the resultant of this attraction combined with the centrifugal force due to the earth's rotation on its axis.^ These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.
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The most obvious method of realizing this direction is by the plumb-line. .In our time, however, this appliance is replaced by either of two others, which admit of much more precise application.^ (Though maybe it's just that D&D admit their bugs more quickly than other people.
  • alt.games.vga-planets FAQ [LONG] 18 January 2010 2:02 UTC www.faqs.org [Source type: General]

^ It looked much like all of the other pictures of the Space Station that I had ever seen before with two exceptions.

^ In my mind it was simply yet-another large scientific meeting, this time spread over too much time (two weeks!

These are the basin of mercury and the spirit-level. The surface of a liquid at rest is necessarily perpendicular to the direction of gravity, and 807 therefore horizontal. Considered as a curved surface, concentric with the earth, a tangent plane to such a surface is the plane of the horizon. .The problem of measuring from an axis perpendicular to this plane is solved on the principle that the incident and reflected rays of light make equal angles with the perpendicular to a reflecting surface.^ The ratio of the amount of light reflected from a surface to the amount of incident light.

^ For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
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^ Spherical Aberration always occurs with rays that are distant from the axis and incident on a spherical mirror or lens.

It follows that if PO (fig. 5) is the direction of a ray, either from a heavenly body or from a terrestrial point, impinging at 0 upon the surface of quicksilver, and reflected in the direction OR, the vertical line is the bisector OZ, of the angle POR. If the point P is so adjusted over the quicksilver that the ray is reflected back Z 0 0 FIG. 5. FIG. 6.
on its own path, P and R lying on the same line above 0, then we know that the line PO is truly vertical. .The zenith-distance of an object is the angle which the ray of light from it makes with the vertical direction thus defined.^ For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal.
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To show the principle involved in the spirit-level let MN (fig. 6) be the tube of such a level, fixed to an axis OZ on which it may revolve. .If this axis is so adjusted that in the course of a revolution around it the bubble of the level undergoes no change of position, we know that the axis is truly vertical.^ What amazed me was how fast the moons rotate around Jupiter, so that their positions noticeably change in just a few hours.

Any slight deviation from verticality is shown by the motion of the bubble during the revolution, which can be measured and allowed for. The level may not be actually attached to an axis, a revolution of 180° being effected round an imaginary vertical axis by turning the level end for end. The motion of the bubble then measures double the inclination of this imaginary axis, or the deviation of a cylinder on which the level may rest from horizontality.
.The problem of determining the zenith distance of a celestial object now reduces itself to that of measuring the angle between the direction of the object and the direction of the vertical line realized in one of these ways.^ A planet is now a celestial object that .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ You can render the influence of the gravity of the Earth unimportant by measuring the gravity horizontally between two objects, and you can measure that gravity by balancing it with a force that you know.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them.
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.This measurement is effected by a combination of two instruments, the telescope and the graduated circle.^ A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful.
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Let OF (fig. 7) be a section of the telescope, MN being its M 0 ______ -? . --os N ', ` FIG. 7.
emanating from the object to be observed, which, for our purpose, object glass. .Let the parallel dotted lines represent rays of light we regard as infinitely distant, a star for example.^ For example, in radioastronomy, spectral line graphs are used to determine the kinematics or relative motions of material at the center of a galaxy or surrounding a star as it is born.
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^ The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.
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^ For example, the optical analog would be to remove the spikes and halos which often appear on images of bright stars because of light scattered by the telescope's internal supports.
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.These rays come to a focus at a point F lying in the focal plane of the telescope.^ The cone of rays through a lens from an off-axis object does not focus at a point.

.In this plane are a pair of cross threads or spider lines which, as the observer looks into the telescope, are seen as AB and CD (fig.^ In the orbit of a Solar-System body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.

^ (The eccentricity of the ellipse is zero - i.e., a circle - for a star on the ecliptic pole; for a star on the ecliptic plane the ellipse degenerates into a straight line.

8). .If the telescope is so pointed that the image of the star is seen in coincidence with the cross threads, as represented in fig.^ The central spot in the diffraction pattern of the image of a star at the focus of a telescope.

^ In the orbit of a Solar-System body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.

^ In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar ).
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.8, then we know that the star is exactly in the line of sight of the telescope, defined as the line joining the centre of the object glass, and the point of intersection of the cross threads.^ [A84] (b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.

^ In the orbit of a Solar-System body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.

.If the telescope is moved around so that the images of two distant points are successively brought into coincidence with the cross threads, we know that the angle between the directions of D these points is equal to that through FIG. 8. which the telescope has been turned.^ The direction in the sky to which the telescope is pointed.
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^ Between the two the image appears blurred.

^ The interval (27.555 days) between two successive perigee passages of the Moon.

This angle is measured by means of a graduated circle, rigidly attached to the tube of the telescope in a plane parallel to the line of sight. .When the telescope is turned in this plane, the angular motion of the line of sight is equal to that through which the circle has turned.^ The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle.
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R
.Stripped of all unnecessary adjuncts, and reduced to a geometric form, the ideal method by which the zenith distance of a heavenly body is determined by the combination which we have described is as follows: - Let OP (fig.^ [A84] (b) The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith.

^ Together, these methods form the “Cosmic Distance Ladder,” allowing astronomers to measure the distances to everything from nearby stars,… .

.9) be the direction of a celestial body at which a telescope, supplied with a graduating circle, is pointed.^ [A84] (b) The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith.

^ Many planetarium programs enable you to tie the viewing direction to a particular celestial body such as a planet or a star .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ [A84] (b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.

.Let OZ be an axis, as nearly vertical as it can easily be set, round which () N C A B the entire instrument may revolve through 180°.^ After the first few times of accidentally seeing the fog we did, one fine day, systematically search through the entire data set.

.After the image of the body is brought into coincidence with the cross threads, the instrument is turned through 180° on the axis, which results in the line of sight of the telescope pointing in a certain direction OQ, determined by the condition QOZ = ZOP. The telescope is then a second time pointed at the object by being moved through the angle QOP. Either of the angles QOZ and ZOP is then one half that through which the telescope has been turned, which may be measured by a graduated circle, and which is the zenith distance of the object measured from the direction of the axis OZ. This axis may not be exactly vertical.^ The distance to an object which has a parallax of one arc second.
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^ The attractor may be a point, a line, or a fractal.

^ If the object (or a point on it) moves from point P 1 to point P 2 in a plane perpendicular to the axis, is the angle P 1 OP 2 , where O is the point at which the perpendicular plane meets the axis.

.Its deviation from the vertical line is determined by the motion of the bubble of a spirit-level rigidly attached either to the axis, or to the telescope.^ For example, in radioastronomy, spectral line graphs are used to determine the kinematics or relative motions of material at the center of a galaxy or surrounding a star as it is born.
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Applying this deviation to the measured arc, the true zenith distance of the body is found.
.When the basin of quicksilver is used, the telescope, either before or after being directed toward P, is pointed directly downwards, so that the observer mounting above it looks through it into the reflecting surface.^ The direction in the sky to which the telescope is pointed.
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^ I started thinking about where 2009 YE7 is in the sky, what telescopes I could use to point at it, how to time the observations.

^ When you are looking at the shadow of your own head, you are looking, by necessity, directly in the opposite direction of the sun.

He then adjusts the instrument so that the cross threads coincide with their images reflected from the surface of the quicksilver. .The angular motion of the telescope in passing from this position to that when the celestial object is in the line of sight is the distance (ND) of the body from the nadir.^ The apparent angular displacement of the observed position of a celestial body produced by the motion of the observer and the actual motion of the observed object.

^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

^ The mean anomaly is the angular distance (measured in the same manner) between periastron and a fictitious body in the direction of the star, which is moving in a circular orbit with a period equal to that of the star.

Subtracting 90° from (ND) gives the altitude; and subtracting (ND) from 180° gives the zenith distance.
In the measurement of equatorial co-ordinates, the polar distance is determined in an analogous way. .We determine the apparent position of an object near the pole on the celestial sphere at any moment, and again at another moment, twelve hours later, when, by the diurnal motion, it has made half a revolution.^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
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^ An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them.
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.The angle through the celestial pole, between these two positions, is double the polar distance.^ [A84] (b) The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith.

^ Asymptotic freedom is the principle which says that these forces become weaker for very close encounters between quarks, so that the quarks become `free' of the forces at very short distances.

^ Hubble's law (E.P. Hubble; 1925) A relationship between a galaxy's distance from us and its velocity through space.
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The pole is the point midway between them. .This being ascertained by one or more stars near it, may be used to determine by direct measurements the polar distances of other bodies.^ A planisphere of the stars used in determining the longitude .
  • Astronomy,Antiquarian Books, Rare Books,antique maps,antique globes,historical prints, travel guides, atlases, gazetteers 19 January 2010 9:52 UTC www.murrayhudson.com [Source type: General]

^ The fog doesn’t appear to prefer hanging around the one big south polar lake or even around the other dark areas that people think might be lakes.

The preceding methods apply mainly to the latitudinal co-ordinate. .To measure the difference between the longitudinal co-ordinates of two objects by means of a graduated circle the instruments must turn on an axis parallel to the principal axis of the system of coordinates, and the plane of the graduated circle must be at right angles to that axis, and, therefore, parallel to the principal co-ordinate plane.^ Mathematically, a singularity is a condition when equations do not give a valid value, and can sometimes be avoided by using a different coordinate system.
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^ The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place.
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^ Rather, we would talk of the great difference between giant planets and terrestrial planets, we would talk of the band of asteroids, and we would talk of the ever-increasing number of tiny icy objects out there on the very edge of the solar system.

.The telescope, in order that it may be pointed in any direction, must admit of two motions, one round the principal axis, and the other round an axis at right angles to it.^ The direction in the sky to which the telescope is pointed.
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^ You can keep the planets in the right order, but give up on showing their true distances from each other.

^ It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward.
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.By these two motions the instrument may be pointed first at one of the objects and then at the other.^ Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.

^ The point in an orbit when two objects are closest together.
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^ Newton's laws of motion (Sir I. Newton) Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.
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.The motion of the graduated circle in passing from one pointing to the other is the measure of the difference between the longitudinal co-ordinates of the two objects.^ You can render the influence of the gravity of the Earth unimportant by measuring the gravity horizontally between two objects, and you can measure that gravity by balancing it with a force that you know.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The difficulty is to measure the gravity, because here on Earth the gravity of the Earth is very much greater than the gravity between any two things that we can handle in a laboratory.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Lagrange points Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance.
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.In the equatorial system this co-ordinate (the right ascension) is measured in a different way, by making the rotating earth perform the function of a graduated circle.^ The apparent motion of the Sun , Moon , planets , and stars in the sky can be explained in two ways: (1) the Sun , Moon , planets , and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ They both have that ability to make me re-remember my astronaut-yearning days, but each in very different ways.

^ Right ascension is analogous to longitude for locating positions on the Earth.
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.The unceasing diurnal motion of the image of any heavenly body relative to the cross threads of a telescope makes a direct accurate measure of any co-ordinate except the declination almost impossible.^ A measure of the directivity of a radio telescope.

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
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^ Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.

Before the position of a star can be noted, it has passed away from the cross threads. This troublesome result is utilized and made a means of measurement. .Right ascensions are now determined, not by measuring the angle between one star and another, but, by noting the time between the transits of successive stars over the meridian.^ Rather, irregularly shaped grains of carbon and/or silicates measuring a fraction of a micron across which are found between the stars.
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^ The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place.
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^ Astronomically, it is half the angle which a a star appears to move as the earth moves from one side of the sun to the other.
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.The difference between these times, when reduced to an angle, is the difference of the right ascensions of the stars.^ As seen from the planet , the Sun returns to (about) the same place between the stars after this much time .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What is different this time is that these people sound truly worried.

^ There is no clear difference (regarding the measurements of the celestial body itself) between large planets and small stars , or between small planets and large asteroids or moons .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The principle is the same as that by which the distance between two stations may be determined by the time required for a train moving at a uniform known speed to pass from one station to the other.^ The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all frames of reference moving at a uniform velocity and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving.
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^ It looked much like all of the other pictures of the Space Station that I had ever seen before with two exceptions.

^ Tell me more about GRIS. gas One of the three states of matter, in which atoms, molecules, or ions move freely and are not bound to each other.
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.The uniform speed of the diurnal motion is 15° per hour.^ The Earth's circumference at the equator is about 40000 km or 24900 miles and the Earth rotates once per 24 hours , so the rotation speed at the equator is 40000/24 = 1670 km/h or 24900/24 = 1038 mph.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

We have already mentioned that in astronomical practice right ascensions are expressed in time, so that no multiplication by 15 is necessary.
.Measures made on the various systems which we have described give the apparent direction of a celestial object as seen by the observer.^ [C95] (b) Measure of the observed brightness of a celestial object as seen from the Earth.

^ [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer.

^ The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic ; moving in the opposite sense from the great majority of solar system bodies.
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.But this is not the true direction, because the ray of light from the object undergoes refraction in passing through the atmosphere.^ Dark lines superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source.

^ The top layer of the ground could have been chemically changed by sunlight (especially the ultraviolet light , which is not kept from the surface of Mars, because Mars lacks an ozone layer) or by chemical reactions with the atmosphere .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ [A84] (b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.

It is therefore necessary to correct the observation for this effect. .This is one of the most troublesome problems in astronomy because, owing to the ever varying density of the atmosphere, arising from differences of temperature, and owing to the impossibility of determining the temperature with entire precision at any other point than that occupied by the observer, the amount of refraction must always be more or less uncertain.^ Only near the equator does the temperature sometimes get above freezing, but the atmosphere is so dry there than any open water would evaporate very quickly.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It is likely that the atmospheres of Venus and the Earth started out similar, but today the atmosphere of Venus is very different from that of Earth: It has a humongous greenhouse effect , surface temperatures of about 450 ℃ or 800 ℉ higher than on Earth and atmospheric pressure at the surface that is about 90 times as great as on Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Difference in a star's brightness when measured on two selected wavelengths, in order to determine the star's temperature.

The complexity of the problem will be seen by reflecting that the temperature of the air inside the telescope is not without its effect. .This temperature may be and commonly is somewhat different from that of the observing room, which, again, is commonly higher than the temperature of the air outside.^ It is likely that the atmospheres of Venus and the Earth started out similar, but today the atmosphere of Venus is very different from that of Earth: It has a humongous greenhouse effect , surface temperatures of about 450 ℃ or 800 ℉ higher than on Earth and atmospheric pressure at the surface that is about 90 times as great as on Earth.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The uncertainty thus arising in the amount of the refraction is least near the zenith, but increases more and more as the horizon is approached.
.The result of astronomical observations which is ordinarily wanted is not the direction of an object from the observer, but from the centre of the earth.^ Astronomers often express units for other objects in terms of solar units, since it makes the resulting numbers smaller and easier to deal with.
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^ Astronomers often express units for other objects in terms of solar units, which makes the resulting numbers smaller and easier to deal with.
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Thus a reduction for parallax is required. Having effected this reduction, and computed the correction to be applied to the observation in order to eliminate all known errors to which the instrument is liable, the work of the practical astronomer is completed.
.The instruments used in astronomical research are described under their several names.^ It was named after A. J. Ångström (1814-1874), the Scandinavian scientist who used units of 10 -10 m to describe wavelengths in his classical map of the Solar spectrum made in 1868.

.The following are those most used in astrometry: The equatorial telescope is an instrument which can be directed to any point in the sky, and which derives its appellation from its being mounted on an axis parallel to that of the earth.^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I started thinking about where 2009 YE7 is in the sky, what telescopes I could use to point at it, how to time the observations.

^ But, really, for most of my life, I’ve been just as guilty when it comes to those other things that occupy our night skies: the satellites.

.By revolving on this axis it follows a star in its diurnal motion, so that the star is kept in the field of view notwithstanding that motion.^ The apparent motion of the Sun , Moon , planets , and stars in the sky can be explained in two ways: (1) the Sun , Moon , planets , and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Next in extent of use are the transit instrument and the meridian circle, which are commonly united in a single instrument, the transit circle, known also as the meridian circle.^ The distance to the Sun remained essentially unknown (except that it was known to be "large") until 1761 when a transit of Venus could be used to estimate it.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.This instrument moves only in the plane of the meridian on a horizontal east and west axis, and is used to determine the right ascensions and declinations of stars.^ A planisphere of the stars used in determining the longitude .
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^ A superior planet (one that is further away from the Sun than the Earth is) usually moves towards the east between the stars , but around its opposition it temporarily moves to the west and then makes a kind of loop between the stars , before continuing with its usual eastward motion.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ At certain latitudes , clouds and the weather tend to move to the east, but at other latitudes they tend to move to the west.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

These two instruments or combinations are a necessary part of the outfit of every important observatory. An adjunct of prime importance, which is necessary to their use, is an accurate clock, beating seconds.

Use of Photography

.Before the development of photography, there was no possible way of making observations upon the heavenly bodies except by the eye.^ There’s only one way to make the fog stick around on the ground for any amount of time, and that is to both add humidty and cool the air just a little.

^ When the first stars were formed there were no elements around except for the three lightest ones (hydrogen, helium, and lithium), and you can't make large molecules or earthlike planets from those, so there could not be any life as we know it then.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ There is no choice except to dispense with trying to depict both the distances between planets and the sizes of planets on the same scale.

.Since the middle of the 19th century the system of photographing the heavenly bodies has been introduced, step by step, so that it bids fair to supersede eye observations in many of the determinations of astronomy.^ VLBI is a means in radio astronomy by which simultaneous observations of an object by many telescopes are combined into one image.

(See Photography: Celestial.) The field of practical astronomy includes an extension which may be regarded as making astronomical science in a certain sense universal. The science is concerned with the heavenly bodies. .The earth on which we live is, to all intents and purposes, one of these bodies, and, so far as its relations to the heavens are concerned, must be included in astronomy.^ Though many planets have an atmosphere , there is only one that has an atmosphere in which we can live, and that is the Earth .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ After all, the planet Neptune was also discovered from its gravitational tugging on the other planets, even though those planets remain so far from Neptune that the gravitational acceleration that Neptune causes in these other planets is less than the gravitational acceleration that the anti-Earth would cause in the terrestrial planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The atmospheres of the other planets that have one contain far too little oxygen or none at all.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The processes of measuring great portions of the earth, and of determining geographical positions, require both astronomical observations proper, and determinations made with instruments similar to those of astronomy.^ A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful.
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^ Astrometrists measure parallaxes and proper motions, which allow astronomers to determine the distances and velocities of the stars.

^ They may also have a rocky core far below the layers of gas, and that core is then probably made of similar elements as the earth-like planets .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

Hence geodesy may be regarded as a branch of practical astronomy. (S. N.) History of Astronomy. A practical acquaintance with the elements of astronomy is indispensable to the conduct of human life. Hence it is most widely diffused among uncivilized peoples, whose existence depends upon immediate and unvarying submission to the dictates of external nature. Having no clocks, they regard instead the face of the sky; the stars serve them for almanacs; they hunt and fish, they sow and reap in correspondence with the recurrent order of celestial appearances. But these, to the untutored imagination, present a mystical, as well as a mechanical aspect; and barbaric familiarity with the heavens developed at an early age, through the promptings of superstition, into a fixed system of observation. In China, Egypt and Babylonia, strength and continuity were lent to this native tendency by the influence of a centralized authority; considerable proficiency was attained in the arts of observation; and from millennial stores of accumulated data, empirical rules were deduced by which the scope of prediction was widened and its accuracy enhanced. But no genuine science of astronomy was founded until the Greeks sublimed experience into theory.
Already, in the third millennium B.C., equinoxes and solstices were determined in China by means of culminating stars. This is known from the orders promulgated by the emperor Yao about 2300 B.C., as recorded in the Shu Chung, a collection of do'.uments antique in the time of Confucius (550-478 B.C.). And Yao was merely the renovator of a system long previously established. .The Chung further relates the tragic fate of the official astronomers, Hsi and Ho, put to death for neglecting to perform the rites customary during an eclipse of the sun, identified by Professor S. E. Russell' with a partial obscuration visible in northern China 2136 B.C. The date cannot be far wrong, and it is by far the earliest assignable to an event of the kind.^ An eclipse of the Sun in which the Moon is too far from Earth to block out the Sun completely, so that a ring of sunlight appears around the Moon.

There is, however, no certainty that the Chinese were then capable of predicting The Observatory, Nos. 2 3 1 - 2 34, 1895.
0 FIG. 9.
eclipses. .They were, on the other hand, probably acquainted, a couple of millenniums before Meton gave it his name, with the nineteen-year cycle, by which solar and lunar years were harmonized; 1 they immemorially made observations in the meridian; regulated time by water-clocks, and used measuring instruments of the nature of armillary spheres and quadrants.^ Why are they so attached to the 18 th largest object in the solar system when they probably can’t even name all of the 17 larger things?

^ They use a clock to represent our current time, where midnight is complete catastrophe.

^ I started thinking about where 2009 YE7 is in the sky, what telescopes I could use to point at it, how to time the observations.

.In or near Iioo B.C., Chou Kung, an able mathematician, determined with surprising accuracy the obliquity of the ecliptic; but his attempts to estimate the sun's distance failed hopelessly as being grounded on belief in the flatness of the earth.^ Mean distance between the Earth and the Sun: 149,598,500km.

^ [H76] (d) The mean distance from the earth to the sun, equal to 92.81 million miles or 499.012 light-seconds.

^ [C95] (c) The mean distance between the Earth and the Sun.

.From of old, in China, circles were divided into 3654 parts, so that the sun described daily one Chinese degree; and the equator began to be employed as a line of reference, concurrently with the ecliptic, probably in the second century B.C. Both circles, too, were marked by star-groups more or less clearly designated and defined.^ A particle counter in which the circuit has been designed so as not to register the passage of an ionizing particle through more than one counting tube.

^ (The eccentricity of the ellipse is zero - i.e., a circle - for a star on the ecliptic pole; for a star on the ecliptic plane the ellipse degenerates into a straight line.

^ More than 1800 have been catalogued, and probably millions of smaller ones exist, but their total mass would probably be less than 3 percent that of the Moon.

.Cometary records of a vague kind go back in China to 2296 B.C.; they are intelligible and trustworthy from 611 B.C. onward.^ The senior member of the team agreed that it seemed unlikely that their method was going to work and he said they would discuss and get back to me.

.Two instruments constructed at the time of Kublai Khan's accession in 1280 were still extant at Peking in 1881. They were provided with large graduated circles adapted for measurements of declination and right ascension, and prove the Chinese to have anticipated by at least three centuries some of Tycho Brahe's most important inventions.^ The image reveals the presence of two giant bright spots, whose size is equivalent to the distance Earth-Sun: they cover a large fraction of the surface.

2 The native astronomy was finally superseded in the 17th century by the scientific teachings of Jesuit missionaries from Europe.
Astrolatry was, in Egypt, the prelude to astronomy. The stars were observed that they might be duly worshipped. .The importance of their heliacal risings, or first visible appearances at dawn, for the purposes both of practical life and of ritual observance, caused them to be systematically noted; the length of the year was accurately fixed in connexion with the annually recurring Nile-flood; while the curiously precise orientation of the Pyramids affords a lasting demonstration of the high degree of technical skill in watching the heavens attained in the third millennium B.C. The constellational system in vogue among the Egyptians appears to have been essentially of native origin; but they contributed little or nothing to the genuine progress of astronomy.^ Point out your halo to any else and they will see precisely the same thing: a halo around their own heads and nothing around yours.

^ Venus today has such a giant greenhouse effect that its surface temperature is 470 degrees Centigrade ― a bit on the high side for life.
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^ The so-called seas on the Moon are volcanic in origin, but they are thousands of millions of years old, and the lava probably came from cracks in the ground rather than from volcanoes.
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With the Babylonians the case was different, although their science lacked the vital principle of growth imparted to it by their successors. From them the Greeks derived their first notions of astronomy. They copied the Ba by Ionian asterisms, appropriated Babylonian knowledge of the planets and their courses, and learned to predict eclipses by means of the " Saros." This is a cycle of 18 years II days, or 223 lunations, discovered at an unknown epoch in Chaldaea, at the end of which the moon very nearly returns to her original position with regard as well to the sun as to her own nodes and perigee. .There is no getting back to the beginning of astronomy by the shores of the Euphrates.^ When the fluid gets to rest again (because of friction with the cup) then some of the fluid flows back to the middle again, and then there is a balance once more.
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Records dating from the reign of Sargon of Akkad (3800 B.C.) imply that even then the varying aspects of the sky had been long under expert observation. Thus early, there is reason to suppose, the stargroups with which we are now familiar began to be formed. .They took shape most likely, not through one stroke of invention, but incidentally, as legends developed and astrological persuasions became defined.^ When the Voyager spacecrafts flew by, they took pictures of Titan which look like a big orange billiard ball.

^ When these stars explode, they spread the newly made elements through the Universe , and only after this could earth-like planets be made.
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3 The zodiacal series in particular seem to have been reformed and reconstructed at wide intervals of time (see ZoDIAc). Virgo, for example, is referred by P. Jensen, on the ground of its harvesting associations, to the fourth millennium B.C., while Aries (according to F. K. Ginzel) was interpolated at a comparatively recent time. .In the main, however, the constellations transmitted to the West from Babylonia by Aratus and Eudoxus must have been arranged very much in their present order about 2800 B.C. E. W. Maunder's argument to this effect is unanswerable.'^ There is not much of a greenhouse effect on Mars today, because there are not enough greenhouse gases in its atmosphere, which is very thin anyway.
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For the space of the 1 Observations of Comets, translated from the Chinese Annals by John Williams, F.S.A. (1871).
J. L. E. Dreyer, Proc. Roy. Irish Acad. vol. iii. No. 7 (December 1881).
F. K. Ginzel, " Die astronomischen Kenntnisse der Babylonier," C. F. Lehmann, Beitriige zur alten Geschichte, Heft i. p. 6 (1901). Knowledge and Scientific News, vol. i. pp. 2, 228.
southern sky left .blank of stellar emblazonments was necessarily centred on the pole; and since the pole shifts among the stars through the effects of precession by a known annual amount, the ascertainment of any former place for it virtually fixes the epoch.^ An ephemeris of a Solar-System body in which the tabulated positions are essentially comparable to catalog mean places of stars at a standard epoch.

It may then be taken as certain that the heavens described by Aratus in 270 B.C. represented approximately observations made some 2500 years earlier in or near north latitude 400.
In the course of ages, Babylonian astronomy, purified from the astrological taint, adapted itself to meet the most refined needs of civil life. .The decipherment and interpretation by the learned Jesuits, Fathers Epping and Strassmeier, of a number of clay tablets preserved in the British Museum, have supplied detailed knowledge of the methods practised in Mesopotamia in the 2nd century B.C. 5 They show no trace of Greek influence, and were doubtless the improved outcome of an unbroken tradition.^ However, conjunctions of the planets have no influence on Earth and are not important, except that they provide opportunities to take nice pictures of them.
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^ Undiscovered planets could only be hiding between the other planets if they have no measurable influence on the other planets.
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' How protracted it had been, can be in a measure estimated from the length of the revolutionary cycles found for the planets. .The Babylonian computers were not only aware that Venus returns in almost exactly eight years to a given starting-point in the sky, but they had established similar periodic relations in 4 6, 59, 79 and 83 years severally for Mercury, Saturn, Mars and Jupiter.^ On Mercury, a day lasts longer than a year , and in fact exactly twice as long.
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^ If, for example, Mars took exactly 2 years to go around the Sun, and Jupiter exactly 12 years , then after every 12 years the Earth would have gone around the Sun exactly 12 times, Mars exactly 6 times, and Jupiter exactly 1 time, so then all three of them would be in the same relative positions again.
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^ The sidereal orbital period (planet year ) is given in Earth years of 365.25 days (i.e., Julian years ) and in planet days (sols).
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.They were accordingly able to fix in advance the approximate positions of these objects with reference to ecliptical stars which served as fiducial points for their determination.^ The branch of astronomy that deals with measuring the positions of celestial objects, especially stars.

^ You should therefore take these reported sizes with a grain of salt, except if they have been determined in an independent manner (for example, through a measurement of the temperature of the object).
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^ Position of a rocket with respect to the horizon or some other fixed reference plane.

.In the Ephemerides published year by year, the times of new moon were given, together with the calculated intervals to the first visibility of the crescent, from which the beginning of each month was reckoned; the dates and circumstances of solar and lunar eclipses were predicted; and due information was supplied as to the forthcoming heliacal risings and settings, conjunctions and oppositions of the planets.^ If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?
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^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
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^ Roughly speaking, the perihelion passage of the Earth will be around the average time when it is New Moon or Full Moon , will be about 30 hours early (i.e., usually on 2 January) if it is near Last Quarter, and about 30 hours late (i.e., usually on 4 January) if it is near First Quarter.
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.The Babylonians knew of the inequality in the daily motion of the sun, but misplaced by to' the perigee of his orbit.^ The apparent motion of the Sun , Moon , planets , and stars in the sky can be explained in two ways: (1) the Sun , Moon , planets , and stars orbit around the Earth once a day, or (2) the Earth rotates around its axis once a day.
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.Their sidereal year was 4zm too long, s and they kept the ecliptic stationary among the stars, making no allowance for the shifting of the equinoxes.^ It does not matter where those stars are in the sky, as long as they are above the horizon , so people noticed this retrograde motion already thousands of years ago.
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The striking discovery, on the other hand, has been made by the Rev. F. X. Kugler 7 that the various periods underlying their lunar predictions were identical with those heretofore believed to have been independently arrived at by Hipparchus, who accordingly must be held to have borrowed from Chaldaea the lengths of the synodic, sidereal, anomalistic and draconitic months.
.A steady flow of knowledge from East to West began in the 7th century B.C. A Babylonian sage named Berossus founded a school about 640 B.C. in the island of Cos, and perhaps counted Thales of Miletus (c. 639-548) among his astro- pupils.^ The first ones that we know about are Hiketas of Syracuse (a city on the island of Sicily) and Herakleides of Pontus (a region that is now in Turkey), who both studied in the school founded by Pythagoras (582 - 496 BC).
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^ The columns "East" and "West" show whether the Sun rises or sets in about that direction.
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^ For example, the Earth rotates clockwise if you look at it from above the south pole, and as seen from Earth the Sun rises in about the east and sets in about the west.
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.The famous " eclipse of Thales " in 585 B.C. has not, it is true, been authenticated by modern research 8; yet the story told by Herodotus appears to intimate that a knowledge of the Saros, and of the forecasting facilities connected with it, was possessed by the Ionian sage.^ When galaxies were known as spiral nebulae and their nature was not yet understood, avoidance was thought by some researchers to indicate a connection between them and the Milky Way.

Pythagoras of Samos (fl. .540-510 B.C.) learned on his travels in Egypt and the East to identify the morning and evening stars, to recognize the obliquity of the ecliptic, goras. and to regard the earth as a sphere freely poised in space.^ The rate of diurnal motion undergoes seasonal variation because of the obliquity of the ecliptic and because of the eccentricity of the Earth's orbit.

^ Thus to an Earth-based observer a star describes an ellipse on the celestial sphere with a semimajor axis of 20'.49.

The tenet of its axial movement was held by many of his followers - in an obscure form by Philolaus of Crotona after the middle of the 5th century B.C., and more explicitly by Ecphantus and Hicetas of Syracuse (4th century B.C.), and by Heraclides of Pontus. .Heraclides, who became a disciple of Herac- Plato in 360 B.C., taught in addition that the sun, fides. while circulating round the earth, was the centre of revolution to Venus and Mercury.^ If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively.

^ [REVOLUTION OF THE EARTH AROUND THE SUN IN A YEAR].
  • Astronomy,Antiquarian Books, Rare Books,antique maps,antique globes,historical prints, travel guides, atlases, gazetteers 19 January 2010 9:52 UTC www.murrayhudson.com [Source type: General]

^ CHAMBERS, W. & R. ATLAS TO ACCOMPANY CHAMBERS'S ENCYCLOPAEDIA : A Series Of Thirty-Nine Colored Maps And A Map Of The ANNUAL REVOLUTION OF THE EARTH ROUND THE SUN. J.B. LIPPINCOTT & CO./W. & R. CHAMBERS. PHILADELPHIA/EDINBURGH. 1869.
  • Astronomy,Antiquarian Books, Rare Books,antique maps,antique globes,historical prints, travel guides, atlases, gazetteers 19 January 2010 9:52 UTC www.murrayhudson.com [Source type: General]

9 A genuine heliocentric system, developed by Aristarchus of Samos (fl. 280-264 B.C.), was described by Archimedes in his Arenarius, only to be set aside Astronomisches aus Babylon (Freiburg im Breisgau, 1889). 6 Ginzel, loc. cit. Heft ii. p. 204.
Die babylonische Mondrechnung, p. 50 (1900) .
8 S. Newcomb, Astr. Nach. No. 3682; P. H. Cowell, Month. Notices Roy. Astr. Soc. lxv. 867.
9 G. V. Schiaparelli, I Precursori del Copernico, pp. 23-28, Pubbl. del R. Osservatorio di Brera, No. iii. (1873).
Egyptian nomy. with disapproval. The long-lived conception of a series of crystal spheres, acting as the vehicles of the heavenly bodies, and attuned to divine harmonies, seems to have originated with Pythagoras himself.
The first mathematical theory of celestial appearances was devised by Eudoxus of Cnidus (4 08 -355 B.C.). .1 The problem he attempted to solve was so to combine uniform circular movements as to produce the resultant effects actually observed.^ The apparent angular displacement of the observed position of a celestial body produced by the motion of the observer and the actual motion of the observed object.

.The sun and moon and the five planets were, with this end in view, accommodated each with a set of variously revolving spheres, to the total number of 27. The Eudoxian or " homocentric " system, after it had been further elaborated by Callippus and Aristotle, was modified by Apollonius of Perga (fl.^ [F88] (b) Divination using the positions of the planets, the Sun and the Moon as seen against the stars in the constellations of the zodiac - a "science" almost as old as homo sapiens.

^ October (1) Moon shadows galore ► September (5) The End What is a dwarf planet?

^ The sun, though it has set over the horizon, is directly behind you as you face the full moon.

250-220 B.C.) into the hypothesis of deferents and epicycles, which held the field for 1800 years as the characteristic embodiment of Greek ideas in astronomy. Eudoxus further wrote two works descriptive of the heavens, the Enoptron and Phaenomena, which, substantially preserved in the Phaenomena of Aratus (fl. 270 B.C.), provided all the leading features of modern stellar nomenclature.
Greek astronomy culminated in the school of Alexandria. It was, soon after its foundation, illustrated by the labours of Aristyllus and Timocharis (c. 320-260 B.C.), who School of constructed the first catalogue giving star -positions as Alex- g g g p andria. measured from a reference-point in the sky. This fundamental advance rendered inevitable the detection of precessional effects. .Aristarchus of Samos observed at Alexandria 280-264 B.C. His treatise on the magnitudes and distances of the sun and moon, edited by John Wallis in 1688, describes a theoretically valid method for determining the relative distances of the sun and moon by measuring the angle between their centres when half the lunar disk is illuminated; but the time of dichotomy being widely indeterminate, no useful result was thus obtainable.^ [F88] (b) Divination using the positions of the planets, the Sun and the Moon as seen against the stars in the constellations of the zodiac - a "science" almost as old as homo sapiens.

^ Mean distance between the Earth and the Sun: 149,598,500km.

^ [C95] (b) The absolute magnitude ( g ) of a Solar-System body such as an asteroid is defined as the brightness at zero phase angle when the object is 1 AU from the Sun and 1 AU from the observer.

.Aristarchus in fact concluded the sun to be not more than twenty times, while it is really four hundred times farther off than our satellite.^ But, really, for most of my life, I’ve been just as guilty when it comes to those other things that occupy our night skies: the satellites.

^ It’s not that I don’t see them all the time when I am looking at the sky, but I never think of them as anything more than spots of light moving across the heavens.

^ For some time after I first moved in I tried to remember to bring a flashlight with me to light my way, but more often than not I forgot.

His general conception of the universe was comprehensive beyond that of any of his predecessors.
Eratosthenes (276-196 B.C.), a native of Cyrene, was summoned from Athens to Alexandria by Ptolemy Euergetes to take charge of the royal library. He invented, or improved armillary spheres, the chief implements of ancient astrometry, determined the obliquity of the ecliptic at 23° 51' (a value 5' too great), and introduced an effective mode of arc-measurement. .Knowing Alexandria and Syene to be situated 5000 stadia apart on the same meridian, he found the sun to be 7° 12' south of the zenith at the northern extremity of this arc when it was vertically overhead at the southern extremity, and he hence inferred a value of 252,000 stadia for the entire circumference of the globe.^ The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern.

^ Because when it was night time in the Canary Islands the sun was still high overhead in southern California.

This is a very close approximation to the truth, if the length of the unit employed has been correctly assigned.2 Among the astronomers of antiquity, two great men stand out with unchallenged pre-eminence. .Hipparchus and Ptolemy entertained the same large organic designs; they worked on similar methods; and as the outcome, their performances fitted so accurately together that between them they re-made celestial science.^ The senior member of the team agreed that it seemed unlikely that their method was going to work and he said they would discuss and get back to me.

^ Celestial objects such as Quaoar and Sedna are presented as possible planets because they are presumably quite similar to Pluto: They have a similar size as Pluto, probably about the same composition, and are all beyond the orbit of Neptune.
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^ If you look carefully you will find that while the details of the cloud maps are different between the two papers, the overall conclusions are largely the same.

.Hipparchus fixed the chief data of astronomy - the lengths of the tropical and sidereal years, of the various months, and of the synodic periods of the five planets; determined the obliquity of the ecliptic and of the moon's path, the place of the sun's apogee, the eccentricity of his orbit, and the moon's horizontal parallax; all with approximate accuracy.^ If P moon is the sidereal period of such a moon (in its orbit around Jupiter), and P J is the sidereal period of Jupiter (in its orbit around the Sun), then the synodical period P syn of the moon is equal to .
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^ This period is also called the planet year .
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^ This is also called just the synodical period of the planet .
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His loans from Chaldaean experts appear, indeed, to have been numerous; but were doubtless independently verified. His supreme merit, however, consisted in the establishment of astronomy on a sound geometrical basis. His acquaintance with trigonometry, a branch of science initiated by 1 G. V. Schiaparelli, I Precursori del Copernico, pp. 23-28, Pubbl. del R. Osservatorio di Brera, No. ix.
2 Marie, Hist. des sciences, t. i. p. 79; P. Tannery, Hist. de l'astronomie ancienne, ch. v. p. 215.
him, together with his invention of the planisphere, enabled him to solve a number of elementary problems; and he was thus led to bestow especial attention upon the position of the equinox, as being the common point of origin for measures both in right ascension and longitude. Its steady retrogression among the stars became manifest to him in 130 B.C., on comparing his own observations with those made by Timocharis a century and a half earlier; and he estimated at not less than 36" (the true value being so") the annual amount of " precession." The choice made by Hipparchus of the geocentric theory of the universe decided the future of Greek astronomy. .He further elaborated it by the introduction of " eccentrics," which accounted for the changes in orbital velocity of the sun and moon by a displacement of the earth, to a corresponding extent, from the centre of the circles they were assumed to describe.^ The orbits of Mars and the Earth are not perfect circles but rather like circles that are a little squashed, and the orbits are also shifted a little so that the Sun is not quite in the center of the orbits.
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^ I think that a planet has a greater chance of having moons if the planet has more mass (i.e., is larger) and if the planet is further away from the Sun .
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^ (The strength of the seasons on Earth cannot change that much, because the gravity of the Moon prevents it.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

This gave the elliptic inequality known as the " equation of the centre," and no other was at that time obvious. .He attempted no detailed discussion of planetary theory; but his catalogue of 1 080 stars, divided into six classes of brightness, or " magnitudes," is one of the finest monuments of antique astronomy.^ A star that is one absolute magnitude brighter than another (e.g., +4 versus +5) is 2.5 times intrinsically brighter; a star that is 5 absolute magnitudes brighter is 100 times intrinsically brighter; and a star that is 10 absolute magnitudes brighter is 10,000 times intrinsically brighter.

^ Bright binary star in which both components contribute to a magnitude of -0.27: it is also the nearest of the bright stars (at a distance of 4.3 light years).

^ Based on its brightness it might well be a perfect size to test one of my new theories about medium-sized Kuiper belt objects.

It is substantially embodied in Ptolemy's Almagest (see Ptolemy).
An interval of 250 years elapsed before the constructive labours of Hipparchus obtained completion at Alexandria. His observations were largely, and somewhat arbi- Ptolemy. trarily, employed by Ptolemy. .Professor Newcomb, who has compiled an instructive table of the equinoxes severally observed by Hipparchus and Ptolemy, with their errors deduced from Leverrier's solar tables, finds palpable evidence that the discrepancies between the two series were artificially reconciled on the basis of a year 6 m too long, adopted by Ptolemy on trust from his predecessor.^ If you look carefully you will find that while the details of the cloud maps are different between the two papers, the overall conclusions are largely the same.

.He nevertheless holds the process to have been one that implied no fraudulent intention.^ What’s more, of the many comments I had gotten from outside the official review process, no one had quite said “incomprehensible.” So what was going on here?

The Ptolemaic system was, in a geometrical sense, defensible; it harmonized fairly well with appearances, and physical reasonings had not then been extended to the heavens. To the ignorant it was recommended by its conformity to crude common sense; to the learned, by the wealth of ingenuity expended in bringing it to perfection. The Almagest was the consummation of Greek astronomy. .Ptolemy had no successor; he found only commentators, among the more noteworthy of whom were Theon of Alexandria (fl.^ What’s more, of the many comments I had gotten from outside the official review process, no one had quite said “incomprehensible.” So what was going on here?

A.D. 400) and his daughter Hypatia (370-415). With the capture of Alexandria by Omar in 641, the last glimmer of its scientific light became extinct, to be rekindled, a century and a half later, on the banks of the Tigris. .The first Arabic translation of the Almagest was made by order of Harun al-Rashid about the year Boo; others followed, and the Caliph Arah al-Mamun built in 829 a grand observatory at astro- Bagdad.^ In a leap year , there is extra delay of about 9 hours, and in a year directly following a leap year the perihelion is 9 hours earlier (and in the following years it is about 3 hours early and 3 hours late, respectively).
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.Here Albumazar (805-885) watched the skies Homers. and cast horoscopes; here Tobit ben Korra (836 901) developed his long unquestioned, yet misleading theory of the " trepidation " of the equinoxes; Abd-ar-rahman al-Safi 9 0 3-9 86) revised at first hand the catalogue of Ptolemy; 3 and Abulwefa (939-998), like al-Safi, a native of Persia, made continuous planetary observations, but did not (as alleged by L. Sedillot) anticipate Tycho Brahe's discovery of the moon's variation.^ We were waiting for Titan (the moon of Saturn) to rise high enough in the sky that night and watching over the operators’ shoulders as they spied on satellites.

Ibn Junis (c. 950-1008), although the scene of his activity was in Egypt, falls into line with the astronomers of Bagdad. He compiled the Hakimite Tables of the planets, and observed at Cairo, in 977 and 978, two solar eclipses which, as being the first recorded with scientific accuracy, 4 were made available in fixing the amount of lunar acceleration. Nasir ud-din (1201-1274) drew up the Ilkhanic Tables, and determined the constant of precession at 51". He directed an observatory established by Hulagu Khan (d. 1265) at Maraga in Persia, and equipped with a mural quadrant of 12 ft. radius, besides altitude and azimuth instruments. Ulugh Beg (1394-1449), a grandson of Tamerlane, was the illustrious personification of Tatar Published by H. C. Schjellerup in a French translation (St Petersburg, 1874).
Newcomb, Researches on the Motion of the Moon, Washington Observations for 1875, Appendix ii. p. 20.
Eudoxus.
Aristarchus. Eratosthenes. astronomy. He founded about 1420 a splendid observatory at Samarkand, in which he re-determined nearly all Ptolemy's stars, while the Tables published by him held the primacy for two centuries.' Arab astronomy, transported by the Moors to Spain, flourished temporarily at Cordova and Toledo. From the latter city the Toletan Tables, drawn up by Arzachel in 1080, took their name; and there also the Alfonsine Tables, > published in 1252, were prepared under the authority of Alphonso X. of Castile. Their appearance signalized the dawn of European science, and was nearly coincident with that of the Sphaera Mundi, a text-book of spherical astronomy, written by a Yorkshireman, John Holywood, known as Sacro Bosco (d. 1256). It had an immense vogue, perpetuated by the printing-press in fifty-nine editions. In Germany, during the 15th century, a brilliant attempt was made to patch up the flaws in Ptolemaic doctrine. George Purbach (1423-1461) introduced into Europe the method of determining time by altitudes employed by Ibn Junis. He lectured with applause at Vienna from 1450; was joined there in 1452 by Regiomontanus; and was on the point of starting for Rome to inspect a manuscript of the Almagest when he died suddenly at the age of thirty-eight. His teachings bore fruit in the work of Regiomontanus, and of. .Bernhard Walther of Nuremberg (1430-1504), who fitted up an observatory with clocks driven by weights, and developed many improvements in practical astronomy.^ Many people who claim to know no constellations in the sky can look up and identify Orion in the winter sky.

Meantime, a radical reform was being prepared in Italy. .Under the searchlights of the new learning, the dictatorship of Ptolemy appeared no more inevitable than that of Aristotle; advanced thinkers like Domenico Maria Novara (1454-1504) promulgated sub rosa what were called Pythagorean opinions; and they were eagerly and fully appropriated by Nicolaus Copernicus during his student-years (1496-1505) at Bologna and Padua.^ The Earth does not have a lot more mass than Venus and is only about 40 % further from the Sun than Venus is, so perhaps it is stranger that the Earth has a large moon than that Venus has no moons.
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^ In April 2005 I still believed it possible that they were all 3 larger than Pluto and that they would eventually be called the 10 th , 11 th , and 12 th planets.

^ Cooler stars emit more light at longer wavelengths (and so appear redder than hot stars).

He laid the groundwork of his heliocentric theory between 1506 and 1512, and brought it to completion in De Revolutionibus Orbium Coelestium (1543) The colossal task of remaking astronomy on an inverted design was, in this treatise, virtually accomplished. Its reasonings were solidly founded on the principle of the relativity of motion.
A continuous shifting of the standpoint was in large measure substituted for the displacements of the objects viewed, which thus acquired a regularity and consistency heretofore lacking to them. .In the new system, the sphere of the fixed stars no longer revolved diurnally, the earth rotating instead on an axis directed towards the celestial pole.^ In 1543, Nicolaus Copernicus (1473 - 1543) published a book in which he swept away Ptolemy's ideas and said that the Earth rotates around its axis, the stars are fixed, and the Earth orbits around the Sun as well.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Moons whose orbit are not in the equatorial plane of their planet or whose direction of orbiting around their planet is opposite to the direction in which the planet rotates around its axis are probably examples of type 2.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ I can tell you who the first people were of whom we know that they wrote that the Earth rotates around its axis.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.The sun too remained stationary, while the planets, including our own globe, circulated round him.^ The planets and the Earth all orbit around the Sun , each at its own speed, so the distance of a planet from the Earth is not always the same.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ All planets orbit around the Sun and turn around their own axis.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ Rule 3 means that some round celestial bodies that orbit directly around the Sun are yet not planets.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.By this means, the planetary " retrogradations " were explained as simple perspective effects due to the combination of the earth's revolutions with those of her sister orbs.^ The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due to gravitational interactions with other planets and/or other effects (such as those due to general relativity).

^ Now the effect is understood to be due to dark clouds of dust and gas in our galaxy, which obscure our view of the Universe beyond in those quarters of the sky.

The retention, however, by Copernicus of the antique postulate of uniform circular motion impaired the perfection of his plan, since it involved a partial survival of the epicyclical machinery. .Nor was it feasible, on this showing, to place the sun at the true centre of any of the planetary orbits; so that his ruling position in the midst of them was illusory.^ The point in a planetary orbit that is at the greatest distance from the Sun.

The reformed scheme was then by no means perfect. Its simplicity was only comparative; many outstanding anomalies compromised its harmonious working. Moreover, the absence of sensible parallaxes in the stellar heavens seemed inconsistent with its validity; and a mobile earth outraged deep-rooted prepossessions. Under these disadvantageous circumstances, it is scarcely surprising that the heliocentric theory, while admired as a daring speculation, won its way slowly to acceptance as a truth.
.The Tabulae Prutenicae, calculated on Copernican principles by Erasmus Reinhold (1511-1553), appeared in 1551. Although they represented celestial movements far better than the Alfonsine Tables, large discrepancies were still apparent, and the desirability of testing the novel hypothesis upon which they were based by more refined observations prompted a reform of 1 F. Bally, Memoirs Roy.^ I do have something I want to tell you, which is maybe better – or at least more rare – than advice.

^ Titan is the only place in the solar system other than the earth that appears to have large quantities of liquid sitting on the surface.

^ I suspect they let their egos and emotions allow them to care more about publishing a paper in Nature than whether or not that paper was correct.

Astr. Society,
vol. xiii. p. 19.
methods, undertaken almost simultaneously by the landgrave William IV. of Hesse-Cassel (1532-1592), and by Tycho Brahe. The landgrave built at Cassel in 1561 the first observatory with a revolving dome, and worked for some years at a star-catalogue finally left incomplete. Christoph Rothmann and Joost Biirgi (1552-1632) became his assistants in 1577 and 1579 respectively; and through the skill of Biirgi, time-determinations were made available for measuring right ascensions. At Cassel, too, the altitude and azimuth instrument is believed to have made its first appearance in Europe.' Tycho's labours were both more strenuous and more effective. He perfected the art of pre-telescopic observation. His instru ments were on a scale and of a type unknown since the days of Nasir ud-din. At Augsburg, in 1569, he ordered the construction of a 19-ft. quadrant, and of a celestial globe 5 ft. in diameter; he substituted equatorial for zodiacal armillae, thus definitively establishing the system of measurements in right ascension and declination; and improved the graduation of circular arcs by adopting the method of " transversals." By these means, employed with consummate skill, he attained an unprecedented degree of accuracy, and as an incidental though valuable result, demonstrated the unreality of the supposed trepidation of the equinoxes.
.No more congruous arrangement could have been devised than the inheritance by Johann Kepler of the wealth of materials amassed by Tycho Brahe.^ The Earth does not have a lot more mass than Venus and is only about 40 % further from the Sun than Venus is, so perhaps it is stranger that the Earth has a large moon than that Venus has no moons.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

The younger man's genius supplied what was wanting to his predecessor. Tycho's endowments were of the practical order; yet he had never designed his observations to be an end in themselves. .He thought of them as means towards the end of ascertaining the true form of the universe.^ Art of bringing parts of the Universe to the perfect state toward which they were thought to aspire - e.g., gold for metals, immortality for human beings.

.His range of ideas was, however, restricted; and the attempt embodied in his ground-plan of the solar system to revive the ephemeral theory of Heraclides failed to influence the development of thought.^ Most people have absolutely no idea what the solar system actually looks like.

^ However, planets are (it is thought) formed by the collision and sticking together of every larger proto-planets, so the manner in which the last few collisions happened to happen has great influence on the rotation of the newly formed planet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The idea of an attractor for a system comes from chaos theory.

Kepler, on the contrary, was endowed with unlimited powers of speculation, but had no mechanical faculty. He found in Tycho's ample legacy of first-class data precisely what enabled him to try, by the touchstone of fact, the successive hypotheses that he imagined; and his untiring patience in comparing and calculating the observations at his disposal was rewarded by a series of unique discoveries. .He long adhered to the traditional belief that all celestial revolutions must be performed equably in circles; but a laborious computation of seven recorded oppositions of Mars at last persuaded him that the planet travelled in an ellipse, one focus of which was occupied by the sun.^ Do all planets travel around the Sun in the same direction?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ The orbits of all planets are ellipses.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ With that definition, all planets except Mercury and (probably) Pluto have an atmosphere, and the moon Titan of Saturn has one, too.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.Pursuing the inquiry, he found that its velocity was uniform with respect to no single point within the orbit, but that the areas described, in equal times, by a line drawn from the sun to the planet were strictly equal.^ Planets orbit around the Sun .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ If P moon is the sidereal period of such a moon (in its orbit around Jupiter), and P J is the sidereal period of Jupiter (in its orbit around the Sun), then the synodical period P syn of the moon is equal to .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ It says that the distance a n of each planet from the Sun is equal to about .
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.These two principles he extended, by direct proof, to the motion of the earth; and, by analogy, to that of the other planets.^ In other words, if there is some guiding principle that makes most planets follow a Titius-Bode-like law, then astronomers haven't found it yet.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What are the results of the impact of a meteorite on Earth and the other planets?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ When these stars explode, they spread the newly made elements through the Universe , and only after this could earth-like planets be made.
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

.They were published in 1609 in De Motibus Stellae Martis. The announcement of the third of " Kepler's Laws " was made ten years later, in De Harmonice Mundi. It states that the squares of the periods of circulation round the sun of the several planets are in the same ratio as the cubes of their mean distances.^ How can you use Kepler's Law to find the distance to a planet?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]

^ What are the least and greatest distances of the planets from the Sun?
  • Astronomy Answers: AstronomyAnswerBook: Planets 18 January 2010 2:02 UTC www.astro.uu.nl [Source type: Original source]