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The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres (238,857 mi), about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located at about 1,700 kilometres (1,100 mi)—a quarter the Earth's radius—beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days[nb 4] (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the phases of the Moon, which repeat every 29.5 days[nb 5] (the synodic period).
The Moon's diameter is 3,474 kilometres (2,159 mi),[6] a little more than a quarter of Earth's. Thus, the Moon's surface area is less than a tenth of the Earth (about a quarter of Earth's land area), and its volume is about 2 percent that of Earth. The pull of gravity at its surface is about 17 percent of that at the Earth's surface.
The Moon is the only celestial body on which human beings have made a manned landing. While the Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft, the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972–the first being Apollo 11 in 1969. Human exploration of the Moon temporarily ceased with the conclusion of the Apollo program, although a few robotic landers and orbiters have been sent to the Moon since that time. The U.S. had committed to return to the Moon by 2018,[7][8][9] however that commitment has been put into jeopardy by the proposed 2011 budget, which will cancel Constellation, NASA's project to send humans back to the moon by 2020. On November 13, 2009, NASA announced the discovery of proof that water exists on the Moon, based on data obtained from its LCROSS lunar impact mission.[10]
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The proper English name for Earth's natural satellite is, simply, the Moon (capitalized).[11][12] Moon is a Germanic word, related to the Latin mensis and Ancient Greek μήνας (mēnas) both meaning month, and Μήνη (Mēnē) (alternate name for Selēnē in Ancient Greek).[13][14] It is ultimately a derivative of the Proto-Indo-European root me-, also represented in measure[15] (time), with reminders of its importance in measuring time in words derived from it like Monday, month and menstrual. The related adjective is lunar, as well as an adjectival prefix seleno- and suffix -selene (from selēnē, σελήνη, the Ancient Greek word for the Moon). In English, the word moon exclusively meant "the Moon" until 1665, when it was extended to refer to the recently discovered natural satellites of other planets.[15] Subsequently, these objects were given distinct names to avoid confusion.[12] The Moon is occasionally referred to by its Latin name Luna, primarily in science fiction.
The Moon is in synchronous rotation, which means it rotates about its axis in about the same time it takes to orbit the Earth. This results in it nearly always keeping the same face turned towards the Earth. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.[16] The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often inaccurately called the "dark side," but in fact, it is illuminated exactly as often as the near side: once per lunar day, during the new Moon phase we observe on Earth when the near side is dark.[17] The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959.[18] One distinguishing feature of the far side is its almost complete lack of maria.[19]
The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims. Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side.[1]
The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (singular mare), Latin for seas, since they were believed by ancient astronomers to be filled with water. These are now known to be vast solidified pools of ancient basaltic lava. The majority of these lavas erupted or flowed into the depressions associated with impact basins that formed by the collisions of meteors and comets with the lunar surface. (Oceanus Procellarum is a major exception in that it does not correspond to a known impact basin.) Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered patches covering about 2% of its surface,[20] compared with about 31% on the near side.[6] The most likely explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer.[21][22] Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.[23]
The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are hypothesized to be the surviving remnants of the impact basin's outer rims.[24] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[25]
From images taken by the Clementine mission in 1994, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the entire lunar day. These peaks of eternal light are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for about 80% of the lunar day. Other consequences of the Moon's small axial tilt are regions that remain in permanent shadow at the bottoms of many polar craters.[26]
The surface of Earth's Moon is marked by impact craters[27] which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the Moon.[citation needed] Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.
The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System,[28] is the South Pole-Aitken basin. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth.[29] Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.
Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and "impact gardened" surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands.[30] Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is many kilometres thicker and comprises highly fractured bedrock.[31]
Astronauts have reported that the dust from the surface felt like snow and smelled like spent gunpowder.[32] The dust is mostly made of silicon dioxide glass (SiO2), most likely created from the meteors that have crashed into the Moon's surface. It also contains calcium and magnesium.
The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time because of the Moon's weak gravity. However, the current obliquity of the Moon means that the Sun never rises above 1.85° at the poles, and the axial tilt of the Moon has remained at its present orientation for the past two billion years, allowing the craters at the poles to remain in permanent shadow for that length of time.[33] Water molecules that ended up in these craters could be stable for long periods of time. Before that point, the Moon had much larger values for its obliquity, possibly reaching angles as high as 77° for periods of several hundred thousand years.[34]
Clementine has mapped craters at the lunar south pole[35] that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow.[26] Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions.[36] Estimate for the quantity of water on the Moon is 32 ounces per short ton (1 kg/Mg) of the top layer of Moon's surface.
Water ice could be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as indicating water ice might instead be a result of rocks ejected from young impact craters.[37] In July 2008, small amounts of water were found in the interior of volcanic pearls from the Moon (brought to Earth in 1971 by the Apollo 15 astronauts).[38][39]
On September 24, 2009, the Indian Space Research Organisation (ISRO) reported that their first lunar mission, Chandrayaan-1 using NASA's Moon Mineralogy Mapper, found evidence of large quantities of water on the Moon's surface, and that water is still presently being formed.[40][41] The instrument observed an absorption line in the spectrum of sunlight reflected from the Moon, indicating that light of a particular wavelength (around 2.8 micrometres) is being absorbed more readily than other nearby wavelengths. The position and shape of the line indicate the absorption is due to water. A nearby line also revealed the presence of the closely-related molecule hydroxyl. The exact abundance of water was not determined, but the team believed it could be as high as 1,000 parts per million in the top layer of Lunar soil. On November 13, 2009, NASA announced the results of the Lunar Crater Observation and Sensing Satellite, saying that "not just water, but lots of water" had been found by the mission near the southern pole.[42]
The Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and core. This structure is hypothesized to have resulted from the fractional crystallization of a magma ocean shortly after its formation, at about 4.4 billion years ago[43]. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have created a mafic mantle and a plagioclase-rich crust (see Origin and geologic evolution below).
Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition,[5] consistent with the magma ocean hypothesis. In terms of elements, the crust is composed primarily of oxygen (41% to 46% by mass), silicon (21%), magnesium (6%), iron (13%), calcium (8%), and aluminium (7%).[44][45] Based on geophysical techniques, its thickness is estimated to be on average about 50 km.[1]
Partial melting within the mantle of the Moon caused the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals olivine, orthopyroxene and clinopyroxene, and that the lunar mantle is more iron rich than that of the Earth. Some lunar basalts contain high abundances of titanium (present in the mineral ilmenite), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about a thousand kilometres below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth.[1]
The Moon has a mean density of 3 346.4 kg/m³, making it the second densest moon in the Solar System after Io. Nevertheless, several lines of evidence imply that the core of the Moon is small, with a radius of about 350 km or less.[1] This corresponds to only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotation indicate that the core is at least partly molten.[46]
The gravitational field of the Moon has been determined through tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the Doppler effect, whereby the spacecraft acceleration in the line-of-sight direction can be determined by means of small shifts in frequency of the radio signal, and the distance from the spacecraft to a station on Earth. However, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon, and the farside gravity field is thus only poorly characterised.[47]
The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins.[48] These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. The mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves cannot explain the entirety of the gravitational signature, and uplift of the crust-mantle interface is required as well. Based on Lunar Prospector gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism.[49] The huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.
The Moon has an external magnetic field of the order of one to a hundred nanoteslas—less than one-hundredth that of the Earth, which is 30–60 microteslas. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin.[50] One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.[51]
The Moon has an atmosphere so thin as to be almost negligible, with a total atmospheric mass of less than 104 kg.[52] The effective surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa).[53] This pressure varies with the diurnal moon cycle. One source of its atmosphere is outgassing—the release of gases such as radon that originate by radioactive decay processes within the crust and mantle.[54] Another important source is generated through the process of sputtering, which involves the bombardment of micrometeorites, solar wind ions, electrons, and sunlight.[5] Gases that are released by sputtering can either reimplant into the regolith as a result of the Moon's gravity, or can be lost to space either by solar radiation pressure or by being swept away by the solar wind magnetic field if they are ionised. The elements sodium (Na) and potassium (K) have been detected using earth-based spectroscopic methods, whereas the element radon-222 (222Rn) and polonium-210 (210Po) have been inferred from data obtained from the Lunar Prospector alpha particle spectrometer.[55] Argon-40 (40Ar), helium-4 (4He), oxygen (O2) and/or methane (CH4), nitrogen (N2) and/or carbon monoxide (CO), and carbon dioxide (CO2) were detected by in-situ detectors placed by the Apollo astronauts.[56]
During the lunar day, the surface temperature averages 107 °C, and during the lunar night, it averages −153 °C.[57]
The Moon has the coldest place in the Solar System measured by a spacecraft.
NASA's Lunar Reconnaissance Orbiter has used its Diviner instrument to probe the insides of permanently shadowed craters on Earth's satellite.[58] It found mid-winter, night-time surface temperatures inside the coldest craters in the northern polar region can dip as low as minus 249 °C (26 kelvins). The Diviner instrument observed the lowest summer temperatures in the darkest craters at the southern pole to be about 35 K (−238 °C); but in the north, close to the winter solstice the instrument recorded a temperature of just 26 K on the south-western edge of the floor of Hermite Crater.
Calculations suggest one would have to travel to a distance beyond the Kuiper Belt—well beyond the orbit of Neptune—to find objects with surfaces this cold. The discovery adds further weight to the idea that some craters on the Moon could harbour water-ices for extended periods, and more volatile substances that require even colder storage temperatures.
Several mechanisms have been suggested for the Moon's formation. The formation of the Moon is hypothesized to have occurred 4.527 ± 0.010 billion years ago, about 30–50 million years after the origin of the Solar System.[59]
As a result of the large amount of energy converted during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly hypothesized that a large portion of the Moon was once initially molten. The molten outer portion of the Moon at this time is referred to as a magma ocean, and estimates for its depth range from about 500 km to the entire radius of the Moon.[21]
As the magma ocean cooled, it fractionally crystallised and differentiated, creating a geochemically distinct crust and mantle. The mantle is inferred to have formed largely by the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene. After about three-quarters of magma ocean crystallisation was complete, the mineral anorthite is inferred to have precipitated and floated to the surface because of its low density, forming the crust.[21]
The final liquids to crystallise from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym KREEP, for potassium (K), rare earth elements (REE), and phosphorus (P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small geologic province that encompasses most of Oceanus Procellarum and Mare Imbrium on the near side of the Moon.[1]
A large portion of the Moon's post–magma-ocean geologic (or, more properly, "selenologic") evolution was dominated by impact cratering. The lunar geologic timescale is largely divided in time based on prominent basin-forming impact events, such as Nectaris, Imbrium, and Orientale. These impact structures are characterised by multiple rings of uplifted material, and are typically hundreds to thousands of kilometres in diameter. Each multi-ring basin is associated with a broad apron of ejecta deposits that forms a regional stratigraphic horizon. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages on stratigraphic grounds. The continuous effects of impact cratering are responsible for forming the regolith.
The other major geologic process that affected the Moon's surface was mare volcanism. The enhancement of heat-producing elements within the Procellarum KREEP Terrane is thought to have caused the underlying mantle to heat up, and eventually, to partially melt. A portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon.[21] Most of the Moon's mare basalts erupted during the Imbrian period in this geologic province 3.0–3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years,[66] and the youngest eruptions, based on the method of crater counting, are hypothesized to have occurred only 1.2 billion years ago.[67]
There has been controversy over whether features on the Moon's surface undergo changes over time. Some observers have claimed that craters either appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or the inadequacy of earlier drawings. Nevertheless, it is known that the phenomenon of outgassing does occasionally occur, and these events could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.[68][69]
Moon rocks fall into two main categories, based on whether they underlie the lunar highlands (terrae) or the maria. The lunar highlands rocks are composed of three suites: the ferroan anorthosite suite, the magnesian suite, and the alkali suite (some consider the alkali suite to be a subset of the mg-suite). The ferroan anorthosite suite rocks are composed almost exclusively of the mineral anorthite (a calic plagioclase feldspar), and are hypothesized to represent plagioclase flotation cumulates of the lunar magma ocean. The ferroan anorthosites have been dated using radiometric methods to have formed about 4.4 billion years ago.[66][67]
The mg- and alkali-suite rocks are predominantly mafic plutonic rocks. Typical rocks are dunites, troctolites, gabbros, alkali anorthosites, and more rarely, granite. In contrast to the ferroan anorthosite suite, these rocks all have relatively high Mg/Fe ratios in their mafic minerals. These rocks generally represent intrusions into the already-formed highlands crust (though a few rare samples appear to represent extrusive lavas), and they have been dated to have formed about 4.4–3.9 billion years ago. Many of these rocks have high abundances of, or are genetically related to, the geochemical component KREEP.
The lunar maria consist entirely of mare basalts. While similar to terrestrial basalts, they have much higher abundances of iron, are completely lacking in hydrous alteration products, and have a large range of titanium abundances.[70][71]
The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days[nb 4](its sidereal period). However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days[nb 5] (its synodic period).[6] Unlike most satellites of other planets, the Moon orbits near the ecliptic and not the Earth's equatorial plane.
The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. The Moon's surface area is less than one-tenth that of the Earth; about a quarter of the Earth's land area. It is the largest moon in the solar system relative to the size of its planet (although Charon is larger relative to the dwarf planet Pluto.) However, the Earth and Moon are still considered a planet-satellite system, rather than a double-planet system, as their barycentre, the common centre of mass, is located about 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.
In 1997, the asteroid 3753 Cruithne was found to have an unusual Earth-associated horseshoe orbit. However, astronomers do not consider it to be a second moon of Earth, and its orbit is not stable in the long term.[72] Three other near-Earth asteroids with orbits similar to Cruithne's have since been discovered: 54509 YORP, (85770) 1998 UP1 and 2002 AA29.[73]
The tides on the Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of the Earth to the other, the tidal forces, with a smaller contribution by the Sun's gravity. These tidal forces form two tidal bulges on the Earth, most clearly seen in elevated sea level, one in the direction of the Moon and one in the opposite direction. Since the Earth spins about 27 times faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, rotating around the Earth once a day as it spins on its axis. The ocean tides, effects of the two ever-moving bulges and the massive ocean currents chasing them, are magnified by an interplay of other effects: frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on the Earth's oceans is almost half that of the Moon, and this interplay is responsible for spring and neap tides.
Gravitational coupling between the Moon and the bulge nearest the Moon affects its orbit. From the Moon's perspective, the center of mass of the near-Moon tidal bulge is perpetually slightly ahead of the point about which it is orbiting. In the same way, the bulge farthest from the Moon lags behind, but as it is 12,756 km farther away, it has slightly less gravitational coupling to the Moon. This gravitational asymmetry of forces attracts the Moon forward in its orbit. This gravitational coupling acts like a torque on the Earth's rotation, and drains angular momentum and rotational kinetic energy from the Earth's spin. In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period. As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down. Measurements from lunar ranging experiments with laser reflectors left during the Apollo missions have showed that, at present, the Moon's distance to the Earth increases by 38 mm per year[74] (though this is only 0.10 ppb/year of the radius of the Moon's orbit). Cumulatively, this effect becomes ever more significant over time. Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year,[75] requiring the occasional addition of a leap second to the calendar. This tidal drag will continue until the spin of the Earth has slowed to match the orbital period of the Moon, at which time the tidal effect of the Sun will dominate, further slowing the Earth and thereafter causing the orbit of the Moon to steadily shrink. However, long before this could happen, the sun will have become a red giant.
The lunar surface also experiences tides, of amplitude ~10 cm over 27 days, with two components: a fixed one due to the Earth, as they are in synchronous rotation, and a varying component from the Sun. The Earth-induced component arises from libration, the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would only be solar tides. Libration also changes the angle from which the Moon is seen, allowing about 59% of its surface to be seen from the Earth (but only half at any instant).[6] The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1,000 kilometres in depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day.[76]
Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line. Solar eclipses occur near a new Moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur near a full Moon, when the Earth is between the Sun and Moon.
The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible.[78] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that hundreds of millions of years ago the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, about 600 million years from now (if the angular diameter of the Sun does not change), the Moon will no longer cover the Sun completely, and only annular eclipses will occur.[79]
Because the Moon's orbit around the Earth is inclined by about 5° to the orbit of the Earth around the Sun, eclipses do not occur at every full and new Moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.[79] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by the Earth, is described by the saros cycle, which has a period of approximately 6 585.3 days (18 years 11 days 8 hours).[80]
As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to the Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Due to the precession of the lunar orbit, each year different stars are occulted.[81]
During its brightest phase, at "full Moon", the Moon has an apparent magnitude of about −12.7. By comparison, the Sun has an apparent magnitude of −26.7. When the Moon is in a quarter phase, its brightness is not half of a full Moon, but only about a tenth. This is because the lunar surface is not a perfect Lambertian reflector. When the Moon is full the opposition effect makes it appear brighter, but away from full there are shadows projected onto the surface which diminish the amount of reflected light.
On average, the Moon covers an area of 0.21078 square degrees on the night sky.[82] The Moon does appear larger when close to the horizon, but this is a purely psychological effect: the Moon illusion.
The Moon appears as a relatively bright object in the sky, in spite of its low albedo. The Moon is about the poorest reflector in the solar system and reflects only about 7% of the light incident upon it (about the same proportion as is reflected by a lump of coal). However, the Moon is not a Lambertian scatterer and reflects more light back towards the Sun (albedo of 13.6%[3]) than in other directions because of the spherical glass beads in the moondust. This increases the brightness of a full Moon.[83] It also has the effect of making the edges of a full Moon seem about as bright as the centre. Besides this, color constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, and since the surrounding sky is comparatively dark the sunlit Moon is perceived as a bright object.
The highest altitude of the Moon on a day varies and has nearly the same limits as the Sun. It also depends on the Earth season and lunar phase, with the full Moon being highest in winter. Moreover, the 18.6 year nodes cycle also has an influence, as when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28° each month (which happened most recently in 2006). This results that the Moon can go overhead on latitudes up to 28 degrees from the equator (e.g. Florida, Canary Islands or in the southern hemisphere Brisbane). Slightly more than 9 years later (next time in 2015) the declination reaches only 18° N or S each month. The orientation of the Moon's crescent also depends on the latitude of the observation site. Close to the equator, an observer can see a boat Moon.[84]
Like the Sun, the Moon can cause atmospheric effects, including a 22° halo ring, and the smaller coronal rings seen more often through thin clouds.
The first leap in lunar observation was prompted by the invention of the telescope. Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon's surface.
The Cold War-inspired space race between the Soviet Union and the U.S. led to an acceleration of interest in the Moon. Unmanned probes, both flyby and impact/lander missions, were sent once allowed by launcher capabilities. The Soviet Union's Luna program was the first to reach the Moon with unmanned spacecraft. The first man-made object to escape Earth's gravity and pass near the Moon was Luna 1, the first man-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966.[6] Rock samples were brought back to Earth by three Luna missions (Luna 16, 20, and 24) and the Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing).
The landing of the first humans on the Moon in 1969 is seen by many as the culmination of the space race.[85] Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on July 21, 1969. The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry and atmospheric re-entry technology.
Today, 37 years, 95 days have passed since Eugene Cernan and Harrison Schmitt, as part of the mission Apollo 17, left the surface of the Moon on December 14, 1972 (Cernan being the last to enter the LM): no one has set foot on it since.
Scientific instrument packages were installed on the lunar surface during all the Apollo missions. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 due to budgetary considerations,[86][87] but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.[88]
From the mid-1960s to the mid-1970s, there were 65 instances of artificial objects reaching the Moon (both manned and robotic, with ten in 1971 alone); the last was Luna 24, in 1976. Only 18 of these were controlled Moon landings, with nine completing a round trip from Earth and returning rock samples. The Soviet Union then turned its primary attention to Venus and space stations, and the U.S. to Mars and beyond.[citation needed]
Post-Apollo and Luna, many more countries have become involved in direct exploration of the Moon.
In 1990, Japan orbited the Moon with the Hiten spacecraft, becoming the third country to place a spacecraft into lunar orbit. The spacecraft released a smaller probe, Hagormo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.
In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. This was followed by the Lunar Prospector mission in 1998. The neutron spectrometer on Lunar Prospector indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. On January 14, 2004, U.S. President George W. Bush called for a plan to resume manned missions to the Moon by 2020 (see Vision for Space Exploration).[89] NASA is now planning for the construction of a permanent outpost at one of the lunar poles.[90]
The European spacecraft Smart 1 was launched September 27, 2003 and was in lunar orbit from November 15, 2004 to September 3, 2006. China has expressed ambitious plans for exploring the Moon and has started the Chang'e program for lunar exploration, successfully launching its first spacecraft, Chang'e-1, on October 24, 2007. On September 14, 2007 the Japan Aerospace Exploration Agency launched SELENE, also known as Kaguya, a lunar orbiter fitted with a high-definition camera and two small satellites.[91] On October 22, 2008 India launched the Chandrayaan I (a Sanskrit word meaning 'Moon-craft') lunar orbiter. The country plans to launch Chandrayaan II in 2010 or 2011, which is slated to include a robotic lunar rover, and has also expressed its hope for a manned mission to the Moon by 2020.[92] The U.S. co-launched the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite (LCROSS) on June 18, 2009.
Russia also announced intentions to resume its previously frozen project Luna-Glob, consisting of an unmanned lander and orbiter, which is slated to land in 2012.[93] The Google Lunar X Prize, announced September 13, 2007, hopes to boost and encourage privately funded lunar exploration. The X Prize Foundation is offering anyone US$20 million who can land a robotic rover on the Moon and meet other specified criteria.[citation needed]
The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5000-year-old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered.[94] In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today.
In Mesopotamia, Babylonian astronomers by the early first millenium BC had discovered a repeating 18-year cycle of lunar eclipses. They had also known by this time that 19 solar years is about equal to 235 lunar months. In the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were caused by the Moon, although he believed that the interaction was mediated by the Earth's atmosphere. According to Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.[95]
Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras (d. 428 BC), who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile.[96] In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether. This separation was held to be part of Aristotelian physics for many centuries after.[97] Aristarchus went a step further and computed the distance from Earth, together with its size, obtaining a value of 20 times the Earth radius for the distance (the real value is 60; the Earth radius was roughly known since Eratosthenes).
During the Warring States of China, astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses based on the relative positions of the Moon and Sun.[98] Although the Chinese of the Han Dynasty (202 BC–202 AD) believed the Moon to be energy equated to qi, their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun (mentioned by Anaxagoras above).[99] This was supported by mainstream thinkers such as Jing Fang (78–37 BC) and Zhang Heng (78–139 AD), but it was also opposed by the influential philosopher Wang Chong (27–97 AD).[99] Jing Fang noted the sphericity of the Moon, while Zhang Heng accurately described a lunar eclipse and solar eclipse.[99][100] These assertions were supported by Shen Kuo (1031–1095) of the Song Dynasty (960–1279) who created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.[101] He also noted that the reason for the Sun and Moon not eclipsing every time their paths met was because of a small obliquity in their orbital paths.[101]
Between 825 and 835 AD, the Persian astronomer, Habash al-Hasib al-Marwazi, conducted various observations at the Al-Shammisiyyah observatory in Baghdad.[102] Using these observations, he estimated the Moon's diameter as 3,037 km (equivalent to 1,519 km radius) and its distance from the Earth as 215,209 miles, which come close to the currently accepted values.[103] In 1021, the Islamic physicist, Alhazen, accurately explained the Moon illusion in the Book of Optics, which stated that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer. With the Moon, there are no intervening objects, therefore since the size of an object depends on its observed distance, which is in this case inaccurate, the Moon appears larger on the horizon. Through Alhazen's work, the Moon illusion gradually came to be accepted as a psychological phenomenon.[104] He also investigated moonlight, which he proved through experimentation that it originates from sunlight and correctly concluded that it "emits light from those portions of its surface which the sun's light strikes."[105]
By the Middle Ages, before the invention of the telescope, an increasing number of people began to recognise the Moon as a sphere, though many believed that it was "perfectly smooth".[106] In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today.
On maps, the dark parts of the Moon's surface were called maria (singular mare) or seas, and the light parts were called terrae or continents. The possibility that the Moon contains vegetation and is inhabited by selenites was seriously considered by major astronomers even into the first decades of the 19th century. The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others.
In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon.[107] Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.
Although several pennants of the Soviet Union were scattered by Luna 2 in 1959 and by later landing missions, and U.S. flags have been symbolically planted on the Moon, no nation currently claims ownership of any part of the Moon's surface. Russia and the U.S. are party to the Outer Space Treaty, which places the Moon under the same jurisdiction as international waters (res communis). This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction.[108]
A second treaty, the Moon Treaty, was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations. Several individuals have made claims to the Moon in whole or in part, although none of these are generally considered credible.[109]
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The tides on Earth are mostly generated by the Moon's gravitation, with a less significant contribution by the Sun. These gravitational effects are specifically manifested as tidal forces. The combination of the two is responsible for spring and neap tides. Two tidal bulges, one in the direction of the Moon, and one in the opposite direction (figure 1) form as a result of the tidal forces. The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth.
Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" (figure 2). Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to its orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotational energy from the Earth's spin to the Moon's orbital movement.
Because the Moon is accelerated in the forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down (figure 3). Measurements reveal that, at present, the Moon's distance to the Earth increases by 38 mm per year (as determined by lunar ranging experiments with laser reflectors left during the Apollo missions. Atomic clocks also show that the Earth's day lengthens by about 15 microseconds (µs) every year. The formation of tidal bulges on Earth (and the resulting frictional energy loss) is irregular because continents on Earth hamper the buildup of tidal bulges (figure 4).
This tidal drag will continue until the spin of the Earth has slowed to match the orbital period of the Moon, at which time the tidal effect of the Sun will dominate, further slowing the Earth and thereafter causing the orbit of the Moon to steadily shrink. However, long before this could happen, the sun will have become a red giant and either engulfed the earth and moon or blown them into the outer solar system.
The lunar surface is also subjected to tides from earth, and rises and falls by around 10 cm over 27 days. Because the Moon keeps the same face turned to the Earth but not to the Sun, the lunar tides are comprised of a selenographically changing component (due to the Sun), and a fixed one (due to Earth). The vertical motion of the Earth-induced component arises from the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would be solar tides only.
The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1,000 kilometres in depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day; for Venus the Love number is also 0.3.[1]
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