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Earth  Astronomical symbol of Earth
High-resolution global composites of Moderate Resolution Imaging Spectroradiometer
Pronunciation /ˈɝθ/  ( listen)[1]
Adjective earthly, tellurian, telluric, terran, terrestrial.
Epoch J2000.0[note 1]
Aphelion 152,097,701 km
1.0167103335 AU
Perihelion 147,098,074 km
0.9832898912 AU
Semi-major axis 149,597,887.5 km
1.0000001124 AU
Eccentricity 0.016710219
Orbital period 365.256366 days
1.0000175 yr
Average orbital speed 29.783 km/s
107,218 km/h
Inclination 1.57869°[2]
to Invariable plane
Longitude of ascending node 348.73936°
Argument of perihelion 114.20783°
Satellites 1 (the Moon)
Physical characteristics
Mean radius 6,371.0 km[3]
Equatorial radius 6,378.1 km[4]
Polar radius 6,356.8 km[5]
Flattening 0.0033528[4]
Circumference 40,075.02 km (equatorial)
40,007.86 km (meridional)
40,041.47 km (mean)
Surface area 510,072,000 km²[6][7][note 2]
148,940,000 km² land  (29.2 %)
361,132,000 km² water (70.8 %)
Volume 1.0832073 × 1012 km3
Mass 5.9736 × 1024 kg[8]
Mean density 5.515 g/cm3
Equatorial surface gravity 9.780327 m/s²[9]
0.99732 g
Escape velocity 11.186 km/s
Sidereal rotation
0.99726968 d[10]
23h 56m 4.100s
Equatorial rotation velocity 1,674.4 km/h (465.1 m/s)
Axial tilt 23.439281°
Albedo 0.367[8]
Surface temp.
min mean max
184 K 287 K 331 K
−89 °C 14 °C 57.7 °C
Surface pressure 101.3 kPa (MSL)
Composition 78.08% nitrogen (N2)
20.95% oxygen (O2)
0.93% argon
0.038% carbon dioxide
About 1% water vapor (varies with climate)[8]
Earth (or the Earth) is the third planet from the Sun, and the fifth-largest of the eight planets in the Solar System. It is also the largest, most massive, and densest of the Solar System's four terrestrial planets. It is sometimes referred to as the World, the Blue Planet,[note 3] or by its Latin name, Terra.[note 4]
.Home to millions of species,[11] including humans, Earth is the only place in the Universe where life is known to exist.^ Uganda's "Impenetrable Forest" -- home to the world's largest population of Mountain Gorillas, is also a hotbed for a number of deadly diseases that cross the species barrier from animals to humans.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ I pledge devotion to the earth, our one and only home, and to the life this earth sustains; one nation, one spirit indivisible, with freedom and fulfillment for all.
  • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

The planet formed 4.54 billion years ago,[12] and life appeared on its surface within a billion years. .Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.^ The human family faces imminent and (Copenhagen would suggest) inevitable collapse of the biosphere – the thin layer of life upon an otherwise lifeless planet – that makes Earth habitable.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ Life on soma, together with early behavioural conditioning, leaves them oblivious to the true welfare of others.
  • Aldous Huxley : Brave New World 11 January 2010 15:57 UTC [Source type: Original source]

[13] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. .The world is expected to continue supporting life for another 1.5 billion years, after which the rising luminosity of the Sun will eliminate the biosphere.^ In contrast to Brave New World, however, the death of ageing won't be followed by our swift demise after a sixty-odd year life-span .
  • Aldous Huxley : Brave New World 11 January 2010 15:57 UTC [Source type: Original source]

^ This is not what you'd expect to find in Afghanistan, a country that is still one of the poorest in the world and remains an unsettled and perilous place after 25 years of war.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ Construction of the reactor is expected to cost $5 billion with its operation is expected to cost another $5 billion over twenty years, according to officials of the reactor project.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

.Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years.^ About 1 million curies of tritium was handled safely over a three year period during over 1,000 DT experiments.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

.About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface.^ Over 7,000 gallons of water a year can be harvested from the rooftop and other surfaces around a typical home.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ This late in her long life, Jen Wolling figured she knew just about all of them.
  • David Brin's Official Web Site: Earth (sample chapters) 27 January 2010 23:47 UTC [Source type: Original source]

^ The groundwater level may be too deep, groundwater may be contaminated with minerals and chemicals such as arsenic or salt, surface water may be contaminated with faeces or chemicals.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

[note 5][note 6] Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.
Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This is a sidereal year, which is equal to 365.26 solar days.[note .7] The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,[15] producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).^ One day in the boy’s tenth year, Larch came upon Immiker slicing stripes down the stomach of a rabbit that was colored like the sky.
  • Fire - Kristin Cashore - Penguin Group (USA) 18 January 2010 9:25 UTC [Source type: Original source]

^ One of the things I enjoy about digging (and there are lots of things I enjoy about it) is the smell of the earth that is released by the spade cutting in and lifting clods that have been buried for a year.
  • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

^ The California Solar Initiative aims to install solar panels on the rooftops of one (m) million homes, businesses and public buildings over the next ten years.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

.Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation.^ With the satellite in a sun-synchronous, nearly circular orbit at about 760 km, mission parameters call for a check-out period of six months followed by an operational period of 2.5-4.5 years.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ One of the things I enjoy about digging (and there are lots of things I enjoy about it) is the smell of the earth that is released by the spade cutting in and lifting clods that have been buried for a year.
  • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

.Between approximately 4.1 and 3.8 billion years ago, asteroid impacts during the Late Heavy Bombardment caused significant changes to the surface environment.^ Just a year ago, in November 2004, Ukranians poured into Kyiv's Independence Square, demanding democratic change.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

.Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population.^ URN is for a Universal Resource Number, and is not currently used or supported, and is a Level 1 attribute.

^ Such a global species-project does not have the desperate moral urgency of eliminating the phenomenon of Darwinian pain - both " mental " and " physical ", human and non-human alike.
  • Aldous Huxley : Brave New World 11 January 2010 15:57 UTC [Source type: Original source]

^ It struck Alex that both objects were products of fiery crucibles -- one mineral, the other social.
  • David Brin's Official Web Site: Earth (sample chapters) 27 January 2010 23:47 UTC [Source type: Original source]

The inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade and military action. .Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth or in Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.^ While reuse of human biowaste/ biosolids (along the lines urine feces) for fertilizing is generating great excitement in the developing world, the United States is cautious in its embrace.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ Dispatches From a Small Planet: Election 2004 FRONTLINE/World scours the planet to bring you global views on the U.S. presidential race.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ The National Endowment for Humanities (NEH) has awarded a Challenge Grant to The Center for the History and Ethics of Public Health at Columbia University's Mailman School of Public Health...
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]



Scientists have been able to reconstruct detailed information about the planet's past. .The earliest dated Solar System material is dated to 4.5672 ± 0.0006 billion years ago,[16] and by 4.54 billion years ago (within an uncertainty of 1%)[12] the Earth and the other planets in the Solar System formed out of the solar nebula—a disk-shaped mass of dust and gas left over from the formation of the Sun.^ The volcanic eruptions thought responsible for Earth's largest mass extinction - which killed more than 70 percent of plants and animals 250 million years ago - is still taking lives today...
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ American Water has set a goal to lower its greenhouse gas emissions per volume of water produced by 16 percent by the year 2017.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ These observations give scientists a way to pick out planets beyond our solar system (extrasolar planets) that are likely to have expanses of liquid, and so stand a better chance of having life.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

This assembly of the Earth through accretion was largely completed within 10–20 million years.[17] .Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere.^ Water is the only element that naturally changes before our eyes from liquid, to gaseous, to solid forms.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

The Moon formed shortly thereafter, 4.53 billion years ago.[18]
The current consensus model[19] for the formation of the Moon is the giant impact hypothesis, in which the Moon formed as a result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass[20] impacting the Earth in a glancing blow.[21] .In this model, some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.^ Some forms of extreme pain, for instance, are so terrible to experience that one would sacrifice the whole world to get rid of the agony.
  • Aldous Huxley : Brave New World 11 January 2010 15:57 UTC [Source type: Original source]

Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.[22] .The newly-formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid—a contradiction dubbed the faint young Sun paradox.^ Physical activity also may protect against lower back pain and some forms of cancer (for example, breast cancer), but the evidence is not yet conclusive.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

^ Water is the only element that naturally changes before our eyes from liquid, to gaseous, to solid forms.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

A combination of greenhouse gases and higher levels of solar activity served to raise the Earth's surface temperature, preventing the oceans from freezing over.[23]
Two major models have been proposed for the rate of continental growth:[24] steady growth to the present-day[25] and rapid growth early in Earth history.[26] .Current research shows that the second option is most likely, with rapid initial growth of continental crust[27] followed by a long-term steady continental area.^ This review which is stipulated by the EURATOM FP7 Specific Programme on Fusion Research has the “motivations to support the rapid and efficient development of fusion as an energy source and to maintain in the programme the facilities needed to fulfil its medium and long term objectives”.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

^ Tomorrow's "personality pills" permit us to become the kind of people we'd most like to be - to fulfil our second-order desires.
  • Aldous Huxley : Brave New World 11 January 2010 15:57 UTC [Source type: Original source]

[28][29][30] On time scales lasting hundreds of millions of years, the surface continually reshaped as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. .Roughly 750 million years ago (Ma), one of the earliest known supercontinents, Rodinia, began to break apart.^ Almost exactly three years ago I wrote one of those pieces.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ The California Solar Initiative aims to install solar panels on the rooftops of one (m) million homes, businesses and public buildings over the next ten years.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

The continents later recombined to form Pannotia, 600–540 Ma, then finally Pangaea, which broke apart 180 Ma.[31]

Evolution of life

.At present, Earth provides the only example of an environment that has given rise to the evolution of life.^ In the following example, the page has several places where links are present, but only one of those places is considered a navigation section.
  • HTML 5 20 September 2009 9:28 UTC [Source type: Reference]

[32] .Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.^ However, the removal of all material may not be completed until 2013.Pretty dramatic aerial footage comparing last year to this past December.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ For the last year and a half, reporter Martin Smith has been investigating global climate change for Heat , a two-hour FRONTLINE broadcast to air this fall.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ All it took was a few sentences in a Japanese history textbook last year to spark the biggest protests China had seen since 1989.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

[33] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed in a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[34] .True multicellular organisms formed as cells within colonies became increasingly specialized.^ Units may be specified using the standard defined units suffixes plus the special suffix of the percent sign indicating the percentage of offset within the cell from the beginning of the cell.

^ In addition, the special suffix of the percent sign may be used to indicate the percentage of offset within the cell from the beginning of the cell.

Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[35]
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[36]
Following the Cambrian explosion, about 535 Ma, there have been five mass extinctions.[37] .The last extinction event was 65 Ma, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews.^ The shift to large-cap stocks that began last fall has helped lower the prices of many small-cap names.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

.Over the past 65 million years, mammalian life has diversified, and several million years ago, an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.^ The total cost of the TFTR program (design, construction, operation and decommissioning ) was $1.65 B. The US fusion program expended $2.5 B during the past 10 years.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

^ Photographer Dong Lin has visited North Korea several times in recent years trying to glimpse life in this secretive state.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ The volcanic eruptions thought responsible for Earth's largest mass extinction - which killed more than 70 percent of plants and animals 250 million years ago - is still taking lives today...
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

[38] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. .The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[39] affecting both the nature and quantity of other life forms.^ Marshes and rivers and forests and fish are far more than resources – they and all natural ecosystems are a necessity for humanity’s existence upon Earth.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

The present pattern of ice ages began about 40 Ma and then intensified during the Pleistocene about 3 Ma. .The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years.^ Early this year, amidst military preparations for a war in Iraq, the United States announced it was sending 3,000 soldiers to Mindanao, the southernmost region of the Philippines.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

.The last ice age ended 10,000 years ago.^ Over the last 10 years, actual contract allocations have exceeded the December… We All Live Downstream .
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]



The life cycle of the Sun.
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Gyr (1.1 billion years) and by 40% over the next 3.5 Gyr.[41] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.[42]
.The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to lethal levels for plants (10 ppm for C4 photosynthesis) in approximately 900 million years.^ The volcanic eruptions thought responsible for Earth's largest mass extinction - which killed more than 70 percent of plants and animals 250 million years ago - is still taking lives today...
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ DOE science programs are funded at $3.6 billion, an increase of $168.2 million over the budget request and $168.6 million above fiscal year 2004.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

.The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.^ Effective treatment of many chronic diseases and disorders has resulted in more years of life, but many of these extra years are spent with disabling conditions that prevent independent living and reduce the quality of life.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

^ Photographer Dong Lin has visited North Korea several times in recent years trying to glimpse life in this secretive state.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ The volcanic eruptions thought responsible for Earth's largest mass extinction - which killed more than 70 percent of plants and animals 250 million years ago - is still taking lives today...
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

[43] .After another billion years all surface water will have disappeared[14] and the mean global temperature will reach 70 °C[43](158 °F).^ Over 7,000 gallons of water a year can be harvested from the rooftop and other surfaces around a typical home.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ Construction of the reactor is expected to cost $5 billion with its operation is expected to cost another $5 billion over twenty years, according to officials of the reactor project.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

^ Some say it’s the most magical time of the year, and I’d agree, although I’d also point out that water is pretty magical too, and that’s all year round.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

.The Earth is expected to be effectively habitable for about another 500 million years,[44] although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere.^ Amazingly, an army of 400-500 workers have removed two-thirds of the spill material from the river and are overall halfway through the $1-billion cleanup.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ About 1 million curies of tritium was handled safely over a three year period during over 1,000 DT experiments.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

^ DOE science programs are funded at $3.6 billion, an increase of $168.2 million over the budget request and $168.6 million above fiscal year 2004.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

[45] Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism,[46] and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.[47]
The Sun, as part of its evolution, will become a red giant in about 5 Gyr. .Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).^ With the satellite in a sun-synchronous, nearly circular orbit at about 760 km, mission parameters call for a check-out period of six months followed by an operational period of 2.5-4.5 years.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

[41][48] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. Therefore, the planet is expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life will be destroyed because of the Sun's increased luminosity.[41] However, a more recent simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be destroyed.[48]

Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation.[49] It also is the only terrestrial planet with active plate tectonics.[50]


Size comparison of inner planets (left to right): Mercury, Venus, Earth and Mars
The shape of the Earth is very close to that of an oblate spheroid, a sphere squished along the orientation from pole to pole such that there is a bulge around the equator.[51] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the pole to pole diameter.[52] .The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.^ The budget request proposed $12,000,000 for the International Thermonuclear Experimental Reactor (ITER), but did so by displacing $10,800,000 of ongoing domestic fusion research.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

.Local topography deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.^ During the isolation process, about 5 nearby homes will be without water service for less than an hour.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

^ Even though the importance of water pricing has been previously discussed by these super bloggers, I still thought it deserved more than a retweet.
  • Alltop - Top Water News 28 January 2010 1:01 UTC [Source type: General]

[54] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the feature farthest from the center of the Earth is Mount Chimborazo in Ecuador.[55][56]
Chemical Composition of the Crust[57]
Compound Formula Composition
Continental Oceanic
silica SiO2 60.2% 48.6%
alumina Al2O3 15.2% 16.5%
lime CaO 5.5% 12.3%
magnesia MgO 3.1% 6.8%
iron(II) oxide FeO 3.8% 6.2%
sodium oxide Na2O 3.0% 2.6%
potassium oxide K2O 2.8% 0.4%
iron(III) oxide Fe2O3 2.5% 2.3%
water H2O 1.4% 1.1%
carbon dioxide CO2 1.2% 1.4%
titanium dioxide TiO2 0.7% 1.4%
phosphorus pentoxide P2O5 0.2% 0.3%
Total 99.6% 99.9%
Size comparison of Earth and Uranus

Chemical composition

The mass of the Earth is approximately 5.98 × 1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. .Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.^ Description
The H6 element identifies text to be separated and displayed less prominently than H5 but more prominently than default text.

^ Description
The SMALL element changes the physical rendering of the contents of the element to a smaller font than normal text, if practical.

^ Description
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.The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen.^ Throughout the developing world, more than 3 million women suffer from a little known yet devastating medical condition called obstetric fistula.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

^ RFC 1866 describes it as a preferred, more persistent identifier than the value of HREF. TITLE is little used or supported, and is a Level 1 attribute, but is expected to be the title of the HREF document.

.The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%.^ In this order: optionally a caption element, followed by either zero or more colgroup elements, followed optionally by a thead element, followed optionally by a tfoot element, followed by either zero or more tbody elements or one or more tr elements, followed optionally by a tfoot element (but there can only be one tfoot element child in total).
  • HTML 5 Reference 11 September 2009 3:51 UTC [Source type: Reference]

^ Description
The H6 element identifies text to be separated and displayed less prominently than H5 but more prominently than default text.

^ Since 1997, more than 25,000 farmers have committed suicide, many drinking the chemical that was supposed to make their crops more, not less, productive.
  • FRONTLINE/WORLD . Stories by Date | PBS 11 January 2010 15:57 UTC [Source type: News]

The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right). All the other constituents occur only in very small quantities.[note 8]

Internal structure

.The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties.^ Since most chemical properties result from outer electron interactions, this tends to explain why elements in the same group exhibit similar physical and chemical properties.
  • Periodic table encyclopedia topics | 9 February 2010 16:16 UTC [Source type: Reference]

^ To maximize classroom time, instruction on physical activity also can be integrated into the lesson plans of other school subjects, such as mathematics, biology, and language arts.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

^ It was their unusual coloration that identified them as monsters, because in every other physical particular they were like normal Dellian animals.
  • Fire - Kristin Cashore - Penguin Group (USA) 18 January 2010 9:25 UTC [Source type: Original source]

The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[59] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[60]
Geologic layers of the Earth[61]

Earth cutaway from core to exosphere. Not to scale.
Component Layer Density
0–60 Lithosphere[note 9]
0–35 ... Crust[note 10] 2.2–2.9
35–60 ... Upper mantle 3.4–4.4
35–2890 Mantle 3.4–5.6
100–700 ... Asthenosphere
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1


Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[63] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[64] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[65] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. .This extra heat production, twice present-day at approximately 3 billion years ago,[63] would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.^ DOE science programs are funded at $3.6 billion, an increase of $168.2 million over the budget request and $168.6 million above fiscal year 2004.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

Present-day major heat-producing isotopes[67]
Isotope Heat release
W/kg isotope

Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle
238U 9.46 × 10-5 4.47 × 109 30.8 × 10-9 2.91 × 10-12
235U 5.69 × 10-4 7.04 × 108 0.22 × 10-9 1.25 × 10-13
232Th 2.64 × 10-5 1.40 × 1010 124 × 10-9 3.27 × 10-12
40K 2.92 × 10-5 1.25 × 109 36.9 × 10-9 1.08 × 10-12
Total heat loss from the earth is 4.2 × 1013 Watts.[68] A portion of the core's thermal energy is transported toward the crust by Mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[69] More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. .The final major mode of heat loss is through conduction through the lithosphere, majority of which occurs in the oceans because the crust there is much thinner than that of the continents.^ Data demonstrate that major decreases in vigorous physical activity occur during grades 9 through 12.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]


Tectonic plates

Earth's main plates[70]
Tectonic plates (empty).svg
Plate name Area
106 km²
African Plate[note 11] 78.0
Antarctic Plate 60.9
Australian Plate 47.2
Eurasian Plate 67.8
North American Plate 75.9
South American Plate 43.6
Pacific Plate 103.3
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[71] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,[72] and their motion is strongly coupled with patterns convection inside the Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. .The combination of these processes continually recycles the oceanic crust back into the mantle.^ Helium-3 and mantle volatiles in the ocean and the oceanic crust.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

.Because of this recycling, most of the ocean floor is less than 100 million years in age.^ Even among children aged 3 to 4 years, those who were less active tended to remain less active after age 3 years than most of their peers.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

.The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years.^ About 1 million curies of tritium was handled safely over a three year period during over 1,000 DT experiments.
  • The FIRE Place 18 January 2010 9:25 UTC [Source type: News]

[73][74] .By comparison, the oldest dated continental crust is 4030 million years old.^ Last Modified: 7 December 1995 (The date is correct, this document is more than two years old.

Other notable plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[76] and the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.[77]


The Earth's terrain varies greatly from place to place. About 70.8%[78] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[52] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.
The planetary surface undergoes reshaping over geological time periods because of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[79] also act to reshape the landscape.
The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[80] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[81] .The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both.^ None of the elements authorized to exist in the contents of the HEAD are displayed; the displayed material is found within the BODY .

The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[82] Common carbonate minerals include calcite (found in limestone), aragonite and dolomite.[83]
The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[7] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 × 107 km² of cropland and 3.4 × 107 km² of pastureland.[84]
The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[85]


Elevation histogram of the surface of the Earth. Approximately 71% of the Earth's surface is covered with water.
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. .The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m.^ Iodine in the surface water of the ocean.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

^ Rhenium: its concentration in Pacific Ocean surface waters.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

^ Influence of Amazon River discharge of surface salinity and dissolved silicate concentration in the Caribbean Sea.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

.The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.^ Mason, R. P. and W. F. Fitzgerald, 1993, The distribution and biochemical cycling of mercury in the equatorial Pacific Ocean: Deep-Sea Res., v.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

[note 12][86] The average depth of the oceans is 3,800 m, more than four times the average height of the continents.[85]
The mass of the oceans is approximately 1.35 × 1018 metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 × 109 km3. If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 13] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.[87]
The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35 ).[88] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[89] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[90] .Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.^ The influence of organisms on the composition of sea water.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

^ On the composition of sea-water in the different parts of the ocean.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

[91] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[92]


The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[8] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation due to weather and seasonal factors.[93]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks ultraviolet solar radiation, permitting life on land. .Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.^ These benefits are particularly important for older people—a good quality of life means being functionally independent and being able to perform the activities of daily living.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

^ Activities of daily living have been identified as a scale to measure dependencies in basic self-care and other functions important for independent living and to avoid institutionalization.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

^ Engaging in moderate physical activity for at least 30 minutes per day will help ensure that sufficient calories are used to provide health benefits.
  • 22 Physical Activity and Fitness 20 September 2009 11:18 UTC [Source type: Academic]

[94] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. .Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth's atmosphere.^ The dissolved gases - carbon dioxide.
  • Periodic Table of the Elements for Seawater(JavaScript) 9 February 2010 16:16 UTC [Source type: Academic]

Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.[78]

Weather and climate

The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.[95]
The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[96] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.[97]
Source regions of global air masses
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.[95] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[98]
The Earth can be sub-divided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[99] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. .The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.^ These relationships are more commonly identified in the HEAD of the document using the LINK element.

^ Further, the old IUPAC system was more frequently used in Europe while the CAS system was most common in America.
  • Periodic table encyclopedia topics | 9 February 2010 16:16 UTC [Source type: Reference]

^ Each element in HTML falls into zero or more categories that group elements with similar characteristics together.
  • HTML 5 Reference 11 September 2009 3:51 UTC [Source type: Reference]


Upper atmosphere

This view from orbit shows the full Moon partially obscured by the Earth's atmosphere. NASA image.
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[94] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind.[100] An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. .The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.^ The CELLSPACING attribute defines spacing between cells and CELLPADDING defines spacing within cells.

^ Description
The PRE element defines a separated multi-line set of text to be rendered as it exists in the source document with the same line breaks and spaces.

Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.[102] The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[103] Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.[104] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[105]

Magnetic field

The Earth's magnetic field, which approximates a dipole.
The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic, and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.[106][107]
The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[108]

Orbit and rotation


Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit.
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time. Each second is slightly longer than an SI second because Earth's solar day is now slightly longer than it was during the 19th century due to tidal acceleration.[109]
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s. [110][note 14] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s).[110] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[111] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[112] and 1962–2005.[113]
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes; the apparent sizes of the Sun and the Moon are approximately the same.[114][115]


Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12 hours. Because of this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.[8]
The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counter-clockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth appears to revolve in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees from the perpendicular to the Earth–Sun plane, and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[8][116]
The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.[117][note 15] This is maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.
Illustration of the Milky Way Galaxy, showing the location of the Sun.
Earth, along with the Solar System, is situated in the Milky Way galaxy, orbiting about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galaxy's equatorial plane in the Orion spiral arm.[118]

Axial tilt and seasons

Because of the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This results in seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.
Earth and Moon from Mars, imaged by Mars Reconnaissance Orbiter. From space, the Earth can be seen to go through phases similar to the phases of the Moon.
By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23.[119]
The angle of the Earth's tilt is relatively stable over long periods of time. However, the tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[120]
In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. However, these dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth-Sun distance results in an increase of about 6.9%[121] in solar energy reaching the Earth at perihelion relative to aphelion. .Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year.^ Last Modified: 7 December 1995 (The date is correct, this document is more than two years old.

However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.[122]


Diameter 3,474.8 km
2,159.2 mi
Mass 7.349 × 1022 kg
8.1 × 1019 (short) tons
Semi-major axis 384,400 km
238,700 mi
Orbital period 27 d 7 h 43.7 m
The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. 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. The natural satellites orbiting other planets are called "moons" after Earth's Moon.
The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.
Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes.[123] During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.[124]
The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[125] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[126] If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result from the resulting extreme seasonal differences. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life.[127] However, this is a controversial subject, and further studies of Mars—which has a similar rotation period and axial tilt as Earth, but not its large Moon or liquid core—may settle the matter.
Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[115] This allows total and annular eclipses to occur on Earth.
A scale representation of the relative sizes of, and average distance between, Earth and Moon.
The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.[128]


A range of theoretical habitable zones with stars of different mass (our Solar System at center). Not to scale.
A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment where complex organic molecules can assemble and sufficient energy to sustain metabolism.[130] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and sustain life on this planet.[131]


The planet's life forms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion years ago. Earth is the only place in the universe where life is known to exist. Some scientists believe that Earth-like biospheres might be rare.[132]
The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land primarily latitude and height above the sea level separates biomes. Terrestrial biomes lying within the Arctic, Antarctic Circle or in high altitudes are relatively barren of plant and animal life, while the greatest latitudinal diversity of species is found at the Equator.[133]

Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.
Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics.[134] .These bodies form concentrated sources for many metals and other useful elements.^ Many other elements automatically imply a text separation, such as headings, list elements, blockquotes, etc.

^ Description
The FRAMESET element is used instead of the BODY element.

^ Version 3 proposes using associated style sheets and the CLASS attribute to handle these, and other, options to numbering style.

The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[135] Humans also live on the land by using building materials to construct shelters. In 1993, human use of land is approximately:
Land use Percentage
Arable land 13.13%[7]
Permanent crops 4.71%[7]
Permanent pastures 26%
Forests and woodland 32%
Urban areas 1.5%
Other 30%
The estimated amount of irrigated land in 1993 was 2,481,250 km².[7]

Natural and environmental hazards

Large areas are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, and other calamities and disasters.
Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.
A scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.[136]

Human geography

North America South America Antarctica Africa Europe Asia Oceania Oceans Continents vide couleurs.png
About this image
Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
Earth has approximately 6,803,000,000 human inhabitants as of December 12, 2009.[137] Projections indicate that the world's human population will reach seven billion in 2013 and 9.2 billion in 2050.[138] Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.[139]
.It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%),[140] high mountains (27%),[141] or other less suitable terrain.^ In this order: optionally a caption element, followed by either zero or more colgroup elements, followed optionally by a thead element, followed optionally by a tfoot element, followed by either zero or more tbody elements or one or more tr elements, followed optionally by a tfoot element (but there can only be one tfoot element child in total).
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^ Example markup is provided for both HTML and XHTML. In some cases, the markup is the same and thus only one example is needed, but in others there may be differences syntactic differences.
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The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[142] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)
The Earth at night, a composite of DMSP/OLS ground illumination data on a simulated night-time image of the world. This image is not photographic and many features are brighter than they would appear to a direct observer.
Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2007 there are 201 sovereign states, including the 192 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities.[7] Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.[143]
The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.[144] It is not, however, a world government. The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.[145]
The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.[146] In total, about 400 people visited outer space and reached Earth orbit as of 2004, and, of these, twelve have walked on the Moon.[147][148][149] Normally the only humans in space are those on the International Space Station. The station's crew, currently six people, is usually replaced every six months.[150] Humans traveled the farthest from the planet in 1970, when Apollo 13 crew was 400,171 km away from Earth.[151][152]

Cultural viewpoint

The first photograph ever taken by astronauts of an "Earthrise", from Apollo 8
Unlike the rest of the planets in the Solar System, mankind didn't perceive the Earth as a planet until the sixteenth century.[153] The name "Earth" derives from the Anglo-Saxon word erda, which means ground or soil. It became eorthe later, and then erthe in Middle English.[154] The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.[155]
Earth has often been personified as a deity, in particular a goddess. In many cultures the mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism[156] or Islam,[157] assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.[158] Such assertions are opposed by the scientific community[159][160] and by other religious groups.[161][162][163] A prominent example is the creation-evolution controversy.
.In the past there were varying levels of belief in a flat Earth,[164] but this was displaced by the concept of a spherical Earth due to observation and circumnavigation.^ There are heading elements for marking up different levels of headings, a paragraph (p) element for paragraph, various list elements for marking up different types of lists, and a table elements for marking up tables.
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[165] The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective.[166][167] This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet.[168]

See also


  1. ^ All astronomical quantities vary, both secularly and periodically. The quantities given are the values at the instant J2000.0 of the secular variation, ignoring all periodic variations.
  2. ^ Due to natural fluctuations, ambiguities surrounding ice shelves, and mapping conventions for vertical datums, exact values for land and ocean coverage are not meaningful. Based on data from the Vector Map and Global Landcover datasets, extreme values for coverage of lakes and streams are 0.6% and 1.0% of the earth’s surface. Note that the ice shields of Antarctica and Greenland are counted as land, even though much of the rock which supports them lies below sea level.
  3. ^ Blue Planet is used as the title of several films Blue Planet and The Blue Planet, in the Life issue The Incredible Year '68 featuring the Earthrise photo with lines from poet James Dickey Behold/The blue planet steeped in its dream/Of reality [1] page 7-8 [2], and in the title of the European Space Agency bulletin report Exploring the water cycle of the 'Blue Planet' [3]
  4. ^ By International Astronomical Union convention, the term terra is used only for naming extensive land masses on celestial bodies other than the Earth. Cf. Blue, Jennifer (2007-07-05). "Descriptor Terms (Feature Types)". Gazetteer of Planetary Nomenclature. USGS. Retrieved 2007-07-05. 
  5. ^ Other planets in the Solar System are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See:
  6. ^ As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: Tinetti, G; Vidal-Madjar, A; Liang, MC; Beaulieu, JP; Yung, Y; Carey, S; Barber, RJ; Tennyson, J et al. (July 2007). "Water vapour in the atmosphere of a transiting extrasolar planet". Nature 448 (7150): 169–171. doi:10.1038/nature06002. PMID 17625559. 
  7. ^ The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.
  8. ^ This article incorporates text from the article "Petrology" in the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.
  9. ^ Locally varies between 5 and 200 km.
  10. ^ Locally varies between 5 and 70 km.
  11. ^ Including the Somali Plate, which is currently in the process of formation out of the African Plate. See: Chorowicz, Jean (October 2005). "The East African rift system". Journal of African Earth Sciences 43 (1–3): 379–410. doi:10.1016/j.jafrearsci.2005.07.019. 
  12. ^ This is the measurement taken by the vessel Kaikō in March 1995 and is believed to be the most accurate measurement to date. See the Challenger Deep article for more details.
  13. ^ The total volume of the Earth's oceans is: 1.4 × 109 km3. The total surface area of the Earth is 5.1 × 108 km². So, to first approximation, the average depth would be the ratio of the two, or 2.7 km.
  14. ^ Aoki, the ultimate source of these figures, uses the term "seconds of UT1" instead of "seconds of mean solar time".—Aoki, S. (1982). "The new definition of universal time". Astronomy and Astrophysics 105 (2): 359–361. Retrieved 2008-09-23. 
  15. ^ For the Earth, the Hill radius is
    \begin{smallmatrix} R_H = a\left ( \frac{m}{3M} \right )^{\frac{1}{3}} \end{smallmatrix},
    where m is the mass of the Earth, a is an Astronomical Unit, and M is the mass of the Sun. So the radius in A.U. is about: \begin{smallmatrix} \left ( \frac{1}{3 \cdot 332,946} \right )^{\frac{1}{3}} = 0.01 \end{smallmatrix}.


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Further reading

External links


Up to date as of January 14, 2010

From Wikiquote

Earth is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. Home to myriad species including humans, it is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", and "the World".


  • This poor world, the object of so much insane attachment, we are about to leave; it is but misery, vanity, and folly; a phantom, — the very fashion of which "passeth away."
    • François Fénelon, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 206.
  • We are pilgrims, not settlers; this earth is our inn, not our home.
    • J. H. Vincent, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 206.
  • “ But the nations became wrathful, and your own wrath came, and the appointed time for the dead to be judged, and to give [their] reward to your slaves the prophets and to the holy ones and to those fearing your name, the small and the great, and to bring to ruin those ruining the earth.”


  • And among His Signs Is the creation of the heavens And the earth, and the variations In your languages And your colors; verily In that are Signs For those who know
  • the late eighteenth century scientists knew very precisely the shape and dimensions of the earth and its distance from the Sun and planets; and now Cavendish, without even leaving home, had given them its weight. So you might think that determining the age of the Earth would be relatively straightforward. After all, the necessary materials were literally at their feet. But no. Human beings would split the atom and invent television, nylon, and instant coffee before they could figure out the age of their own planet.
  • Earth provides enough to satisfy every man's need, but not every man's greed
  • Every Winter, When the great sun has turned his face away, The earth goes down into a vale of grief, And fasts, and weeps, and shrouds herself in sables
  • I tell you, we are here on Earth to fart around, and don't let anybody tell you different.
  • It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small.
  • Take what you can use and let the rest go by
  • The best investment on earth is earth.
    • Louis J. Glickman
  • The church says the earth is flat, but I know that it is round, for I have seen the shadow on the moon, and I have more faith in a shadow than in the church
    • Ferdinand Magellan This quote should be attributed to Robert Green Ingersoll who is the real author of the quote. There is no document where you can find Magellan uttered or wrote those words.
  • The earth, that is sufficient, I do not want the constellations any nearer, I know they are very well where they are, I know they suffice for those who belong to them
  • The earth was made so various, that the mind Of desultory man, studious of change, And pleased with novelty, might be indulged
  • The earth we abuse and the living things we kill will, in the end, take their revenge; for in exploiting their presence we are diminishing our future
    • Marya Mannes, American Writer (1904-1990)
  • The surface of the Earth is the shore of the cosmic ocean. From it we have learned most of what we know. Recently, we have waded a little out to sea, enough to dampen our toes or, at most, wet our ankles. The water seems inviting. The ocean calls.
  • Those who contemplate the beauty of the earth find resources of strength that will endure as long as life lasts
  • Those who dwell among the beauties and mysteries of the earth are never alone or weary of life.
  • To see the earth as it truly is, small and blue in that eternal silence where it floats, is to see riders on the earth together, brothers on that bright loveliness in the eternal cold - brothers who know now they are truly brothers.
  • Earth's creation is not a piece of knowledge, it is what we rely on.
    • Steven Rawn, Plutonium Party politician

External Links

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

Travel guide

Up to date as of January 14, 2010

From Wikitravel

Sol : Earth
This was an April Fool's Day joke article. We amused ourselves tremendously in making it. Please refrain from dragging out the silliness and plunge forward on some real articles!
Lage von Earth
Quick Facts
Capital New York City
Government Primitive hierarchical
Currency Dried cellulose pulp, minted pieces of metal
Area total: 510,065,600 km2
land: 148,939,100 km2
water: 361,126,400 km2
Population 6,600,000,000 (March 2006 est.)
Language Earthlingish meat-flapping
Religion Usually
Electricity Non-fusion based
Calling Code +222
Internet TLD .un
Time Zone GTC-61x1037
Earth (literally, "dirt") is the third planet in the Sol system, about 4.3 lightyears from Alpha Centauri. Without significant orbital recreation systems or magma baths, this planet with its pre-warp-drive civilization may not seem to be worth a stop for refueling. But there's more to Earth than meets the eyes, and if you give it a chance, you may find something here that catches your interest. If you do, be sure to tell us.
  • Antarctica -- an icy wasteland only slightly warmer than interstellar space, unspoiled by locals
  • Africa -- oldest populated continent, with the most large land species
  • Asia -- a huge mess of wide open spaces and tightly-packed cities
  • Australia -- former Denebian penal colony, whose descendants include the platypus, koala, and kangaroo
  • Europe -- cramped and crowded, not much of interest
  • North America -- richest source of atmospheric CO2, great for vegetative travelers
  • South America -- fastest-rising major mountain range
  • Oceans -- home to most of the intelligent life
  • Heaven -- mostly full of birds and other winged creatures
  • Hell -- Worth a visit to see what the planet will look like after 50 years of Global Warming.


This sector of the galaxy lacks any major nebula-based or multi-planet metropolis, but there are some quaint villages on Earth that can provide basic amenities for the weary traveler.
  • New York City -- world headquarters of the nascent and dysfunctional planetary government
  • London -- some good beers can be found here
  • Paris -- locals flock for the tacky folk-art and the single (non-operational) zeppelin docking tower
  • Johannesburg -- largest city near Antarctica; good for picking up groceries
  • Roswell -- if you must approach the surface, try this rather-pleasant area of desert.

Other destinations

Earth's metropolitan areas are unbearably dreary and provincial, but you can find some fun in the rural sectors of the planet.
  • Grand Canyon -- excellent for pod-racing
  • Bermuda Triangle -- fishing, hurricanes, freighter abduction
  • Iceland -- tasty fresh hot sulfur springs
  • Lake Baikal -- invigorating spot for a cold swim, and for the adventurous, there's a thriving sturgeon brothel
  • Luna -- the smaller partner of this dual-planet system is ideal for golf
  • California - home of the last surviving California Raisin, the Governator, and the Gates to Hell, just off the I-5 in Aneheim.



Belched as a cloud of particles from the core of its central star, Sol, sometime about 4 billion years ago, the Earth coalesced into a roughly sphere-shaped lump soon afterwards (relatively speaking). A thin layer of goo formed on its outer crust about 1 billion years ago, and that goo jiggles and sloshes around every once in a while. There have been some awesome meteor collisions, especially in the areas of Yucatan and Tugunska, and the polar areas have formed nifty frost patterns if you look closely enough. Other than that, not much has happened on Earth except the Cincinnati Bengals going to the playoffs in 2005.


70.8% of the Earth's surface is covered by a noxious solution of dihydrogen monoxide and sodium chloride. The rest is largely silicate, with occasional ferrous deposits.

Flora and fauna

Earth is inhabited by Earthlings, which come in various shapes and sizes. While simple arachnids are present in fairly large numbers, especially in cheaper lodging establishments, the formation of a proper, civilized Hive mind has been inhibited by a hard core that prevents burrowing to suitable depths. Instead, the dominant type of Earthling appears to be a hairless bipedal sack of protoplasm known as a Homo, which has recently invented the digital wristwatch and primitive computing tools with speeds at a paltry ~3-4Ghz.


Earth has an unstable oxygen-rich atmosphere that would produce widespread combustion if not for the annoyingly frequent precipitation of dihydrogen monoxide in its liquid and solid forms. Temperatures generally vary between 223-323°K. You may hear of rumors of "global warming", but this is simply a myth to scare off monsters from far off galaxies, who tend to emigrate to colder locales. Compared to its nearest planetary neighbor Venus, Earth is quite cool.

Get in

There are no regularly scheduled spaceship services to Earth, as the Vogons have yet to complete construction of the new hyperspace bypass that would make access easier. Hitchhikers have occasionally been successful, although getting a ride back can be exceedingly time-consuming.
If you charter a spaceship, be aware that decent landing facilities are woefully absent and the natives are afraid of flying saucers. Orbital stations are amazingly primitive and do not have modern docking facilities. Area 51 is widely considered the best option for cheap parking.

Get around

Local transportation relies mostly on the combustion of fossil fuels and is grotesquely unsafe and slow, with speeds rarely exceeding 0.002 lightseconds/hour. Earthlings have a primitive taboo about tunneling directly through the planet to other locations, and tend to take the long way around following the surface instead. Consequently, travel times can can be considerably longer.
Jet packs, quantum disappearance drives, and ultranic hyperspace fizzout devices remain undiscovered by Earth's primitive technological priest-caste. Strangely enough, use of these tools will not be greeted with curiosity, but with hostility and fear. It may be difficult to employ them with Earthlings about.


Amazingly, most Earthlings can only communicate by pushing air through flaps of meat, producing series of vaguely musical squeaks and burbles. Utilization of radio for communication is limited to broadcasting political propaganda and mating songs. However, rudimentary digital conversation is possible anonymously over the Intarweb, accessible at 2.4-2.5 GHz near St. Arbucks outlets.
Earth's best conversationalists
Earth's best conversationalists
Earth's waterbound delphine population, on the other hand flipper, uses a charming series of beeps, clicks, whistles and multi-band telepathy to communicate. Delphine is a pleasant language that can be picked up quite easily by the experienced traveler. Be forewarned that dolphins primarily use their powerful communications skills to tell dirty jokes and to scam their way into cool nightclubs.


There are few objects of interest on Earth, although some visitors have found that Homos make amusing pets. (Ruminants of genus Bos are also quaint, but more difficult to care for.) Try to select your specimens in isolated areas, such as farming communities, to minimize unwanted attention. Turning on your spaceship lights at full blast will usually make them freeze long enough to be captured. Once on-board, be sure to give the Homo a thorough medical examination to ensure it's free of vermin, and if found infested, just drop it off where you took it. Species of the genus Heteros breed at an alarming rate (the process is not pretty), so be careful not to pick up a mating pair of them.


Earthlings are an acquired taste; most find them overly chewy and hairy. Wikitravelers, especially Wikitravel administrators, taste horribly grotesque and bitter beyond belief. It's best to avoid eating any Wikitraveler. Try the fish tacos instead, or a nice tossed salad.


It appears that dihydrogen monoxide is a required liquid to sustain the existence of the semi-intelligent life forms that inhabit the planet. Although commonly available in juice-bearing fruits, many Earthlings instead add bubbles and mix high fructose corn syrup with it before serving. Various forms of fermented plant product (known locally as "beer", "whiskey" or "the sauce") are also consumed in great quantities. These liquids appear to serve an important purpose in the life of the Earthling male, who will frequently buy them for the females as part of the species' bizarre mating ritual.


One place that Earth falls down on the hospitality scale is in sleeping accommodations. Almost all hotels are tailored specifically for the needs of beings the size and shape of the typical Earthling (18 standard units high, or 2 local "meters"). Typical galactic customizations that could be handled in even the most basic of civilized planets (a bed longer than 300 meters, a bed of molten lava, a comfy prism for intelligent shades of the color blue) will elicit only dumb, moist stares from the local hospitality industry. Only beings between, say, 1/4 to double the size of an average human, and that can tolerate liquid-water temperatures, will find professional lodging of any kind.
Of course, there are ample opportunities for sleeping al fresco. Camping on Earth can be highly rewarding, especially if you can find a section of the planet sufficiently remote to go mostly undetected. The locals are easily frightened, however, so avoid any serious modifications of the environment (boring holes into the planet's crust; igniting whole ecosystems; consuming all atmospheric nitrogen). Remember the galactic campers' code: take only cow entrails, leave only crop circles.


Although the planet boasts countless cosmetology schools, none of them are accredited by any galactic boards and their credits will almost assuredly fail to transfer to degree-granting institutions in your quadrant. Don't be fooled by the names of Har-Vard University and Ox-Ford University; these appear to have no relationship with the famous and prestigious Har-Fart University and Ox-Fart University on Neptune. Earth has no other educational opportunities to speak of.


Earth offers few of the work opportunities found in other systems - the need for hyper-drive repair technicians is unknown, galactic refueling operations are non-existent, and crop-art is frowned upon by the locals and unlikely to generate significant patronage. However, for those willing to assume a visage that resembles that of an Earthling some interesting options exist. Hollywood currently employs numerous non-Earthlings in positions within the entertainment industry, including Keith Richards, Prince, and Robert Downey, Jr. Earthlings are also amused by the simplest of anatomical tricks, and the inter-galactic Cirque du Soleil troupe has found great success on the planet. For those looking for quieter employment, the New York subway system is always seeking drivers, and the country of France offers unlimited opportunities.

Stay safe

Mostly harmless. The inhabitants of Earth have recently discovered the weaponry use of primitive fission- and fusion-based devices. Facilities for inter-planetary deployment of such devices remain lacking, and such devices are unlikely to penetrate the armor of spacecrafts conforming to Intergalactic Automotive Associations Standards 345IR-14. Carbon-based travelers should take note that deployment of such devices may cause some discomfort.

Stay healthy

With proper precautions Earth is a generally safe place to visit. The fatalities suffered by Martian visits to England in 1898 and New Jersey in 1938 (naively referred to by Earthlings as the "War of the Worlds" and by Martians as "The War of Earthling Aggression") are easily avoided with use of simple antihistamines. Temperatures on Earth are almost universally lower than on other planets. While these extreme temperatures provide unique opportunities - metals such as lead and tin can actually be seen in their solid form - proper clothing is vital to a pleasant stay on this planet.
Health facilities are completely useless on Earth. Medical technology still utilizes such quaint devices as the tongue depressor (literally a wooden stick used for manipulating organs in the oral cavity) and rubber glove (which Earthing doctors use in unspeakable ways to treat the male of the species). Adding insult to injury, the wait and paperwork required for even this rudimentary level of care is interminable.


Hard as it may be to accept, Earthlings deserve our grudging respect as living, sentient creatures; our brothers from across the stars who may someday join the galactic community as equals and peers. We have so much to learn from each other.
Ha ha! Gotcha! You believed us there for a moment, didn't you? Seriously, you do want to be on your best behavior, as Earthlings are extremely prickly, hidebound, unfun creatures who freak out quite easily at anything they haven't seen before on cable TV. They can get quite cheesed off by pretty much anything you say, do, or radiate, and they get stabby and shooty whenever they're angry. So be careful.
Typical Earthling sore spots include: being proven wrong; consuming any of their close family members; reminding them that they still look an awful lot like monkeys; cool tricks with fusion, subspace, or really anything that shows how backward and remedial their own technology is; making any considerable holes in the planet itself; and the elimination of any native species, however small, inconsequential or squiggly.


Arecibo on the rocky outcrop of Puerto Rico allows you to call home. Alternatively, dial 1-800-CALL-ATT. And to the Earthlings, for future reference, YES we can hear you now...

Study guide

Up to date as of January 14, 2010

From Wikiversity

.Earth is the third planet from the Sun, and the largest of the four terrestrial planets in the Solar System in terms of diameter, mass and density.^ Earth is also the only planet in our Solar System with tectonic plates .
  • Earth | Universe Today 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

^ The Earth is the 3rd planet from the Sun .
  • Earth | Universe Today 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

^ Our planet is the third planet from the Sun and the fifth largest of the eight planets in our Solar System .
  • Earth | Universe Today 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

.It is also referred to as the World, the Blue Planet, and Terra.^ Earth has a number of nicknames including the Blue Planet and Terra.
  • Earth | Universe Today 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

more from Wikipedia



.The Earth, like that of the other terrestrial planets, is chemically divided into layers or spheres.^ Earth's spheres Earth is composed of several layers, or spheres, somewhat like the layers of an onion.
  • NASA - Earth 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

^ For the purposes of what I hope to be a technically and scientifically accurate document, I will define our goal thus: by any means necessary, to change the Earth into something other than a planet or a dwarf planet .
  • How to destroy the Earth @ Things Of Interest 27 January 2010 23:47 UTC [Source type: Original source]

^ But on the other hand, verse 5 of the 104th Psalm reads "He [God] set the Earth on its foundations; it can never be moved", and there are other verses like this, so maybe: .
  • How to destroy the Earth @ Things Of Interest 27 January 2010 23:47 UTC [Source type: Original source]

.Two sets of spheres have been defined: one set above the surface and one set below.^ Thus we have defined the two dimensions of a surface; that is, length and breadth.
  • New elements in HTML 5 20 September 2009 9:28 UTC [Source type: General]

^ Bottled waters mostly come in two kinds, alkaline ones with massive mineral levels just below brackish, and acidic ones with mineral levels just above RO flatness.
  • The Insanely Long Water FAQ - 28 January 2010 1:01 UTC [Source type: FILTERED WITH BAYES]

.Besides the layers above and below the surface other spheres have been identified such as the hydrosphere, the biosphere and the magnetosphere.^ Surface water is exposed to many different contaminants, such as animal wastes, pesticides, insecticides, industrial wastes, algae and many other organic materials.
  • The Water Treatment FAQ 28 January 2010 1:01 UTC [Source type: FILTERED WITH BAYES]

^ Many people are concerned that the ozone layer may become too thin, allowing ultraviolet rays to reach the surface and harm people and other living things.
  • NASA - Earth 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

^ The presence of these organisms in drinking water suggest contamination from a surface or shallow subsurface source such as cesspool leakage, barnyard runoff or other source.
  • The Water Treatment FAQ 28 January 2010 1:01 UTC [Source type: FILTERED WITH BAYES]

Above the surface

Electric currents created in sunward ionosphere.
.The Earth's atmosphere is a layer of gases surrounding the planet that is retained by the Earth's gravity.^ Earth is surrounded by a thin layer of air, the atmosphere.
  • NASA - Earth 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

^ The atmosphere Air surrounds Earth and becomes progressively thinner farther from the surface.
  • NASA - Earth 27 January 2010 23:47 UTC [Source type: FILTERED WITH BAYES]

^ Water vapor, carbon dioxide, methane, and other gases in the atmosphere trap heat from the sun, warming Earth.
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.The fuzzy boundary between the atmosphere and outer space is called the exosphere.^ The atmosphere has no definite outer edge but fades gradually into space.
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^ The boundaries between two plates sliding past each other are called transform faults.
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As we descend toward the surface, we pass through the ionosphere, which includes the exosphere and the thermosphere. .Then we go through increasingly dense layers, the mesosphere, the stratosphere and finally the troposphere which contains the gaseous elements and water vapour that sustain life on the planetary surface.^ Related chapters abundance of elements for life : abundance in universe, planet, solar system, earth, sea, plants, animals (4 pages) composition of sea water : abundance of elements in sea water, with dissolved gases and CO2.

^ Associate all nodes with the heading of lower rank than this one, then set c to be this element, and continue going backwards through the previous siblings.
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^ Earth's final resting place : lumps of heavy elements, torn apart, sinking into the massive cloud layers of Jupiter, never to be seen again.
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The average atmosperic pressure at sea level is 101.325 kPa (1013.25 mbar, or hPa) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg) of mercury. See International Standard Atmosphere for much more information about how atmospheric pressure is calculated and standardized.

Below the surfce

Geology of the interior of Earth:
A: Mohorovičić discontinuity (Moho boundary)
The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.
^ The sun is much larger than Earth.
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^ Beneath Earth's solid crust are the mantle, the outer core, and the inner core.
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^ The outer part of the core is liquid, while the inner part is solid.
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.The Inner Core of the earth is believed to be composed primarily of a nickel-iron alloy, with very small amounts of some other elements.^ Most scientists believe that the extinction was caused by the impact of a small asteroid with Earth.
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^ The immediate user selection changes as the user selects different elements (either by pointing at them with a pointing device, or by selecting them in some other way).
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^ Comments : Earth is believed to have been hit by an object the size of Mars at some point in the distant past before its surface cooled.
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[1] .The Outer Core of the earth is a liquid layer made of iron and nickel.^ The core is made mostly of iron and nickel and possibly smaller amounts of lighter elements, including sulfur and oxygen.
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^ The solid Earth consists of a thin outer layer, the crust, with a thick rocky layer, the mantle, beneath it.
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^ Geologists believe the innermost part of the core, about 1,600 miles (2,600 kilometers) in diameter, is made of a similar material as the outer core, but it is solid.
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On and near the surface

The Earth module is a stub. You can help Wikiversity by expanding it.

Earth and the Cosmos

Earth's tilt, rotation and orbit.
Illustration of the Milky Way Galaxy showing the location of the Earth's Sun.


  1. Lars Stixrude; Evgeny Waserman and Ronald Cohen (November 1997). "Composition and temperature of Earth's inner core". Journal of Geophysical Research 102 (B11): 24729–24740. w:American Geophysical Union. doi:10.1029/97JB02125.  

See also

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. g paq"€, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in the Lat. arare, obsolete Eng. " ear," and Gr. apouv, but this is now considered very doubtful; see G. Curtius, Greek Etymology, Eng. trans., i. 426; Max Milner, Lectures, 8th ed. i. 294). From early times the word " earth " has been used in several connexions - from that of soil or ground to that of the planet which we inhabit, but it is difficult to trace the exact historic sequence of the diverse usages. In the cosmogony of the Pythagoreans, Platonists and other philosophers, the term or its equivalent denoted an element or fundamental quality which conferred upon matter the character of earthiness; and in the subsequent development of theories as to the ultimate composition of matter by the alchemists, iatrochemists, and early phlogistonists an element of the same name was retained (see Element). In modern chemistry, the common term " earth " is applied to certain oxides: - the alkaline earths " are the oxides of calcium (lime), barium. (baryta) and strontium (strontia); the " rare earths " are the oxides of a certain class of rare metals.
THE Earth The terrestrial globe is a member of the Solar system, the third in distance from the Sun, and the largest within the orbit of Jupiter. In the wider sense it may be regarded as composed of a gaseous atmosphere (see Meteorology), which encircles the crust or lithosphere (see Geography), and surface waters or hydrosphere (see Ocean And Oceanography). The description of the surface features is a branch of Geography, and the discussions as to their origin and permanence belongs to Physiography (in the narrower sense), physiographical geology, or physical geography. The investigation of the crust belongs to geology and of rocks in particular to petrology.
In the present article we shall treat the subject matter of the Earth as a planet under the following headings: (1) Figure and Size, (2) Mass and Density, (3) Astronomical Relations, (4) Evolution and Age. These subjects will be treated summarily,. readers being referred to the article Astronomy and to the cross-references for details.
1. Figure and Size. - To primitive man the Earth was a flat disk with its surface diversified by mountains, rivers and seas. In many cosmogonies this disk was encircled by waters, unmeasurable by man and extending to a junction with the sky; and the disk stood as an island rising up through the waters from the floor of the universe, or was borne as an immovable ship on the surface. Of such a nature was the cosmogony of the Babylonians and Hebrews; Homer states the same idea, naming the encircling waters 'f21cEavos; and Hesiod regarded it as a disk midway between the sky and the infernal regions. The theory that the Earth extended downwards to the limit of the universe was subjected to modification when it was seen that the same sun and stars reappeared in the east after their setting in the west. But man slowly realized that the earth was isolated in space, floating freely as a balloon, and much speculation was associated about that which supported the Earth. Tunnels. in the foundations to permit the passage of the sun and stars were suggested; the Greeks considered twelve columns to support the heavens, and in their mythology the god Atlas appears condemned to support the columns; while the Egyptians had the Earth supported by four elephants, which themselves stood on a tortoise swimming on a sea. Earthquakes were. regarded as due to a movement of these foundations; in Japan this was considered to be due to the motion of a great spider, an animal subsequently replaced by a cat-fish; in Mongolia it is a hog; in India, a mole; in some parts of South America, a whale; and among some of the North American Indians,. a giant tortoise.
The doctrine of the spherical form has been erroneously assigned to Thales; but he accepted the Semitic conception of the disk, and regarded the production of springs after earthquakes as due to the inrushing of the waters under the Earth into fissures in the surface. His pupil, Anaximander (610-547), according to Diogenes Laertius, believed it to be spherical (see The Observatory, 1894, P. 208); and Anaximenes probably held a similar view. The spherical form is undoubtedly a discovery of Pythagoras, and was taught by the Pythagoreans and by the Eleatic Parmenides. The expositor of greatest moment was Aristotle; his arguments are those which we employ to-day:- the ship gradually disappearing from hull to mast as it recedes from the harbour to the horizon; the circular shadow cast by the Earth on the Moon during an eclipse, and the alteration in the appearance of the heavens as one passes from point to point on the Earth's surface.' He records attempts made to determine the circumference; but the first scientific investigation in this 1 Aristotle regarded the Earth as having an upper inhabited half and a lower uninhabited one, and the air on the lower half as tending to flow upwards through the Earth. The obstruction of this passage brought about an accumulation of air within the Earth, and the increased pressure may occasion oscillations of the surface, which may be so intense as to cause earthquakes.
From La Grande EncycloQedie. FIG. 2. - Thetis crossing the sea, with the armour of Achilles. Ear-ring from the Crimea, Hermitage museum.
direction was made 150 years later by Eratosthenes. The spherical form, however, only became generally accepted after the Earth's circumnavigation (see Geography).
The historical development of the methods for determining the figure of the Earth (by which we mean a theoretical surface in part indicated by the ocean at rest, and in other parts by the level to which water freely communicating with the oceans by canals traversing the land masses would rise) and the mathematical investigation of this problem are treated in the articles Figure of the Earth, and Geodesy; here the results are summarized. Sir Isaac Newton deduced from the mechanical consideration of the figure of equilibrium of a mass of rotating fluid, the form of an oblate spheroid, the ellipticity of a meridian section being 1/231, and the axes in the ratio 230:231. Geodetic measurements by the Cassinis and other French astronomers pointed to a prolate form, but the Newtonian figure was proved to be correct by the measurement of meridional arcs in Peru and Lapland by the expeditions organized by the French Academy of Sciences. More recent work points to an elliptical equatorial section, thus making the earth pear-shaped. The position of the longer axis is somewhat uncertain; it is certainly in Africa, Clarke placing it in longitude 8° 15' W., and Schubert in longitude 41° 4' E.; W. J. Sollas, arguing from terrestrial symmetry, has chosen the position lat. 6° N., long. 28° E., i.e. between Clarke's and Schubert's positions. For the lengths of the axes and the ellipticity of the Earth, see Figure of the Earth.
Table of contents

2. Mass and Density

The earliest scientific investigation on the density and mass of the Earth (the problem is really single if the volume of the Earth be known) was made by Newton, who, mainly from astronomical considerations, suggested the limiting densities 5 and 6; it is remarkable that this prophetic guess should be realized, the mean value from subsequent researches being about 5.2, which gives for the mass the value 6 X io 2 ' tons. The density of the Earth has been determined by several experimenters within recent years by methods described in the article Gravitation; the most probable value is there stated to be 5.527.

3. Astronomical Relations

The grandest achievements of astronomical science are undoubtedly to be associated with the elucidation of the complex motion of our planet. The notion that the Earth was fixed and immovable at the centre of an immeasurable universe long possessed the minds of men; and we find the illustrious Ptolemy accepting this view in the 2nd century A.D.. and rejecting the notion of a rotating Earth - a theory which had been proposed as early as the 5th century B.C. by Philolaus on philosophical grounds, and in the 3rd century B.C. by the astronomer Aristarchus of Samos. He argued that if the Earth rotated then points at the equator had the enormous velocity of about 1000 m. per hour, and as a consequence there should be terrific gales from the east; the fact that there were no such gales invalidated, in his opinion, the theory. The Ptolemaic theory was unchallenged until 1543, in which year the De Revolutionibus orbium Celestium of Copernicus was published. In this work it was shown that the common astronomical phenomena could be more simply explained by regarding the Earth as annually revolving about a fixed Sun, and daily rotating about itself. A clean sweep was made of the geocentric epicyclic motions of the planets which Ptolemy's theory demanded, and in place there was substituted a procession of planets about the Sun at different distances. The development of the Copernican theory - the corner-stone of modern astronomy - by Johann Kepler and Sir Isaac Newton is treated in the article Astronomy: History; here we shall summarily discuss the motions of our planet and its relation to the solar system.
The Earth has two principal motions - revolution about the Sun, rotation about its axis; there are in addition a number of secular motions.


The Earth revolves about the Sun in an elliptical orbit having the Sun at one focus. The plane of the orbit is termed the ecliptic; it is inclined to the Earth's equator at an angle termed the obliquity, and the points of intersection of the equator and ecliptic are termed the equinoctial points. The major axis of the ellipse is the line of apsides; when the Earth is nearest the Sun it is said to be in perihelion, when farthest it is in aphelion. The mean distance of the Earth from the Sun is a most important astronomical constant, since it is the unit of linear measurement; its value is about 93,000,000 m., and the difference between the perihelion and aphelion distances is about 3,000,000 m. The eccentricity of the orbit is o 016751. A tabular comparison of the orbital constants of the Earth and the other planets is given in the article Planet. The period of revolution with regard to the Sun, or, in other words, the time taken by the Sun apparently to pass from one equinox to the same equinox, is the tropical or equinoctial year; its length is 365 d. 5 hrs. 48 m. 46 secs. It is about 20 minutes shorter than the true or sidereal year, which is the time taken for the Sun apparently to travel from one star to it again. The difference in these two years is due to the secular variation termed precession (see below). A third year is named the anomalistic year, which is the time occupied in the passage from perihelion to perihelion; it is a little longer than the sidereal.


The Earth rotates about an axis terminating at the north and south geographical poles, and perpendicular to the equator; the period of rotation is termed the day, of which several kinds are distinguished according to the body or point of reference. The rotation is performed from west to east; this daily rotation occasions the diurnal motion of the celestial sphere, the rising of the Sun and stars in the east and their setting in the west, and also the phenomena of day and night. The inclination of the axis to the ecliptic brings about the presentation of places in different latitudes to the more direct rays of the sun; this is revealed in the variation in the length of daylight with the time of the year, and the phenomena of seasons.
Although the rotation of the Earth was an accepted fact soon after its suggestion by Copernicus, an experimental proof was wanting until 1851, when Foucault performed his celebrated pendulum experiment at the Pantheon, Paris. A pendulum about 200 ft. long, composed of a flexible wire carrying a heavy iron bob, was suspended so as to be free to oscillate in any direction. The bob was provided with a style which passed over a table strewn with fine sand, so that the style traced the direction in which the bob was swinging. It was found that the oscillating pendulum never retraced its path, but at each swing it was apparently deviated to the right, and moreover the deviations in equal times were themselves equal. This means that the floor of the Pantheon was moving, and therefore the Earth was rotating. If the pendulum were swung in the southern hemisphere, the deviation would be to the left; if at the equator it would not deviate, while at the poles the plane of oscillation would traverse a complete circle in 24 hours.
The rotation of the Earth appears to be perfectly uniform, comparisons of the times of transits, eclipses, &c., point to a variation of less than yhth of a second since the time of Ptolemy. Theoretical investigations on the phenomena of tidal friction point, however, to a retardation, which may to some extent be diminished by the accelerations occasioned by the shrinkage of the globe, and some other factors difficult to evaluate (see Tide).
We now proceed to the secular variations.


The axis of the earth does not preserve an invariable direction in space, but in a certain time it describes a cone, in much the same manner as the axis of a top spinning out of the vertical. The equator, which preserves approximately the same inclination to the ecliptic (there is a slight variation in the obliquity which we shall mention later), must move so that its intersections with the ecliptic, or equinoctial points, pass in a retrograde direction, i.e. opposite to that of the Earth. This motion is termed the precession of the equinoxes, and was observed by Hipparchus in the 2nd century B.C.; Ptolemy corrected the catalogue of Hipparchus for precession by adding 2° 40' to the longitudes, the latitudes being unaltered by this motion, which at the present time is 50.26" annually, the complete circuit being made in about 26,000 years. Owing to precession the signs of the zodiac are traversing paths through the constellations, or, in other words, the constellations are continually shifting with regard to the equinoctial points; at one time the vernal equinox Aries was in the constellations of that name; it is now in Pisces, and will then pass into Aquarius. The pole star, i.e. the star towards which the Earth's axis points, is also shifting owing to precession; in about 2700 B.C. the Chinese observed a Draconis as the pole star (at present a Ursae minoris occupies this position and will do so until 3500); in 13600 Vega (a Lyrae) the brightest star in the Northern hemisphere, will be nearest.
Precession is the result of the Sun and the Moon's attraction on the Earth not being a single force through its centre of gravity. If the Earth were a homogeneous sphere the attractions would act through the centre, and such forces would have no effect upon the rotation about the centre of gravity, but the Earth being spheroidal the equatorial band which stands up as it were beyond the surface of a sphere is more strongly attracted, with the result that the axis undergoes a tilting. The precession due to the Sun is termed the solar precession and that due to the Moon the lunar precession; the joint effect (two-thirds of which is due to the Moon) is the luni-solar precession. Solar precession is greatest at the solstices and zero at the equinoxes; the part of luni-solar precession due to the Moon varies with the position of the Moon in its orbit. The obliquity is unchanged by precession (see Precession Of The Equinoxes).


In treating precession we have stated that the axis of the Earth traces a cone, and it follows that the pole describes a circle (approximately) on the celestial sphere, about the pole of the ecliptic. This is not quite true. Irregularities in the attracting forces which occasion precession also cause a slight oscillation backwards and forwards over the mean precessional path of the pole, the pole tracing a wavy line or nodding. Both the Sun and Moon contribute to this effect. Solar nutation depends upon the position of the Sun on the ecliptic; its period is therefore I year, and in extent it is only I. 2"; lunar nutation depends upon the position of the Moon's nodes; its period is therefore about 18.6 years, the time of revolution of the nodes, and its extent is 9. 2". There is also given to the obliquity a small oscillation to and fro. Nutation is one of the great discoveries of James Bradley (1747).

Planetary Precession

So far we have regarded the ecliptic as absolutely fixed, and treated precession as a real motion of the equator. The ecliptic, however, is itself subject to a motion, due to the attractions of the planets on the Earth. This effect also displaces the equinoctial points. Its annual value is 0.13". The term General Precession in longitude is given to the displacement of the intersection of the equator with the apparent ecliptic on the latter. The standard value is 50.2453", which prevailed in 1850, and the value at 1850+t, i.e. the constant of precession, is 5 0.2 453" + 0.0002225" t. This value is also liable to a very small change. The nutation of the obliquity at time 1850 + t is given by the formula 23° 27' 32. o" -0.47" I. Complete expressions for these functions are given in Newcomb's Spherical Astronomy (1908), and in the Nautical Almanac. The variation of the line of apsides is the name given to the motion of the major axis of the Earth's orbit along the ecliptic. It is due to the general influence of the planets, and the revolution is effected in 21,000 years.
The variation of the eccentricity denotes an oscillation of the form of the Earth's orbit between a circle and ellipse. This followed the mathematical researches of Lagrange and Leverrier. It was suggested by Sir John Herschel in 1830 that this variation might occasion great climatic changes, and James Croll developed the theory as affording a solution of the glacial periods in geology.

Variation of Latitude

Another secular motion of the Earth is due to the fact that the axis of rotation is not rigidly fixed within it, but its polar extremities wander in a circle of about 50 ft. diameter. This oscillation brings about a variability in terrestrial latitudes, hence the name. Euler showed mathematically that such an oscillation existed, and, making certain assumptions as to the rigidity of the Earth, deduced that its period was 305 days; S. C. Chandler, from 1890 onwards, deduced from observations of the stars a period of 428 days;. and Simon Newcomb explained the deviation of these periods by pointing out that Euler's assumption of a perfectly rigid Earth is not in accordance with fact. For details of this intricate subject see the articles Latitude and Figure of the Earth.
4. Evolution and Age. - In its earliest history the mass now consolidated as the Earth and Moon was part of a vast nebulous aggregate, which in the course of time formed a central nucleus - our Sun - which shed its outer layers in such a manner as to form the solar system (see Nebular Theory). The moon may have been formed from the Earth in a similar manner, but the theory of tidal friction suggests the elongation of the Earth along an equatorial axis to form a pear-shaped figure, and that in the course of time the protuberance shot off to form the Moon (see Tide). The age of the Earth has been investigated from several directions, as have also associated questions related to climatic changes, internal temperature, orientation of the land and water (permanence of oceans and continents), &c. These problems are treated in the articles Geology and Geography.
Figure of the Earth. The determination of the figure of the earth is a problem of the highest importance in astronomy, inasmuch as the diameter of the earth is the unit to which all celestial distances must be referred.
Historical. Reasoning from the uniform level appearance of the horizon, the variations in altitude of the circumpolar stars as one travels towards the north or south, the disappearance of a ship standing out to sea, and perhaps other phenomena, the earliest astronomers regarded the earth as a sphere, and they endeavoured to ascertain its dimensions. Aristotle relates that the mathematicians had found the circumference to be 400,000 stadia (about 46,000 miles). But Eratosthenes (c. 250 B.C.) appears to have been the first who entertained an accurate idea of the principles on which the determination of the figure of the earth really depends, and attempted to reduce them to practice. His results were very inaccurate, but his method is the same as that which is followed at the present day - depending, in fact,on the comparison of a line measured on the earth's surface with the corresponding arc of the heavens. He observed that at Syene in Upper Egypt, on the day of the summer solstice, the sun was exactly vertical, whilst at Alexandria at the same season of the year its zenith distance was 7° 12', or one-fiftieth of the circumference of a circle. He assumed that these places were on the same meridian; and, reckoning their distance apart as 5000 stadia, he inferred that the circumference of the earth was 250,000 stadia (about 29,000 miles). A similar attempt was made by Posidonius, who adopted a method which differed from that of Eratosthenes only in using a star instead of the sun. He obtained 240,000 stadia (about 27,600 miles) for the circumference. Ptolemy in his Geography assigns the length of the degree as Soo stadia.
The Arabs also investigated the question of the earth's magnitude. The caliph Abdallah al Mamun (A.D. 814), having fixed on a spot in the plains of Mesopotamia, despatched one company of astronomers northwards and another southwards, measuring the journey by rods, until each found the altitude of the pole to have changed one degree. But the result of this measurement does not appear to have been very satisfactory. From this time the subject seems to have attracted no attention until about 1500, when Jean Fernel (1497-1558), a Frenchman, measured a distance in the direction of the meridian near Paris by counting the number of revolutions of the wheel of a carriage. His astronomical observations were made with a triangle used as a quadrant, and his resulting length of a degree was very near the truth.
Willebrord Snell substituted a chain of triangles for actual linear measurement. He measured his base line on the frozen surface of the meadows near Leiden, and measured the angles of his triangles, which lay between Alkmaar and Bergen-op-Zoom, with a quadrant and semicircles. He took the precaution of Eratosthenes Batavus, seu de terrae ambitus ver y quantitate suscitatus, a Willebrordo Snellio, Lugduni-Batavorum X1617). VIII. 26 comparing his standard with that of the French, so that his result was expressed in toises (the length of the toise is about 6.39 English ft.). The work was recomputed and reobserved by P. von Musschenbroek in 1729. In 1637 an Englishman, Richard Norwood, published a determination of the figure of the earth in a volume entitled The Seaman's Practice, contayning a Fundamentall Probleme in Navigation experimentally verified, namely, touching the Compasse of the Earth and Sea and the quantity of a Degree in our English Measures. He observed on the I rth of June 1633 the sun's meridian altitude in London as 62° 1', and on the 6th of June 1635, his meridian altitude in York as 59° 33'. He measured the distance between these places partly with a chain and partly by pacing. By this means, through compensation of errors, he arrived at 367,176 ft. for the degree - a very fair result.
The application of the telescope to angular instruments was the next important step. Jean Picard was the first who in 1669, with the telescope, using such precautions as the nature of the operation requires, measured an arc of meridian. He measured with wooden rods a base line of 5663 toises, and a second or base of verification of 3902 toises; his triangulation extended from Malvoisine, near Paris, to Sourdon, near Amiens. The angles of the triangles were measured with a quadrant furnished with a telescope having cross-wires. The difference of latitude of the terminal stations was determined by observations made with a sector on a star in Cassiopeia, giving 1° 22' S5" for the amplitude. The terrestrial measurement gave 78,850 toises, whence he inferred for the length of the degree 57,060 toises.
Hitherto geodetic observations had been confined to the determination of the magnitude of the earth considered as a sphere, but a discovery made by Jean Richer (d. 1696) turned the attention of mathematicians to its deviation from a spherical form. This astronomer, having been sent by the Academy of Sciences of Paris to the island of Cayenne, in South America, for the purpose of investigating the amount of astronomical refraction and other astronomical objects, observed that his clock, which had been regulated at Paris to beat seconds, lost about two minutes and a half daily at Cayenne, and that in order to bring it to measure mean solar time it was necessary to shorten the pendulum by more than a line (about 2th of an in.). This fact, which was scarcely credited till it had been confirmed by the subsequent observations of Varin and Deshayes on the coasts of Africa and America, was first explained in the third book of Newton's Principia, who showed that it could only be referred to a diminution of gravity arising either from a protuberance of the equatorial parts of the earth and consequent increase of the distance from the centre, or from the counteracting effect of the centrifugal force. About the same time (1673) appeared Christian Huygens' De Horologio Oscillatorio, in which for the first time were found correct notions on the subject of centrifugal force. It does not, however, appear that they were applied to the theoretical investigation of the figure of the earth before the publication of Newton's Principia. In 1690 Huygens published his De Causa Gravitatis, which contains an investigation of the figure of the earth on the supposition that the attraction of every particle is towards the centre.
Between 1684 and 1718 J. and D. Cassini, starting from Picard's base, carried a triangulation northwards from Paris to Dunkirk and southwards from Paris to Collioure. They measured a base of 7246 toises near Perpignan, and a somewhat shorter base near Dunkirk; and from the northern portion of the arc, which had an amplitude of 2° 12' 9", obtained for the length of a degree 56,960 toises; while from the southern portion, of which the amplitude was 6° 18' 57", they obtained 57,097 toises. The immediate inference from this was that, the degree diminishing with increasing latitude, the earth must be a prolate spheroid. This conclusion was totally opposed to the theoretical investigations of Newton and Huygens, and accordingly the Academy of Sciences of Paris determined to apply a decisive test by the measurement of arcs at a great distance from each other - one in the neighbourhood of the equator, the other in a high latitude. Thus arose the celebrated expeditions of the French academicians. In May 1735 Louis Godin, Pierre Bouguer and Charles Marie de la Condamine, under the auspices of Louis XV., proceeded to Peru, where, assisted by two Spanish officers, after ten years of laborious exertion, they measured an arc of 3° 7', the northern end near the equator. The second party consisted of Pierre Louis Moreau de Maupertuis, Alexis Claude Clairault, Charles Etienne Louis Camus, Pierre Charles Lemonnier, and Reginaud Outhier, who reached the Gulf of Bothnia in July 1736; they were in some respects more fortunate than the first party, inasmuch as they completed the measurement of an arc near the polar circle of 57' amplitude and returned within sixteen months from the date of their departure.
The measurement of Bouguer and De la Condamine was executed with great care, and on account of the locality, as well as the manner in which all the details were conducted, it has always been regarded as a most valuable determination. The southern limit was at Tarqui, the northern at Cotchesqui. A base of 6272 toises was measured in the vicinity of Quito, near the northern extremity of the arc, and a second base of 5260 toises near the southern extremity. The mountainous nature of the country made the work very laborious, in some cases the difference of heights of two neighbouring stations exceeding 1 mile; and they had much trouble with their instruments, those with which they were to determine the latitudes proving untrustworthy. But they succeeded by simultaneous observations of the same star at the two extremities of the arc in obtaining very fair results. The whole length of the arc amounted to 176,945 toises, while the difference of latitudes was 3° 7' 3". In consequence of a misunderstanding that arose between De la Condamine and Bouguer, their operations were conducted separately, and each wrote a full account of the expedition. Bouguer's book was published in 1749; that of De la Condamine in 1751. The toise used in this measure was afterwards regarded as the standard toise, and is always referred to as the Toise of Peru. The party of Maupertuis, though their work was quickly despatched, had also to contend with great difficulties. Not being able to make use of the small islands in the Gulf of Bothnia for the trigonometrical stations, they were forced to penetrate into the forests of Lapland, commencing operations at Tornea, a city situated on the mainland near the extremity of the gulf. From this, the southern extremity of their arc, they carried a chain of triangles northward to the mountain Kittis, which they selected as the northern terminus. The latitudes were determined by observations with a sector (made by George Graham) of the zenith distance of a and b Draconis. The base line was measured on the frozen surface of the river Tornea about the middle of the arc; two parties measured it separately, and they differed by about 4 in. The result of the whole was that the difference of latitudes of the terminal stations was 57' 29". 6, and the length of the arc 55,023 toises. In this expedition, as well as in that to Peru, observations were made with a pendulum to determine the force of gravity; and these observations coincided with the geodetic results in proving that the earth was an oblate and not prolate spheroid.
In 1740 was published in the Paris Memoires an account, by Cassini de Thury, of a remeasurement by himself and Nicolas Louis de Lacaille of the meridian of Paris. With a view to determine more accurately the variation of the degree along the meridian, they divided the distance from Dunkirk to Collioure into four partial arcs of about two degrees each, by observing the latitude at five stations. The results previously obtained by J. and D. Cassini were not confirmed, but, on the contrary, the length of the degree derived from these partial arcs showed on the whole an increase with an increasing latitude. Cassini and Lacaille also measured an arc of parallel across the mouth of the Rhone. The difference of time of the extremities was determined by the observers at either end noting the instant of a signal given by flashing gunpowder at a point near the middle of the arc.
While at the Cape of Good Hope in 1752, engaged in various astronomical observations, Lacaille measured an arc of meridian of 1° 13' 17", which gave him for the length of the degree 57,037 toises - an unexpected result, which has led to the remeasurement of the arc by Sir Thomas Maclear (see Geodesy).
Passing over the measurements made between Rome and Rimini and on the plains of Piedmont by the Jesuits Ruggiero Giuseppe Boscovich and Giovanni Battista Beccaria, and also the arc measured with deal rods in North America by Charles Mason and Jeremiah Dixon, we come to the commencement of the English triangulation. In 1783, in consequence of a representation from Cassini de Thury on the advantages that would accrue from the geodetic connexion of Paris and Greenwich, General William Roy was, with the king's approval, appointed by the Royal Society to conduct the operations on the part of England, Count Cassini, Mechain and Delambre being appointed on the French side. A precision previously unknown was attained by the use of Ramsden's theodolite, which was the first to make the spherical excess of triangles measurable. The wooden rods with which the first base was measured were replaced by glass rods, which were afterwards rejected for the steel chain of Ramsden. (For further details see Account of the Trigonometrical Survey of England and Wales.) Shortly after this, the National Convention of France, having agreed to remodel their system of weights and measures, chose for their unit of length the ten-millionth part of the meridian quadrant. In order to obtain this length precisely, the remeasurement of the French meridian was resolved on, and deputed to J. B. J. Delambre and Pierre Francois Andre Mechain. The details of this operation will be found in the Base du systeme metrique decimale. The arc was subsequently extended by Jean Baptiste Biot and Dominique Francois Jean Arago to the island of Iviza. Operations for the connexion of England with the continent of Europe were resumed in 1821 to 1823 by Henry Kater and Thomas Frederick Colby on the English side, and F. J. D. Arago and Claude Louis Mathieu on the French.
The publication in 1838 of Friedrich Wilhelm Bessel's Gradmessung in Ostpreussen marks an era in the science of geodesy. Here we find the method of least squares applied to the calculation of a network of triangles and the reduction of the observations generally. The systematic manner in which all the observations were taken with the view of securing final results of extreme accuracy is admirable. The triangulation, which was a small one, extended about a degree and a half along the shores of the Baltic in a N.N.E. direction. The angles were observed with theodolites of 12 and 15 in. diameter, and the latitudes determined by means of the transit instrument in the prime vertical - a method much used in Germany. (The base apparatus is described in the article Geodesy.) The principal triangulation of Great Britain and Ireland, which was commenced in 1783 under General Roy, for the more immediate purpose of connecting the observatories of Greenwich and Paris, had been gradually extended, under the successive direction of Colonel E. Williams, General W. Mudge, General T. F. Colby, Colonel L. A. Hall, and Colonel Sir Henry James; it was finished in 1851. The number of stations is about 250. At 32 of these the latitudes were determined with Ramsden's and Airy's zenith sectors. The theodolites used for this work were, in addition to the two great theodolites of Ramsden which were used by General Roy and Captain Kater, a smaller theodolite of 18 in. diameter by the same mechanician, and another of 24 in. diameter by Messrs Troughton and Simms. Observations for determination of absolute azimuth were made with those instruments at a large number of stations; the stars a, b, and X Ursae Minoris and 51 Cephei being those observed always at the greatest azimuths. At six of these stations the probable error of the result is under 0.4", at twelve under 0.5", at thirty-four under 0.7": so that the absolute azimuth of the whole network is determined with extreme accuracy. Of the seven base lines which have been measured, five were by means of steel chains and two with Colby's compensation bars (see Geodesy). The triangulation was computed by least squares. The total number of equations of condition for the triangulation is 920; if therefore the whole had been reduced in one mass, as it should have been, the solution of an equation of 920 unknown quantities would have occurred as a part of the work. To avoid this an approximation was resorted to; the triangulation was divided into twenty-one parts or figures; four of these, not adjacent, were first adjusted by the method explained, and the corrections thus determined in these figures carried into the equations of condition of the adjacent figures. The average number of equations in a figure is 44; the largest equation is one of 77 unknown quantities. The vertical limb of Airy's zenith sector is read by four microscopes, and in the complete observation of a star there are 10 micrometer readings and 12 level readings. The instrument is portable; and a complete determination of latitude, affected with the mean of the declination errors of two stars, is effected by two micrometer readings and four level readings. The observation consists in measuring with the telescope micrometer the difference of zenith distances of two stars which cross the meridian, one to the north and the other to the south of the observer at zenith distances which differ by not much more than io' or is', the interval of the times of transit being not less than one nor more than twenty minutes. The advantages are that, with simplicity in the construction of the instrument and facility in the manipulation, refraction is eliminated (or nearly so, as the stars are generally selected within 25° of the zenith), and there is no large divided circle. The telescope, which is counterpoised on one side of the vertical axis, has a small circle for finding, and there is also a small horizontal circle. This instrument is universally used in American geodesy.
The principal work containing the methods and results of these operations was published in 1858 with the title Ordnance Trigonometrical Survey of Great Britain and Ireland. Account of the observations and calculations of the principal triangulation and of the figure, dimensions and mean specific gravity of the earth as derived therefrom. Drawn up by Captain Alexander Ross Clarke, R.E., F.R.A.S., under the direction of Lieut.-Colonel H. James, R.E., F.R.S., M.R.I.A., &c. A supplement appeared in 1862: Extension of the Triangulation of the Ordnance Survey into France and Belgium, with the measurement of an arc of parallel in 52° N. from Valentia in Ireland to Mount Kemmel in Belgium. Published by. Col. Sir Henry James. Extensive operations for surveying India and determining the figure of the earth were commenced in 1800. Colonel W. Lambton started the great meridian arc at Punnae in latitude 8° 9', and, following generally the methods of the English survey, he carried his triangulation as far north as 20° 30'. The work was continued by Sir George (then Captain) Everest, who carried it to the latitude of 29° 30'. Two admirable volumes by Sir George Everest, published in 1830 and in 1847, give the details of this undertaking. The survey was afterwards prosecuted by Colonel T. T. Walker, R.F., who made valuable contributions to geodesy. The working out of the Indian chains of triangle by the method of least squares presents peculiar difficulties, but, enormous in extent as the work was, it has been thoroughly carried out. The ten base lines on which the survey depends were measured with Colby's compensation bars.
The survey is detailed in eighteen volumes, published at Dehra Dun, and entitled Account of the Operations of the Great Trigonometrical Survey of India. Of these the first nine were published under the direction of Colonel Walker; and the remainder by Colonels Strahan and St G. C. Gore, Major S. G. Burrard and others. Vol. i., 1870, treats of the base lines; vol. ii., 1879, history and general de3criptions of the principal triangulation and of its reduction; vol. v., 1879, pendulum operations (Captains T. P. Basevi and W. T. Heaviside); vols. xi., 1890, and xviii., 5906, latitudes; vols. ix., 1883, x., 1887, xv., 1893, longitudes; vol. xvii., 5905, the Indo-European longitude-arcs from Karachi to Greenwich. The other volumes contain the triangulations.
In 1860 Friedrich Georg Wilhelm Struve published his Arc du meridien de 25° 20' entre le Danube et la Mer Glaciale mesure depuis 1816 jusqu'en 1855. The latitudes of the thirteen astronomical stations of this arc were determined partly with vertical circles and partly by means of the transit instrument in the prime vertical. The triangulation, a great part of which, however, is a simple chain of triangles, is reduced by the method of least squares, and the probable errors of the resulting distances of parallels is given; the probable error of the whole arc in length is 6.2 toises. Ten base lines were measured. The sum of the lengths of the ten measured bases is 29,863 toises, so that the average length of a base line is 19,100 ft. The azimuths were observed at fourteen stations. In high latitudes the determination of the meridian is a matter of great difficulty; nevertheless the azimuths at all the northern stations were successfully determined, - the probable error of the result at Fuglenaes being o".
Before proceeding with the modern developments of geodetic measurements and their application to the figure of the earth, we must discuss the " mechanical theory," which is indispensable for a full understanding of the subject.
Mechanical Theory. Newton, by applying his theory of gravitation, combined with the so-called centrifugal force, to the earth, and assuming that an oblate ellipsoid of rotation is a form of equilibrium for a homogeneous fluid rotating with uniform angular velocity, obtained the ratio of the axes 229:230, and the law of variation of gravity on the surface. A few years later Huygens published an investigation of the figure of the earth, supposing the attraction of every particle to be towards the centre of the earth, obtaining as a result that the proportion of the axes should be 578:579. In 1740 Colin Maclaurin, in his De causa physica fluxus et refluxus maris, demonstrated that the oblate ellipsoid of revolution is a figure which satisfies the conditions of equilibrium in the case of a revolving homogeneous fluid mass, whose particles attract one another according to the law of the inverse square of the distance; he gave the equation connecting the ellipticity with the proportion of the centrifugal force at the equator to gravity, and determined the attraction on a particle situated anywhere on the surface of such a body. In 1743 Clairault published his Theorie de la figure de la terre, which contains a remarkable theorem ("Clairault's Theorem"), establishing a relation between the ellipticity of the earth and the variation of gravity from the equator to the poles. Assuming that the earth is composed of concentric ellipsoidal strata having a common axis of rotation, each stratum homogeneous in itself, but the ellipticities and densities of the successive strata varying according to any law, and that the superficial stratum has the same form as if it were fluid, he proved that _g +e= 2 m, g where g, g' are the amounts of gravity at the equator and at the pole respectively, e the ellipticity of the meridian (or " flattening "), and m the ratio of the centrifugal force at the equator to g. He also proved that the increase of gravity in proceeding from the equator to the poles is as the square of the sine of the latitude. This, taken with the former theorem, gives the means of determining the earth's ellipticity from observation of the relative force of gravity at any two places. P. S. Laplace, who devoted much attention to the subject, remarks on Clairault's work that " the importance of all his results and the elegance with which they are presented place this work amongst the most beautiful of mathematical productions " (Isaac Todhunter's History of the Mathematical Theories of Attraction and the Figure of the Earth, vol. i. p. 229).
The problem of the figure of the earth treated as a question of mechanics or hydrostatics is one of great difficulty, and it would be quite impracticable but for the circumstance that the surface differs but little from a sphere. In order to express the forces at any point of the body arising from the attraction of its particles, the form of the surface is required, but this form is the very one which it is the object of the investigation to discover; hence the complexity of the subject, and even with all the present resources of mathematicians only a partial and imperfect solution can be obtained.
We may here briefly indicate the line of reasoning by which some of the most important results may be obtained. If X, Y, Z be the components parallel to three rectangular axes of the forces acting on a particle of a fluid mass at the point x, y, z, then, p being the pressure there, and p the density, dp = p(Xdx+Ydy+Zdz) and for equilibrium the necessary conditions are, that p(Xdx+ Ydy+Zdz) be a complete differential, and at the free surface Xdx+ Ydy+Zdz=o. This equation implies that the resultant of the forces is normal to the surface at every point, and in a homogeneous fluid it is obviously the differential equation of all surfaces of equal pressure. If the fluid be heterogeneous then it is to be remarked that for forces of attraction according to the ordinary law of gravitation, if X, Y, Z be the components of the attraction of a mass whose potential is V, then Xdx+Ydy+Zdz = dV dV dx dx d-Ty - dy + dz dz, which is a complete differential. And in the case of a fluid rotating with uniform velocity, in which the so-called centrifugal force enters as a force acting on each particle proportional to its distance from the axis of rotation, the corresponding part of Xdx+Ydy+Zdz is obviously a complete differential. Therefore for the forces with which we are now concerned Xdx+Ydy+Zdz=dU, where U is some function of x, y, z, and it is necessary for equilibrium that dp = pdU be a complete differential; that is, p must be a function of U or a function of p, and so also p a function of U. So that dU=0 is the differential equation of surfaces of equal pressure and density.
We may now show that a homogeneous fluid mass in the form of an oblate ellipsoid of revolution having a uniform velocity of rotation can be in equilibrium. It may be proved that the attraction of the ellipsoid x 2 +y 2 +z 2 (1+E 2) =c 2 (I+E 2) upon a particle P of its mass at x, y, z has for components X = - Ax, Y = - Ay, Z = - Cz, where 1 +E2 I) A p / tan-le, 2 /1+€2 1 1 C =4?rk p 0 tan and k 2 the constant of attraction. Besides the attraction of the mass of the ellipsoid, the centrifugal force at P has for components d-xw 2, +yw 2 , o; then the condition of fluid equilibrium is (A - w 2) xdx+ (A - w 2) ydy+Czdz =0, which by integration gives (A - w 2) (x 2 +y 2) +Cz 2 = constant.
This is the equation of an ellipsoid of rotation, and therefore the equilibrium is possible. The equation coincides with that of the surface of the fluid mass if we make A - w2= C /(1 +€2), which gives k2p '3' E 2 In the case of the earth, which is nearly spherical, we obtain by expanding the expression for 3) 2 in powers of E 2, rejecting the higher powers, and remarking that the ellipticity e= w 2 /27rk 2 p =4E 2 /15 = 8e/15.
Now if m be the ratio of the centrifugal force to the intensity of gravity at the equator, and a = c (1 +e), then m =aw 2 /31rk 2 pa, . . 0/27rk 2 p = m.
In the case of the earth it is a matter of observation that m =1/289, hence the ellipticity e=5m/4=1/231, so that the ratio of the axes on the supposition of a homogeneous fluid earth is 230:231, as stated by Newton.
Now, to come to the case of a heterogeneous fluid, we shall assume that its surfaces of equal density are spheroids, concentric and having a common axis of rotation, and that the ellipticity of these surfaces varies from the centre to the outer surface, the density also varying. In other words, the body is composed of homogeneous spheroidal shells of variable density and ellipticity. On this supposition we shall express the attraction of the mass upon a particle in its interior, and then, taking into account the centrifugal force, form the equation expressing the condition of fluid equilibrium. The attraction of the homogeneous spheroid x 2+y2+z2 (1 +2e) = c 2 (1 +2e), where e is the ellipticity (of which the square is neglected), on an internal particle, whose co-ordinates are x = f, y =o, z = h, has for its x and z components X'= pf(1 - fe), Z'= - 431rk2ph(1+5e), the Y component being of course zero. Hence we infer that the attraction of a shell whose inner surface has an ellipticity e, and its outer surface an ellipticity e+de, the density being p, is expressed by dX' = i rk 2 pf de, dZ' = - s - irk 2 ph de. To apply this to our heterogeneous spheroid; if we put c 1 for the semiaxis of that surface of equal density on which is situated the attracted point P, and co for the semiaxis of the outer surface, the attraction of that portion of the body which is exterior to P, namely, of all the shells which enclose P, has for components X o = i i rk2 f j °l op e d c, Zo = - iIirk 2 h j Cl o p-dc, both e and p being functions of c. Again the attraction of a homogeneous spheroid of density p on an external point f, h has the components X" = - 3,rk 2 p fr 3 {c 3 (I +2e) - Xec 5 l, Z" = - 3,rk 2 phr 3 {c 3 (I +2e) - A'ec 5 }, where A= t (4 h2 - f 2) /r 4 , v= 5(Z h 2 - 3f 2)/ r4, and r2=f +h2. Now e being considered a function of c, we can at once express the attraction of a shell (density p) contained between the surface defined by c+dc, e+de and that defined by c, e upon an external point; the differentials with respect to c, viz. dX" dZ", must then be integrated with p under the integral sign as being a function of c. The integration will extend from c = o to c = c l . Thus the components of the attraction of the heterogeneous spheroid upon a particle within its mass, whose co-ordinates are f, o, h, are ? fc X=-3,rk2f y3Jo lp d{c 3 (I+2e)} - y10l pd(eG5) s cl o pde Z=-3,rk'h d{c3(I+2e))-3pd(eG5) +f pde]. We take into account the rotation of the earth by adding the centrifugal force fw 2 = F to X. Now, the surface of constant density upon which the point f, o, h is situated gives (I - 2e) fdf+hdh= o; and the condition of equilibrium is that (X +F)df +Zdh = o. Therefore, (X+F)h=Zf (I - 2e), which, neglecting small quantities of the order e 2 and putting w't 2 = 4,r2k2, gives 2 e c1 3 6 Cc' 5 6% 3,r y3 o pd{c (I-}-2e)) - 5r5Jo pd(ec5) SJci pde= 2 Here we must now put c for c1, c for r; and I +2e under the first integral sign may be replaced by unity, since small quantities of the second order are neglected. Two differentiations lead us to the following very important differential equation (Clairault): d 2 e 2pc 2 de (2 pc 6) dc 2+ fpc 2 dc dc + f pc' dc c2 'e =o.
When p is expressed in terms of c, this equation can be integrated. We infer then that a rotating spheroid of very small ellipticity, composed of fluid homogeneous strata such as we have specified, will be in equilibrium; and when the law of the density is expressed, the law of the corresponding ellipticities will follow.
If we put M for the mass of the spheroid, then M;andm= e 3 t 472 and putting c=co in the equation expressing the condition of equilibrium, we find M(2e - m) =3,r.5 c 2 o pd(ec5). Making these substitutions in the expressions for the forces at the surface, and putting r/c = I +e - e(h/c)2, we get G sin = ak2 I (52-m-2e) h2 S h. Here G is gravity in the latitude 4,, and a the radius of the equator. Since G= Mksec 4, = (c /f) {' + e + ( eh '/ c2)), t) 13 -m+ jm - e) sin e 4, an expression which contains the theorems we have referred to as discovered by Clairault.
The theory of the figure of the earth as a rotating ellipsoid has been especially investigated by Laplace in his Mecanique celeste. The principal English works are: - Sir George Airy, Mathematical Tracts, a lucid treatment without the use of Laplace's coefficients; Archdeacon Pratt's Attractions and Figure of the Earth; and O'Brien's Mathematical Tracts; in the last two Laplace's coefficients are used.
In 1845 Sir G. G. Stokes (Camb. Trans. viii.; see also Camb. Dub. Math. 'Journ., ' 1849, iv.) proved that if the external form of the sea - imagined to percolate the land by canals - be a spheroid with small ellipticity, then the law of gravity is that which we have shown above; his proof required no assumption as to the ellipticity of the internal strata, or as to the past or present fluidity of the earth. This investigation admits of being regarded conversely, viz. as determining the elliptical form of the earth from measurements of gravity; if G, the observed value of gravity in latitude 4), be expressed in the form G = g(sine 0), where g is the value at the equator and a coefficient. In this investigation, the square and higher powers of the ellipticity are neglected; the solution was completed by F. R. Helmert with regard to the square of the ellipticity, who showed that a term with sin 2 24) appeared (see Helmert, Geodasie, ii. 83). For the coefficient of this term, the gravity measurements give a small but not sufficiently certain value; we therefore assume a value which agrees best with the hypothesis of the fluid state of the entire earth; this assumption is well supported, since even at a depth of only 50 km. the pressure of the superincumbent crust is so great that rocks become plastic, and behave approximately as fluids, and consequently the crust of the earth floats, to some extent, on the interior (even though this may not be fluid in the usual sense of the word). This is the geological theory of " Isostasis " (cf. Geology); it agrees with the results of measurements of gravity (vide infra), and was brought forward in the middle of the 19th century by J. H. Pratt, who deduced it from observations made in India.
The sin 2 24) term in the expression for G, and the corresponding deviation of the meridian from an ellipse, have been analytically established by Sir G. H. Darwin and E. Wiechert; earlier and less complete investigations were made by Sir G. B. Airy and O. Callandreau. In consequence of the sin 2 24) term, two parameters of the level surfaces in the interior of the earth are to be determined; for this purpose, Darwin develops two differential equations in the place of the one by Clairault. By assuming Roche's law for the variation of the density in the interior of the Earth, viz. p = p i - k (c/c,) 2, k being a coefficient, it is shown that in latitude 45°, the meridian is depressed about 34 metres from the ellipse, and the coefficient of the term sin'4) cos' 0(=1 sin224)) is - (3 0000295. According to Wiechert the earth is composed of a kernel and a shell, the kernel being composed of material, chiefly metallic iron, of density near 8.2, and the shell, about 900 miles thick, of silicates, &c., of density about 3.2. On this assumption the depression in latitude 45° is 24 metres, and the coefficient of sin e 4) cos 2 4) is, in round numbers, - o 0000280.1 To this additional term in the formula for G, there corresponds an extension of Clairault's formula for the calculation of the flattening from (3 with terms of the higher orders; this was first accomplished by Helmert.
For a long time the assumption of an ellipsoid with three unequal axes has been held possible for the figure of the earth, in consequence of an important theorem due to K. G. Jacobi, who proved that for a homogeneous fluid in rotation a spheroid is not the only form of equilibrium; an ellipsoid rotating round its least axis may with certain proportions of the axes and a certain time of revolution be a form of equilibrium. 2 It has been objected to the figure of three unequal axes that it does not satisfy, in the proportions of the axes, the conditions brought out in Jacobi's theorem (c: a<11 1 2). Admitting this, it has to be noted, on the other hand, that Jacobi's theorem contemplates a homogeneous fluid, and this is certainly far from the actual condition of our globe; indeed the irregular distribution of continents and oceans suggests the possibility of a sensible divergence from a perfect surface of revolution. We may, however, assume the ellipsoid with three unequal axes to be an interpolation form. More plausible forms are little adapted for computation.' Consequently we now generally take the ellipsoid of rotation as a basis, especially so because measurements of gravity have shown that the deviation from it is but trifling.
Local Attraction. In speaking of the figure of the earth, we mean the surface of the sea imagined to percolate the continents by canals. That
1 O. Callendreau, "Memoire sur la theorie de la figure des planetes," Ann. obs. de Paris (1889); G. H. Darwin, "The Theory of the Figure of the Earth carried to the Second Order of Small Quantities," Mon. Not. R.A.S., 1899; E. Wiechert, "Über die Massenverteilung im Innern der Erde," Nach. d. hon. G. d. W. zu Gött., 1897. 2 See I. Todhunter, Proc. Roy. Soc., 1870. 3 J. H. Jeans, "On the Vibrations and Stability of a Gravitating Planet," Proc. Roy. Soc. vol. 71; G. H. Darwin, "On the Figure and Stability of a liquid Satellite," Phil. Trans. 206, p. 161; A. E. H. Love, "The Gravitational Stability of the Earth," Phil. Trans. 207, p. 237; Proc. Roy. Soc. vol. 80.
G cos? = 2 '-' - e- 2m+ ( 2m -2e) c c this surface should turn out, after precise measurements, to be exactly an ellipsoid of revolution is a priori improbable. Although it may be highly probable that originally the earth was a fluid mass, yet in the cooling whereby the present crust has resulted, the actual solid surface has been left most irregular in form. It is clear that these irregularities of the visible surface must be accompanied by irregularities in the mathematical figure of the earth, and when we consider the general surface of our globe, its irregular distribution of mountain masses, continents, with oceans and islands, we are prepared to admit that the earth may not be precisely any surface of revolution. Nevertheless, there must exist some spheroid which agrees very closely with the mathematical figure of the earth, and has the same axis of rotation. We must conceive this figure as exhibiting slight departures from the spheroid, the two surfaces cutting one another in various lines; thus a point of the surface is defined by its latitude, longitude, and its height above the " spheroid of reference." Calling this height N, then of the actual magnitude of this quantity we can generally have no information, it only obtrudes itself on our notice by its variations. In the vicinity of mountains it may change sign in the space of a few miles; N being regarded as a function of the latitude and longitude, if its differential coefficient with respect to the former be zero at a certain point, the normals to the two surfaces then will lie in the prime vertical; if the differential coefficient of N with respect to the longitude be zero, the two normals will lie in the meridian; if both coefficients are zero, the normals will coincide. The comparisons of terrestrial measurements with the corresponding astronomical observations have always been accompanied with discrepancies. Suppose A and B to be two trigonometrical stations, and that at A there is a disturbing force drawing the vertical through an angle S, then it is evident that the apparent zenith of A will be really that of some other place A', whose distance from A is rS, when r is the earth's radius; and similarly if there be a disturbance at B of the amount S', the apparent zenith of B will be really that of some other place B', whose distance from B is re'. Hence we have the discrepancy that, while the geodetic measurements deal with the points A and B, the astronomical observations belong to the points A', B'. Should S, S' be equal and parallel, the displacements AA', BB' will be equal and parallel, and no discrepancy will appear. The non-recognition of this circumstance often led to much perplexity in the early history of geodesy. Suppose that, through the unknown variations of N, the probable error of an observed latitude (that is, the angle between the normal to the mathematical surface of the earth at the given point and that of the corresponding point on the spheroid of reference) be e, then if we compare two arcs of a degree each in mean latitudes, and near each other, say about five degrees of latitude apart, the probable error of the resulting value of the ellipticity will be approximately o 5 o oe, e being expressed in seconds, so that if e be so great as 2" the probable error of the resulting ellipticity will be greater than the ellipticity itself.
It is necessary at times to calculate the attraction of a mountain, and the consequent disturbance of the astronomical zenith, at any point within its influence. The deflection of the plumb-line, caused by a local attraction whose amount is k2A3, is measured by the ratio of k 2 A3 to the force of gravity at the station. Expressed in seconds, the deflection A is A= 12 " 447AS/p, where p is the mean density of the earth, S that of the attracting mass, and A = fs3 xdv, in which dv is a volume element of the attracting mass within the distance s from the point of deflection, and x the projection of s on the horizontal plane through this point, the linear unit in expressing A being a mile. Suppose, for instance, a table-land whose form is a rectangle of 12 miles by 8 miles, having a height of soo ft. and density half that of the earth; let the observer be 2 miles distant from the middle point of the longer side. The deflection then is I" 472; but at z mile it increases to 2" 20.
At sixteen astronomical stations in the English survey the disturbance of latitude due to the form of the ground has been computed, and the following will give an idea of the results. At six stations the deflection is under 2", at six others it is between 2" and 4", and at four stations it exceeds 4". There is one very exceptional station on the north coast of Banffshire, near the village of Portsoy, at which the deflection amounts to 10", so that if that village were placed on a map in a position to correspond with its astronomical latitude, it would be 1000 ft. out of position! There is the sea to the north and an undulating country to the south, which, however, to a spectator at the station does not suggest any great disturbance of gravity. A somewhat rough estimate of the local attraction from external causes gives a maximum limit of 5", therefore we have 5" which must arise from unequal density in the underlying strata in the surrounding country. In order to throw light on this remarkable phenomenon, the latitudes of a number of stations between Nairn on the west, Fraserburgh on the east, and the Grampians on the south, were observed, and the local deflections determined. It is somewhat singular that the deflections diminish in all directions, not very regularly certainly, and most slowly in a southwest direction, finally disappearing, and leaving the maximum at the original station at Portsoy.
The method employed by Dr C. Hutton for computing the attraction of masses of ground is so simple and effectual that it can hardly be improved on. Let a horizontal plane pass through the given station; let r, 0 be the polar co-ordinates of any point in this plane, and r, 0, z, the co-ordinates of a particle of the attracting mass; and let it be required to find the attraction of a portion of the mass contained between the horizontal planes z= o, z = h, the cylindrical surfaces r = r, r = r2, and the vertical planes 0=0 1 ,0=0 2 . The component of the attraction at the station or origin along the line is e h r 2 cos0 k2 (82 f 2 of o (r2 + z2)3 dr dO dz J J =k 2 Sh (sin 02 - sin 0 1) log {r2+(rz +h 2) 1/2 /r i + (r 1 2 +h 2) By taking r 2 - r 1, sufficiently small, and supposing la also small compared with r 1 +r 2 (as it usually is), the attraction is k 2 3(r 2 - r i) (sin 02 - sin 0,) hir, where r=1 (r i d-r 2). This form suggests the following procedure. Draw on the contoured map a series of equidistant circles, concentric with the station, intersected by radial lines so disposed that the sines of their azimuths are in arithmetical progression. Then, having estimated from the map the mean heights of the various compartments, the calculation is obvious.
In mountainous countries, as near the Alps and in the Caucasus, deflections have been observed to the amount of as much as 30", while in the Himalayas deflections amounting to 60" were observed. On the other hand, deflections have been observed in flat countries, such as that noted by Professor K. G. Schweizer, who has shown that, at certain stations in the vicinity of Moscow, within a distance of 16 miles the plumb-line varies 16" in such a manner as to indicate a vast deficiency of matter in the underlying strata; deflections of 20" were observed in the level regions of north Germany.
Since the attraction of a mountain mass is expressed as a numerical multiple of S :p the ratio of the density of the mountain to that of the earth, if we have any independent means of ascertaining the amount of the deflection, we have at once the ratio p :6, and thus we obtain the mean density of the earth, as, for instance, at Schiehallion, and afterwards at Arthur's Seat. Experiments of this kind for determining the mean density of the earth have been made in greater numbers; but they are not free from objection (see Gravitation).
Let us now consider the perturbation attending a spherical subterranean mass. A compact mass of great density at a small distance under the surface of the earth will produce an elevation of the mathematical surface which is expressed by the formula y=aµ{(i - 2u cos 0-Fu2)4 - i}, where a is the radius of the (spherical) earth, a(i - u) the distance of the disturbing mass below the surface, 11 the ratio of the disturbing mass to the mass of the earth, and a9 the distance of any point on the surface from that point, say Q, which is vertically over the disturbing mass. The maximum value of y is at Q, where it is y=aµu(i - u). The deflection at the distance aB is A= 11 u sin B(I - zu cos 9 + u 2) -1, or since 0 is small, putting h+u= 1, we have A=µ 0(h 2 +0 2) 2 . The maximum deflection takes place at a point whose distance from Q is to the depth of the mass as I: J 2, and its amount is 211/3 h2.
If, for instance, the disturbing mass were a sphere a mile in diameter, the excess of its density above that of the surrounding country being equal to half the density of the earth, and the depth of its centre half a mile, the greatest deflection would be 5", and the greatest value of y only two inches. Thus a large disturbance of gravity may arise from an irregularity in the mathematical surface whose actual magnitude, as regards height at least, is extremely small.
The effect of the disturbing mass 11 on the vibrations of a pendulum would be a maximum at Q; if v be the number of seconds of time gained per diem by the pendulum at Q, and a the number of seconds of angle in the maximum deflection, then it may be shown that v/a=7rd/3/Io.
The great Indian survey, and the attendant measurements of the degree of latitude, gave occasion to elaborate investigations of the deflection of the plumb-line in the neighbourhood of the high plateaus and mountain chains of Central Asia. Archdeacon Pratt (Phil. Trans.,18J5 and 1857), in instituting these investigations, took into consideration the influence of the apparent diminution of the mass of the earth's crust occasioned by the neighbouring ocean-basins; he concluded that the accumulated masses of mountain chains, &c., corresponded to subterranean mass diminutions, so that over any level surface in a fixed depth (perhaps loo miles or more) the masses of prisms of equal section are equal. This is supported by the gravity measurements at More in the Himalayas at a height of 4696 metres, which showed no deflection due to the mountain chain (Phil. Trans., 1871); more recently, H. A. Faye (Compt. rend., 1880) arrived at the same conclusion for the entire continent.
This compensation, however, must only be regarded as a general principle; in certain cases, the compensating masses show marked horizontal displacements. Further investigations, especially of gravity measurements, will undoubtedly establish other important facts. Colonel S. G. Burrard has recently recalculated, with the aid of more exact data, certain Indian deviations of the plumb-line, and has established that in the region south of the Himalayas (lat. 24°) there is a subterranean perturbing mass. The extent of the compensation of the high mountain chains is difficult to recognize from the latitude observations, since the same effect may result from different causes; on the other hand, observations of geographical longitude have established a strong compensation.' Meridian Arcs. The astronomical stations for the measurement of the degree of latitude will generally lie not exactly on the same meridian; and it is therefore necessary to calculate the arcs of meridian M which lie between the latitude of neighbouring stations. If S be the geodetic line calculated from the triangulation with the astronomically determined azimuths al and a2, then M = S cos Zia I + i vd 2 s i n 2a. .. } in which 2a =al+a2 - 180°, Oa=a2 - a1-180°.
The length of the arc of meridian between the latitudes 01 and 02 is M= (I sin where a 2 e 2 2 - 2; instead of using the eccentricity e, put the ratio of the axes b: a= i - n: 1+n, then
1 Survey of India, The Attraction of the Himalaya Mountains upon the Plumb Line in India" (1901), p. 98.
__ ? b (I+n)(I - n2)d? M t (I +2ncos20 +n2) This, after integration, gives M/b= (I+fl+flh +fl3) ao - (3n+3n h +n h) ai+ (24n3) where ao=ti - al sin (02-01) ('+431) a 2 = sin 2(0 2 -4 j) cos 2 (02+(I)i) a 3 = sin 3(02-4))) cos 3(4)2+4,1).
The part of M which depends on n 3 is very small; in fact, if we calculate it for one of the longest arcs measured, the Russian arc, it amounts to only an inch and a half, therefore we omit this term, and put for M/b the value I +n+4 7 / 2) am - (3n+3n 2) al+ (In) a2.
Now, if we suppose the observed latitudes to be affected with errors, and that the true latitudes are 01+ xi, 02+x2; and if further we suppose that ni+dn is the true value of a - b: a+b, and that n, itself is merely a very approximate numerical value, we get, on making these substitutions and neglecting the influence of the corrections x on the position of the arc in latitude, i.e. on 4)1+ 02, M/ b= (i+ni +n) ao - (3 n i+3 n) ai+ (nl) a2 + (I+Zn) ao - (3+6n) a.+ (fli) a 2 do -1-n i - 3ndao } dao here dao = x 2 - x i; and as b is only known approximately, put b=b l (I +u); then we get, after dividing through by the co efficient of dao, which is = 11-n i -3n 1 cos (02-01) cos (02+4 m), an equation of the form x 2 =x l +h+fu+gv, where for con venience we put v for dn. Now in every measured arc there are not only the extreme stations determined in latitude, but also a number of intermediate stations so that if there be i+1 stations there will be i equations x2 =xl+ f iu +gly+hl X3 = x l -F-f2u+g2v+h2 x; =xl +fiu+gtiv +h, In combining a number of different arcs of meridian, with the view of determining the figure of the earth, each arc will supply a number of equations in u and v and the corrections to its observed latitudes. Then, according to the method of least squares, those values of u and v are the most probable which render the sum of the squares of all the errors x a minimum. The corrections x which are here applied arise not from errors of observation only. The mere uncertainty of a latitude, as determined with modern instruments, does not exceed a very small fraction of a second as far as errors of observation go, but no accuracy in observing will remove the error that may arise from local attraction. This, as we have seen, may amount to some seconds, so that the corrections x to the observed latitudes are attributable to local attraction. Archdeacon Pratt objected to this mode of applying least squares first used by Bessel; but Bessel was right, and the objection is groundless. Bessel found, in 1841, from ten meridian arcs with a total amplitude of 50° 6: a= 3272077 toises = 6 377397 metres.
e (ellipticity) = (a - b)/a= 1 /299.15 (prob. error =3.2).
The probable error in the length of the earth's quadrant is = 33 6 m.
We now give a series of some meridian-arcs measurements, which were utilized in 1866 by A. R. Clarke in the Comparisons of the Standards of Length, pp. 280-287; details of the calculations are given by the same author in his Geodesy (1880), pp. 311 et seq. The data of the French arc from Formentera to Dunkirk are (L a2 a3r Stations. Astronomical Distance of Latitudes. Parallels.
° Ft.
Formentera .
3 8
Mountjouy .
Barcelona .
Carcassonne .
54'3 0
Pantheon .
4 8
5 0
47'9 8
The distance of the parallels of Dunkirk and Greenwich, deduced from the extension of the triangulation of England into France, in 1862, is 161407.3 ft., which is 3.9 ft. greater than that obtained from Captain Kater's triangulation, and 3.2 ft. less than the distance calculated by Delambre from General Roy's triangulation. The following table shows the data of the English arc with the distances in standard feet from Formentera.
2 7
2 9'5 0
Kellie Law
1 4
53' 60
2 7
37' 21
The latitude assigned in this table to Saxavord is not the directly observed latitude, which is 60° 49' 38.58", for there are here a cluster of three points, whose latitudes are astronomically determined; and if we transfer, by means of the geodesic connexion, the latitude of Gerth of Scaw to Saxavord, we get 60° 49' 36.59"; and if we similarly transfer the latitude of Balta, we get 60° 49' 36.46". The mean of these three is that entered in the above table.
For the Indian arc in long. 77° 40' we have the following data: And, finally, for the Peruvian arc, in long. 281° o', Punnea. Putchapolliam Dodagunta Namthabad. Daumergida. Takalkhera. Kalianpur .
Kaliana The data of the Russian arc (long. 26° 40') taken from Struve's work are as below Ft.
616529.81 1246762-17 1737551.48 2448745.17 3400312.63 4076412.28 4762421'43 5386135'39 631 79 05.67 7486789'97 8530517.90 9257921.06 Thomas Maclear North End. Heerenlogement Berg Royal Observatory .
Zwart Kop.. Cape Point.. .
Tarqui 3 4 32.068 Cotchesqui.. 0 2 31.387 1131036.3 Having now stated the data of the problem, we may seek that oblate ellipsoid (spheroid) which best represents the observations. Whatever the real figure may be, it is certain that if we suppose it an ellipsoid with three unequal axes, the arithmetical process will bring out an ellipsoid, which will agree better with all the observed latitudes than any spheroid would, therefore we do not prove that it is an ellipsoid; to prove this,` arcs of longitude would be required. The result for the spheroid may be expressed thus :- a = 2092-6062 ft. = 6378206.4 metres.
b =2085-5121 ft. =6356583.8 metres. b: a =293.98: 294.98.
As might be expected, the sum of the squares of the 40 latitude corrections, viz. 153.99, is greater in this figure than in that of three axes, where it amounts to 138.30. For this case, in the Indian arc the largest corrections are at Dodagunta, -{- 3'87", and at Kalianpur, - 3.68". In the Russian arc the largest corrections are -}- 3.76", at Tornea, and-3.31", at Staro Nekrasovsk. Of the whole 40 corrections, 16 are under ISO", 10 between 1.0" and 2.O", 10 between 2.0" and 3.O ", and 4 over 3.O". The probable error of an observed latitude is 1.42"; for the spheroidal it would be very slightly larger. This quantity may be taken therefore as approximately the probable amount of local deflection.
If p be the radius of curvature of the meridian in latitude cb, p' that perpendicular to the meridian, D the length of a degree of the meridian, D' the length of a degree of longitude, r the radius drawn from the centre of the earth, V the angle of the vertical with the radius-vector, then Ft.
P = 2089-0606.6 - 106411.5 cos 20 + 225.8 cos 40 2096-1607.3 - 3559 0.9 cos 20 + 4 5.2 cos 40 D = 364609.87 - 1857.14 cos 2 4) + 3'94 cos 40 D'= 3 6 553 8 '4 8 cos - 310.17 cos 30-f0.39 COS 50 Log rla= 9.9992645 -?- .0007374 cos 20 - 0000019 cos 44> V = 700.44" sin 20.- 1 19" sin 40.
A. R. Clarke has recalculated the elements of the ellipsoid of the earth; his values, derived in 1880, in which he utilized the measurements of parallel arcs in India, are particularly in practice. These values are a = 2092-6202 ft. = 6378249 metres.
b = 2085-4895 ft. =6356515 metres. b : a = 292.465: 293.465.
The calculation of the elements of the ellipsoid of rotation from measurements of the curvature of arcs in any given azimuth by means of geographical longitudes, latitudes and azimuths is indicated in the article Geodesy; reference may be made to Principal Triangulation, Helmert's Geodasie, and the publications of the Kgl. Preuss. Geod. Inst. :-Lotabweichungen (1886), and Die europ. Langengradmessung in 52° Br. (1893). For the calculation of an ellipsoid with three unequal axes see Comparison of Standards, preface; and for non-elliptical meridians, Principal Triangulation, P. 733.
Gravitation-Measurements. According to Clairault's theorem (see above) the ellipticity e of the mathematical surface of the earth is equal to the difference 2m-13, where m is the ratio of the centrifugal force at the equator to gravity at the equator, and s is derived from the formula G= g(1 +0 sin 2 0). Since the beginning of the 19th century many efforts have been made to determine the constants of this formula, and numerous expeditions undertaken to investigate the intensity of gravity in different latitudes. If m be known, it is only necessary to determine /3 for the evaluation of e; consequently it is unnecessary to determine G absolutely, for the relative values of G at two known latitudes suffice. Such relative measurements are easier and more exact than absolute ones. In some cases the ordinary thread pendulum, i.e. a spherical bob suspended by a wire, has been employed; but more often a rigid metal rod, bearing a weight and a knifeedge on which it may oscillate, has been adopted. The main point is the constancy of the pendulum. From the formula for the time of oscillation of the mathematically ideal pendulum, t= 27r1/ 11G, 1 being the length, it follows that for two points GI/G2=4/ti.
In 1808 J. B. Biot commenced his pendulum observations at several stations in western Europe; and in 1817-1825 Captain Louis de Freycinet and L. I. Duperrey prosecuted similar observations far into the southern hemisphere. Captain Henry Kater confined himself to British stations (1818-1819); Captain E. Sabine, from 1819 to 1829, observed similarly, with Kater's pendulum, at seventeen stations ranging from the West Indies Ft.
29 44 31 58
33 56
34 13 34 21
17.66 9 11 3.20
32'13 6.26
811507 7 1526386.8 1632583.3
1029174'9 17 5 6562.0 2518 37 6.3 3591788'4 4697329'5 5794695'7 7755835'9 ° 8 Io 12 151821 24 29 9 59 59535 7 30 31.132 42'276 52'165 53 562 15.292 51.532 11.262 48 322 Staro Nekrasovsk Vodu-Luy. Suprunkovzy Kremenets. Byelin. Nemesh. Jacobstadt Dorpat. Hogland Kilpi-maki Tornea Stuor-oivi Fuglenaes From the arc measured in Cape Colony by Sir in long. 18° 30', we have 45 47 48 50 52 54 56 58 60 62 65 68 70 20 455239 30 22 5 38 49 40 40 2.94 24.98 3.04 49.95 42.16 4.16 4.97 47.56 9' 84 5.25 44'57 58.40 11.23 to Greenland and Spitsbergen; and in 1824-1831, Captain Henry Foster (who met his death by drowning in Central America) experimented at sixteen stations; his observations were completed by Francis Baily in London. Of other workers in this field mention may be made of F. B. Lake (1826-1829), a Russian rear-admiral, and Captains J. B. Basevi and W. T. Heaviside, who observed during 1865 to 1873 at Kew and at 29 Indian stations, particularly at More in the Himalayas at a height of 4696 metres. Of the earlier absolute determinations we may mention those of Biot, Kater, and Bessel at Paris, London and Konigsberg respectively. The measurements were particularly difficult by reason of the length of the pendulums employed, these generally being second-pendulums over metre long. In about 1880, Colonel Robert von Sterneck of Austria introduced the half-second pendulum, which permitted far quicker and more accurate work. The use of these pendulums spread in all countries, and the number of gravity stations consequently increased: in 1880 there were about 120, in 1900 there were about 1600, of which the greater number were in Europe. Sir E. Sabine' calculated the ellipticity to be 1/288.5, a value shown to be too high by Helmert, who in 1884, with the aid of 120 stations, gave the value 1/299.26, 2 and in 1901, with about 1400 stations, derived the value 1/298.3. 3 The reason for the excessive estimate of Sabine is that he did not take into account the systematic difference between the values of G for continents and islands; it was found that in consequence of the constitution of the earth's crust (Pratt) G is greater on small I H, and g, the value at sea-level. This is supposed to take into account the attraction of the elevated strata or plateau; but, from the analytical method, this is not correct; it is also disadvantageous since, in general, the land-masses are compensated subterraneously, by reason of the isostasis of the earth's crust.
4, =
F =
- 0.06
- 0.5 4
- 0.90
-o 62
0.5 3
o 56
o 26
In 1849 Stokes showed that the normal elevations N of the geoid towards the ellipsoid are calculable from the deviations Ag of the acceleration of gravity, i.e. the differences between the observed g and the value calculated from the normal G formula. The method assumes that gravity is measured on the earth's surface at a sufficient number of points, and that it is conformably reduced. In order to secure the convergence of the expansions in spherical harmonics, it is necessary to assume all masses outside a surface parallel to the surface of the sea at a depth of 21 km. (= R X ellipticity) to be condensed on this surface (Helmert, Geod. ii. 172). In addition to the reduction with 2gH/R, there still result small reductions with mountain chains and coasts, and somewhat larger ones for islands. The sea-surface generally varies but very little by this condensation. The elevation (N) of the geoid is then equal to N = R ' FG -l og dI, where is the spherical distance from the point N, and Ag,, denotes the mean value of Ag for all points in the same distance (' around; F is a function of tp, and has the following values: - islands of the ocean than on continents by an amount which may approach to 0.3 cm. Moreover, stations in the neighbourhood of coasts shelving to deep seas have a surplus, but a little smaller. Consequently, Helmert conducted his calculations of 1901 for continents and coasts separately, and obtained G for the coasts o 036 cm. greater than for the continents, while the value of (3 remained the same. The mean value, reduced to continents, is G=978.03(1+0.005302 sin 2 4) -0.000007 sin e 2q5)cm/sec2.
The small term involving sin 2 2c/) could not be calculated with sufficient exactness from the observations, and is therefore taken from the theoretical views of Sir G. H. Darwin and E. Wiechert. For the constant g=978.03 cm. another correction has been suggested (1906) by the absolute determinations made by F. Kiihnen and Ph. Furtwangler at Potsdam.4 A report on the pendulum measurements of the 19th century has been given by Helmert in the Comptes rendus des seances de la 13 8 conference generale de l'Association Geod. Internationale d Paris 0900), ii. 139-385.
A difficulty presents itself in the case of the application of measurements of gravity to the determination of the figure of the earth by reason of the extrusion or standing out of the landmasses (continents, &c.) above the sea-level. The potential of gravity has a different mathematical expression outside the masses than inside. The difficulty is removed by assuming (with Sir G. G. Stokes) the vertical condensation of the masses on the sea-level, without its form being considerably altered (scarcely i metre radially). Further, the value of gravity (g) measured at the height H is corrected to sea-level by +2gH/R, where R is the radius of the earth. Another correction, due to P. Bouguer, is - 2g8H/pR, where 8 is the density of the strata of height H, and p the m..-,an density of the earth. These two corrections are represented in " Bouguer's Rule ": g H = g s (I - 2H/R+38H/2pR), where gx is the gravity at height 1 Account of Experiments to Determine the Figure of the Earth by means of a Pendulum vibrating Seconds in Different Latitudes (1825).
2 Helmert, Theorien d. hoheren Geod. ii., Leipzig, 1884.
Helmert, Sitzber. d. kgl. preuss. Ak. d. Wiss. zu Berlin (1901), P. 33 6 4 " Bestimmung der absoluten Grosse der Schwerkraft zu Potsdam mit Reversionspendeln " (Veroffentlichung des kgl. preuss. Geod. Inst., N.F., No. 27).
H. Poincare (Bull. Astr., 1901, p. 5) has exhibited N by means of Lame's functions; in this case the condensation is effected on an ellipsoidal surface, which approximates to the geoid. This condensation is, in practice, the same as to the geoid itself.
If we imagine the outer land-masses to be condensed on the sea-level, and the inner masses (which, together with the outer masses, causes the deviation of the geoid from the ellipsoid) to be compensated in the sea-level by a disturbing stratum (which, according to Gauss, is possible), and if these masses of both kinds correspond at the point N to a stratum of thickness D and density 8, then, according to Helmert (Geod. ii. 260) we have approximately 3 gOD_N}.
g 2 R p J Since N slowly varies empirically, it follows that in restricted regions (of a few roo km. in diameter) Ag is a measure of the variation of D. By applying the reduction of Bouguer to g, D is diminished by H and only gives the thickness of the ideal disturbing mass which corresponds to the perturbations due to subterranean masses. Ag has positive values on coasts, small islands, and high and medium mountain chains, and occasionally in plains; while in valleys and at the foot of mountain ranges it is negative (up to 0.2 cm.). We conclude from this that the masses of smaller density existing under high mountain chains lie not only vertically underneath but also spread out sideways.
The European Arc of Parallel in 52° Lat. Many measurements of degrees of longitudes along central parallels in Europe were projected and partly carried out as early as the first half of the 19th century; these, however, only became of importance after the introduction of the electric telegraph, through which calculations of astronomical longitudes obtained a much higher degree of accuracy. Of the greatest moment is the measurement near the parallel of 52° lat., which extended from Valentia in Ireland to Orsk in the southern Ural mountains over 69° long. (about 6750 km.). F. G. W. Struve, who is to be regarded as the father of the Russo-Scandinavian latitude-degree measurements, was the originator of this investigation. Having made the requisite arrangements with the VIII. 26 a governments in 1857, he transferred them to his son Otto, who, in 1860, secured the co-operation of England. A new connexion of England with the continent, via the English Channel, was accomplished in the next two years; whereas the requisite triangulations in Prussia and Russia extended over several decennaries. The number of longitude stations originally arranged for was 15; and the determinations of the differences in longitude were uniformly commenced by the Russian observers E. I. von Forsch, J. I. Zylinski, B. Tiele and others; Feaghmain (Valentia) being reserved for English observers. With the concluding calculation of these operations, newer determinations of differences of longitudes were also applicable, by which the number of stations was brought up to 29. Since local deflections of the plumb-line were suspected at Feaghmain, the most westerly station, the longitude (with respect to Greenwich) of the trigonometrical station Killorglin at the head of Dingle Bay was shortly afterwards determined.
The results (1891-1894) are given in volumes xlvii. and 1. of the memoirs (Zapiski) of the military topographical division of the Russian general staff, volume li. contains a reconnexion of Orsk. The observations made west of Warsaw are detailed in the Die europ. Langengradmessung in 52° Br., i. and ii., 1893, 1896, published by the Kgl. Preuss. Geod. Inst.
Feaghmain .
- 10
- 3.3
Killorglin. .
- 9
Haverfordwest .
- 4
Greenwich. .
+ I.5
+ 2
- 1.7
Bonn. .
- 4.4
Göttingen. .
- 2.4
Brocken .
+2' 7
Grossenhain .
+ 1 3
- 2.9
Schneekoppe .
+ 1 7
+ 18
- 0.5
Schonsee .
+ 18
- 2.9
+1 9
+ 2 3
- 2.8
+3 6
3 6
- 2.6
Orenburg .
+5 8
- 8 o
The following figures are quoted from Helmert's report " Die Grosse der Erde " (Sitzb. d. Berl. Akad. d. Wiss., 1906, P. 535):- Easterly Deviation of the Astronomical Zenith. These deviations of the plumb-line correspond to an ellipsoid having an equatorial radius (a) of nearly 6,378,000 metres (prob. error ° 70 metres) and an ellipticity 1/299.15. The latter was taken for granted; it is nearly equal to the result from the gravity-measurements; the value for a then gives /77 2 a minimum (nearly). The astronomical values of the geographical longitudes (with regard to Greenwich) are assumed, according to the compensation of longitude differences carried out by van de Sande Bakhuyzen (Comp. rend. des seances de la commission permanente de l'Association Geod. Internationale a Geneve, 1893, annexe A.I.). Recent determinations (Albrecht, Astr. Nach., 3993/4) have introduced only small alterations in the deviations, a being slightly increased.
Of considerable importance in the investigation of the great arc was the representation of the linear lengths found in different countries, in terms of the same unit. The necessity for this had previously occurred in the computation of the figure of the earth from latitude-degree-measurements. A. R. Clarke instituted an extensive series of comparisons at Southampton (see Comparisons of Standards of Length of England, France, Belgium, Prussia, Russia, India and Australia, made at the Ordnance Survey Office, Southampton, 1866, and a paper in the Philosophical Transactions for 1873, by Lieut.-Col. A. R. Clarke, C.B., R.E., on the further comparisons of the standards of Austria, Spain, the United States, Cape of Good Hope and Russia) and found that 1 toise 6-3945-3348 ft., 1 metre =3.2808-6933 ft.
In 1875 a number of European states concluded the metre convention, and in 1877 an international weights-and-measures bureau was established at Breteuil. Until this time the metre was determined by the end-surfaces of a platinum rod (metre des archives); subsequently, rods of platinum-iridium, of cross-section H, were constructed, having engraved lines at both ends of the bridge, which determine the distance of a metre. There were thirty of the rods which gave as accurately as possible the length of the metre; and these were distributed among the different states (see Weights And Measures). Careful comparisons with several standard toises showed that the metre was not exactly equal to 443,296 lines of the toise, but, in round numbers, 1/75000 of the length smaller. The metre according to the older relation is called the " legal metre," according to the new relation the "international metre." The values are (see Europ. Ldngengradmessung, i. p. 230): Legal metre =3.2808-6933 ft., International metre =3.2808257 ft.
The values of a given above are in terms of the international metre; the earlier ones in legal metres, while the gravity formulae are in international metres.
The International Geodetic Association (Internationale Erdmessung). On the proposition of the Prussian lieutenant-general, Johann Jacob Baeyer, a conference of delegates of several European states met at Berlin in 1862 to discuss the question of a " Central European degree-measurement." The first general conference took place at Berlin two years later; shortly afterwards other countries joined the movement, which was then named " The European degree-measurement." From 1866 till 1886 Prussia had borne the expense incident to the central bureau at Berlin; but when in 1886 the operations received further extension and the title was altered to " The International Earth-measurement " or " International Geodetic Association," the co-operating states made financial contributions to this purpose. The central bureau is affiliated with the Prussian Geodetic Institute, which, since 1892, has been situated on the Telegraphenberg near Potsdam. After Baeyer's death Prof. Friedrich Robert Helmert was appointed director. The funds are devoted to the advancement of such scientific works as concern all countries and deal with geodetic problems of a general or universal nature. During the period 1897-1906 the following twenty-one countries belonged to the association: - Austria, Belgium, Denmark, England, France, Germany, Greece, Holland, Hungary, Italy, Japan, Mexico, Norway, Portugal, Rumania, Russia, Servia, Spain, Sweden, Switzerland and the United States of America. At the present time general conferences take place every three years.' Baeyer projected the investigation of the curvature of the meridians and the parallels of the mathematical surface of the earth stretching from Christiania to Palermo for 12 degrees of longitude; he sought to co-ordinate and complete the network of triangles in the countries through which these meridians passed, and to represent his results by a common unit of length. This proposition has been carried out, and extended over the greater part of Europe; as a matter of fact, the network has, with trifling gaps, been carried over the whole of western and central Europe, and, by some chains of triangles, over European Russia. Through the co-operation of France, the network has been extended into north Africa as far as the geographical latitude of 32 0; in Greece a network, united with those of Italy and Bosnia, has been carried out by the Austrian colonel, Heinrich Hartl; Servia has projected similar triangulations; Rumania has begun to make the triangle measurements, and three base 1 Die KOnigl. Observatorien fiir Astrophysik, Meteorologie and Geodasie bei Potsdam (Berlin, 1890); Verhandlungen der I. Allgemeinen Conferenz der Bevollmachtigten zur mitteleurop. Gradmessung, October, 1864, in Berlin (Berlin, 1865); A. Hirsch, Verhandlungen der VIII. Allg. Conf. der Internationalen Erdmessung, October, 1886, in Berlin (Berlin, 1887); and Verhandlungen der XI. Allg. Conf. d. I. E., October, 1895, in Berlin (1896).
lines have been measured by French officers with Brunner's apparatus. At present, in Rumania, there is being worked a connexion between the arc of parallel in lat. 47°/48° in Russia (stretching from Astrakan to Kishinev) with Austria-Hungary. In the latter country and in southBavaria the connecting triangles for this parallel have been recently revised, as well as the French chain on the Paris parallel, which has been connected with the German net by the co-operation of German and French geodesists. This will give a long arc of parallel, really projected in the first half of the 19th century. The calculation of the Russian section gives, with an assumed ellipticity of 1/299.15, the value a= 637735 0 metres; this is rather uncertain, since the arc embraces only 19° in longitude.
We may here recall that in France geodetic studies have recovered their former expansion under the vigorous impulse of Colonel (afterwards General) Francois Perrier. When occupied with the triangulation of Algeria, Colonel Perrier had conceived the possibility of the geodetic junction of Algeria to Spain, over the Mediterranean; therefore the French meridian line, which was already connected with England, and was thus produced to the 60th parallel, could further be linked to the Spanish triangulation, cross thence into Algeria and extend to the Sahara, so as to form an arc of about 30° in length. But it then became urgent to proceed to a new measurement of the French arc, between Dunkirk and Perpignan. In 1869 Perrier was authorized to undertake that revision. He devoted himself to that work till the end of his career, closed by premature death in February 1888, at the very moment when the Depot de la guerre had just been transformed into the Geographical Service of the Army, of which General F. Perrier was the first director. His work was continued by his assistant, Colonel (afterwards General) J. A. L. Bassot. The operations concerning the revision of the French arc were completed only in 1896. Meanwhile the French geodesists had accomplished the junction of Algeria to Spain, with the help of the geodesists of the Madrid Institute under General Carlos Ibanez (1879), and measured the meridian line between Algiers and El Aghuat (1881). They have since been busy in prolonging the meridians of El Aghuat and Biskra, so as to converge towards Wargla, through Ghardaia and Tuggurt. The fundamental co-ordinates of the Pantheon have also been obtained anew, by connecting the Pantheon and the Paris Observatory with the five stations of Bry-sur-Marne, Morlu, Mont Valerien, Chatillon and Montsouris, where the observations of latitude and azimuth have been effected.' According to the calculations made at the central bureau of the international association on the great meridian arc extending from the Shetland Islands, through Great Britain, France and Spain to El Aghuat in Algeria, a = 6377935 metres, the ellipticity being assumed as 1/299.15. The following table gives the difference: astronomical-geodetic latitude. The net does not follow the meridian exactly, but deviates both to the west and to the east; actually, the meridian of Greenwich is nearer the mean than that of Paris (Helmert, Grosse d. Erde). West Europe-Africa Meridian-arc.' Name. Latitude. A.-G. o Saxavord 60 49.6 Balta. 60 45.0 Ben Hutig. 58 33'1 Cowhythe. 57 41.1 Great Stirling 57 27.8 Kellie Law. 56 14'9 Calton Hill. 55 57'4 Durham 54 46.1 Burleigh Moor 54 34.3 Clifton Beacon 53 27'5 1 Ibanez and Perrier, Jonction geod. et astr. de l'Algerie avec l'Espagne (Paris, 1886); Memorial du depot general de la guerre, t. xii.: Nouvelle meridienne de France (Paris, 1885, 1902, 2904); Comptes rendus des seances de la 12 e -19 e conference generale de l'Assoc. Geod. Internat., 1898 at Stuttgart, 1900 at Paris, 1903 at Copenhagen, 1906 at Budapest (Berlin, 1899, 1901, 2904, 1908); A. Ferrero, Rapport sur les triangulations, pres. a la Il e conf. gen. 1898. 2 R. Schumann, C. r. de Budapest, p. 244.
Arbury Hill
Lihons .
52 1 3.4
51 28.6
51 7.8
51 2.7
49 49'9
48 50.8
0 5
Saligny le Vif
1 3 7
Puy de Dome
4 6 '5
Roden .
21 ' 4
+ 1 '7
1 3'3
Lerida .
-0 2
Javalon .
13 8
5' 0
Chinchilla. .
Mola de Formentera
- 1.2
Tetica. .
15 2
Roldan. .
Conjuros .
3 6
Mt. Sabiha .
39' 6
Nemours .
5' 8
3 6
4 8 ' 0
+2 '9
Algiers (Voirol)
3 6
Guelt es Stel.
- 1.0
El Aghuat .
West Europe-Africa Meridian-arc (contd.). 'sasavord' to Hutig ?
Stirling ellie Law C to ill ss Clif t on on urleigh Moor Arbury Hill reenwich Nieuport Rosendai:l Lihons Pantheon Chevry Saligny le Vif Arpheuille Puy de Dime ' ' Rodes Carcassonne Rivesaltes Montolar Javalon Chinchilla > Desierto Lerida 0 ' 'v Mole de Formentera TetfcaConjuros Roldan Algiers Wok Bouzareah Guelt es Stet ElAghuat Mt. Sabiha Nemours 4-0 -6.1 +0.3 +7'3 -2.3 -3'7 +3'5 - 0.9 +2' 1 +1.3 While the radius of curvature of this arc is obviously not uniform (being, in the mean, about 600 metres greater in the northern than in the southern part), the Russo-Scandinavian meridian arc (from 45° to 70°), on the other hand, is very uniformly curved, and gives, with an ellipticity of 1/299.15, a = 6 37 8 455 metres; this arc gives the plausible value 1/298.6 for the ellipticity. But in the case of this arc the orographical circumstances are more favourable.
The west-European and the Russo-Scandinavian meridians indicate another anomaly of the geoid. They were connected at the Central Bureau by means of east-to-west triangle chains (principally by the arc of parallel measurements in lat. 52°); it was shown that, if one proceeds from the west-European meridian arcs, the differences between the astronomical and geodetic latitudes of the Russo-Scandinavian arc become some 4" greater.' The central European meridian, which passes through Germany and the countries adjacent on the north and south, is under review at Potsdam (see the publications of the Kgl. Preuss. Geod. Inst., Lotabweichungen, Nos. 1-3). Particular notice must be made of the Vienna meridian, now carried southwards to Malta. The Italian triangulation is now complete, and has been joined with the neighbouring countries on the north, and with Tunis on the south.
The United States Coast and Geodetic Survey has published an account of the transcontinental triangulation and measurement of an arc of the parallel of'39°, which extends from Cape May (New Jersey), on the Atlantic coast, to Point Arena (California), on the Pacific coast, and embraces 48° 46' of longitude, with a linear development of about 4225 km. (2625 miles). The triangulation depends upon ten base-lines, with an aggregate length of 86 km. the longest exceeding 17 km. in length, which have been measured with the utmost care. In crossing the Rocky Mountains, many of its sides exceed ioo miles in length, and there is one side reaching to a length of 294 km., or 183 miles; the altitude of many of the stations is also considerable, reaching to 4300 metres, or 14,108 ft., in the case of Pike's Peak, and to 14,421 ft. at Elbert Peak, Colo. All geometrical conditions subsisting in the triangulation are satisfied by adjustment, inclusive of the required accord of the base-lines, so that the same length for any given line is found, no matter from what line one may start.2 Over or near the arc were distributed 109 latitude stations, occupied with zenith telescopes; 73 azimuth stations; and 29 telegraphically determined longitudes. It has thus been possible to study in a very complete manner the deviations of the vertical, which in the mountainous regions sometimes amount to 25 seconds, and even to 29 seconds.
With the ellipticity 1/299.15, a= 6377897 = 65 metres (prob. error); in this calculation, however, some exceedingly perturbed stations are excluded; for the employed stations the mean perturbation in longitude is =4.9" (zenith-deflection east-to west t 3.8").
The computations relative to another arc, the " eastern oblique arc of the United States," are also finished. 3 It extends from Calais (Maine) in the north-east, to the Gulf of Mexico, and terminates at New Orleans (Louisiana), in the south. Its length is 2612 km. (1623 miles), the difference of latitude 15° 1', and of longitude 22° 47'. In the main, the triangulation follows the Appalachian chain of mountains, bifurcating once, so as to leave an oval space between the two branches. It includes among its stations Mount Washington (1920 metres) and Mount Mitchell (2038 metres). It depends upon six base-lines, and the adjustment is effected in the same manner as for the arc of the 1 O. and A. Borsch, " Verbindung d. russ.-skandina y. mit der franz.-engl. Breitengradmessung " (Verhandlungen der 9. Allgem. Conf. d. I. E. in Paris, 1889, Ann. xi.).
U.S. Coast and Geodetic Survey; H. S. Pritchett, superintendent. The Transcontinental Triangulation and the American Arc of the Parallel, by C. A. Schott (Washington, 1900).
U.S. Coast and Geodetic Survey; O. H. Tittmann, superintendent. The Eastern Oblique Arc of the United States, by C. A. Schott (1902).
parallel. The astronomical data have been afforded by 71 latitude stations, 17 longitude stations, and 56 azimuth stations, distributed over the whole extent of the arc. The resulting dimensions of an osculating spheroid were found to be a= 6378157 metres =90 (prob. error), e(ellipticity) =1 /3 0 4.5 =1.9 (prob. error).
With the ellipticity 1/399.15, a = 6378041 metres = 80 (prob. er.).
During the years 1903-1906 the United States Coast and Geodetic Survey, under the direction of O. H. Tittmann and the special management of John F. Hayford, executed a calculation of the best ellipsoid of rotation for the United States. There were 507 astronomical determinations employed, all the stations being connected through the net-work of triangles. The observed latitudes, longitude and azimuths were improved by the attractions of the earth's crust on the hypothesis of isostasis for three depths of the surface of 114, 121 and 162 km., where the isostasis is complete. The land-masses, within the distance of 4126 km., were taken into consideration. In the derivation of an ellipsoid of rotation, the first case proved itself the most favourable, and there resulted: a = 6378283 metres '74(,ellipticity =1/297.8 = 0.9 (
The most favourable value for the depth of the isostatic surface is approximately 114 km.
The measurement of a great meridian arc, in long. 98° W., has been commenced; it has a range of latitude of 23°, and will extend over 50° when produced southwards and northwards by Mexico and Canada. It may afterwards be connected with the arc of Quito. A new measurement of the meridian arc of Quito was executed in the years 1901-1906 by the Service geographique of France under the direction of the Academie des Sciences, the ground having been previously reconnoitred in 1899. The new arc has an amplitude in latitude of 5° 53' 33", and stretches from Tulcan (lat. o° 48' 25") on the borders of Columbia and Ecuador, through Columbia to Payta (lat. - 5° 5' 8") in Peru. The end-points, at which the chain of triangles has a slight north-easterly trend, show a longitude difference of 3°. Of the 74 triangle points, 64 were latitude stations; 6 azimuths and 8 longitude-differences were measured, three base-lines were laid down, and gravity was determined from six points, in order to maintain indications over the general deformation of the geoid in that region. Computations of the attraction of the mountains on the plumb-line are also being considered. The work has been much delayed by the hardships and difficulties encountered. It was conducted by Lieut. - Colonel Robert Bourgeois, assisted by eleven officers and twenty-four soldiers of the geodetic branch of the Service geographique. Of these officers mention may be made of Commandant E. Maurain, who retired in 1904 after suffering great hardships; Commandant L. Massenet, who died in 1905; and Captains I. Lacombe, A. Lallemand, and Lieut. Georges Perrier (son of General Perrier). It is conceivable that the chain of triangles in longitude 98° in North America may be united with that of Ecuador and Peru: a continuous chain over the whole of America is certainly but a question of time. During the years 1899-1902 the measurement of an arc of meridian was made in the extreme north, in Spitzbergen, between the latitudes 76° 38' and 80° 50', according to the project of P. G. Rosen. The southern part was determined by the Russians - O. Backlund, Captain D. D. Sergieffsky, F. N. Tschernychev, A. Hansky and others - during 1899-1901, with the aid of i base-line, 15 trigonometrical, II latitude and 5 gravity stations. The northern part, which has one side in common with the southern part, has been determined by Swedes (Professors Rosen, father and son, E. Jaderin, T. Rubin and others), who utilized i base-line, 9 azimuth measurements, 18 trigonometrical, 17 latitude and 5 gravity stations. The party worked under excessive difficulties, which were accentuated by the arctic climate. Consequently, in the first year, little headway was made.4 4 Missions scientifiques pour la mesure d'un arc de me'ridien au Spitzberg entreprises en 1899-1902 sous les auspices des gouvernements russe et suedois. Mission russe (St Petersbourg, 1904); Mission suedoise (Stockholm, 1904).
Sir David Gill, when director of the Royal Observatory, Cape Town, instituted the magnificent project of working a latitudedegree measurement along the meridian of 30° long. This meridian passes through Natal, the Transvaal, by Lake Tanganyika, and from thence to Cairo; connexion with the RussoScandinavian meridian arc of the same longitude should be made through Asia Minor, Turkey, Bulgaria and Rumania. With the completion of this project a continuous arc of 105° in latitude will have been measured.' Extensive triangle chains, suitable for latitude-degree measurements, have also been effected in Japan and Australia.
Besides, the systematization of gravity measurements is of importance, and for this purpose the association has instituted many reforms. It has ensured that the relative measurements made at the stations in different countries should be reduced conformably with the absolute determinations made at Potsdam; the result was that, in 1906, the intensities of gravitation at some 2000 stations had been co-ordinated. The intensity of gravity on the sea has been determined by the comparison of barometric and hypsometric observations (Mohn's method). The association, at the proposal of Helmert, provided the necessary funds for two expeditions: - English Channel - Rio de Janeiro, and the Red Sea - Australia - San Francisco - Japan. Dr O. Hecker of the central bureau was in charge; he successfully overcame the difficulties of the work, and established the tenability of the isostatic hypothesis, which necessitates that the intensity of gravity on the deep seas has, in general, the same value as on the continents (without regard to the proximity of coasts).2 As the result of the more recent determinations, the ellipticity, compression or flattening of the ellipsoid of the earth may be assumed to be very nearly 1/298.3; a value determined in 1901 by Helmert from the measurements of gravity. The semimajor axis, a, of the meridian ellipse may exceed 6,378,000 inter. metres by about 200 metres. The central bureau have adopted, :for practical reasons, the value 1/299.15, after Bessel, for which tables exist; and also the value a=6 377397.1 55(1 + 0.0001).
The methods of theoretical astronomy also permit the evaluation of these constants. The semi-axis a is calculable from the parallax of the moon and the acceleration of gravity on the earth; but the results are somewhat uncertain: the ellipticity deduced from lunar perturbations is 1/297.8±2 (Helmert, Geodeisie, ii. pp. 460-473); William Harkness (The Solar Parallax and its related Constants, 1891) from all possible data .derived the values: ellipticity = 1/300.2±3, a = 6377972±125 metres. Harkness also considered in this investigation the relation of the ellipticity to precession and nutation; newer investigations of the latter lead to the limiting values 1/296, 1/298 (Wiechert). It was clearly noticed in this method of determination that the influence of the assumption as to the density of the strata in the interior of the earth was but very slight (Radau, Bull. astr. ii. (1885) 157). The deviations of the geoid from the flattened ellipsoid of rotation with regard to the heights (the;directions of normals being nearly the same) will scarcely exceed +loo metres (Helmert).3 The basis of the degreeand gravity-measurements is actually formed by a stationary sea-surface, which is assumed to be level. However, by the influence of winds and ocean currents the mean surface of the sea near the coasts (which one assumes as the fundamental sea-surface) can deviate somewhat from a level -surface. According to the more recent levelling it varies at the most by only some decimeters.4 ' Sir David Gill, Report on the Geodetic Survey of South Africa, 1833-1892 (Cape Town, 1896), vol. ii. 1901, vol. iii. 2905.
z O. Hecker, Bestimmung der Schwerkraft a. d. Atlantischen Ozean (Veroffentl. d. Kgl. Preuss. Geod. Inst. No. II), Berlin, 1903.
' F. R. Helmert, " Neuere Fortschritte in der Erkenntnis der math. Erdgestalt " (Verhandl. des VII. Internationalen GeographenKongresses, Berlin, 1899), London, 1901.
C. Lallemand, " Rapport sur les travaux du service du nivellement general de la France, de 1900 a 1903 " (Comp. rend. de la 14' .conf. gen. de l'Assoc. Geod. Intern., 1903, p. 178).
It is well known that the masses of the earth are continually undergoing small changes; the earth's crust and sea-surface reciprocally oscillate, and the axis of rotation vibrates relatively to the body of the earth. The investigation of these problems falls in the programme of the Association. By continued observations of the water-level on sea-coasts, results have already been obtained as to the relative motions of the land and sea (cf. GEOLOGY); more exact levelling will, in the course of time, provide observations on countries remote from the sea-coast. Since 1900 an international service has been organized between some astronomical stations distributed over the north parallel of 39° 8', at which geographical latitudes are observed whenever possible. The association contributes to all these stations, supporting four entirely: two in America, one in Italy, and one in Japan; the others partially (Tschardjui in Russia, and Cincinnati observatory). Some observatories, especially Pulkowa, Leiden and Tokyo, take part voluntarily. Since 1906 another station for South America and one for Australia in latitude - 3 1 ° 55' have been added. According to the existing data, geographical latitudes exhibit variations amounting to +0.25", which, for the greater part, proceed from a twelveand a fourteenmonth period. 5 (A. R. C.; F. R. H.


Up to date as of January 14, 2010

Definition from Wiktionary, a free dictionary

See also earth



The Earth



Proper noun

  1. (astronomy) The third planet in order from the Sun, upon which humans live. Represented in astronomy and astrology by and .
  2. (paganism) The Earth Goddess in Heathenry.
    • (A date for this quote is being sought): Tony Linsell
      Allfather had a second child by Night; it was a daughter named Earth.




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Bible wiki

Up to date as of January 23, 2010

From BibleWiki

  1. In the sense of soil or ground, the translation of the word adamah'. In Gen 9:20 "husbandman" is literally "man of the ground or earth." Altars were to be built of earth (Ex 20:24). .Naaman asked for two mules' burden of earth (2Kg 5:17), under the superstitious notion that Jehovah, like the gods of the heathen, could be acceptably worshipped only on his own soil.
  2. As the rendering of 'erets, it means the whole world (Gen 1:2); the land as opposed to the sea (1:10).^ And that is the only worship of God there is.
    • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

    ^ This could be done - there is a definite upper limit on how fast something like the Earth can spin before it breaks apart.
    • How to destroy the Earth @ Things Of Interest 27 January 2010 23:47 UTC [Source type: Original source]

    ^ In a catalogue selling composting equipment I read that two handfuls of healthy soil contain more living organisms than there are people on the earth.
    • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

    .Erets also denotes a country (21:32); a plot of ground (23:15); the ground on which a man stands (33:3); the inhabitants of the earth (6:1; 11:1); all the world except Israel (2Chr 13:9).^ No man but feels more of a man in the world if he have a bit of ground that he can call his own.
    • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

    ^ Fondness for the ground comes back to a man after he has run the round of pleasure and business, eaten dirt, and sown wild oats, drifted about the world, and taken the wind of all its moods.
    • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

    ^ Earth teach me courage as the tree which stands all alone.
    • Earth, Soil, Dirt  -  Quotes, Poems and Wisdom for Gardeners 27 January 2010 23:47 UTC [Source type: Original source]

    In the New Testament "the earth" denotes the land of Judea (Mt 23:35); also things carnal in contrast with things heavenly (Jn 3:31; Col 3:1, 2).
This entry includes text from Easton's Bible Dictionary, 1897.
what mentions this? (please help by turning references to this page into wiki links)
This article needs to be merged with EARTH (Jewish Encyclopedia).


Up to date as of January 31, 2010
(Redirected to Wikia Gaming:Earth article)

From Wikia Gaming, your source for walkthroughs, games, guides, and more!

Note! These pages contain material that are meant to be humorous.
They are not intended, nor should they be used, for any research or serious use.

Doesn't it look so innocent?
Third planet from the sun. It is almost always on a path to destruction where the planet will either fall prey to invading aliens, demons, a vengeful god, a world dominating corporation, or a global government.
So full of things.

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Inhabited from within by

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Simple English

Earth File:Earth
File:The Earth seen from Apollo
Alternative names Blue Planet, Terra, Planet Earth
Orbital characteristics
Epoch J2000.0
Aphelion 152,097,701 km
1.0167103335 AU
Perihelion 147,098,074 km
0.9832898912 AU
Semi-major axis 149,597,887.5 km
1.0000001124 AU
Eccentricity 0.016710219
Orbital period 365.256366 days
1.0000175 yr
Average orbital speed 29.783 km/s
107,218 km/h
Inclination 1°34'43.3"
to Invariable plane
Physical characteristics

Earth is the third planet from the Sun. It is one of the four terrestrial planets in our Solar System. This means most of its mass is solid. The other three are Mercury, Venus and Mars. The Earth is also called Planet Earth, Blue Planet[note 1] and Terra.[note 2]

Earth is home to millions of species of plants and animals,[1] including humans. Earth is the only place in the universe where life has been confirmed to exist.[2] Science shows that the Earth formed around 4.5 billion years ago.[3][4] The things that live on Earth have completely changed its air or atmosphere. This is called a biosphere.[5]

About 71% of Earth's surface is covered in salt water oceans. Earth is the only place in the universe where liquid water is known to exist.[6] The other 29% is made of rocky land in the shape of continents and islands. Earth interacts with other objects in the Solar System, particularly the Sun and the Moon. The Earth orbits or goes around the Sun roughly once every 365.25 days. One spin is called a day and one orbit around the Sun is called a year. This is why we have 365 days in a year. Earth has only one moon, known as the Moon.



See also: Historical geology

What it is made of

File:Terrestrial planet size
A rough size comparison of Earth and the other three terrestrial planets, Mercury, Venus and Mars.

Earth is a terrestrial planet. This means it is made up of solid rock unlike a gas giant such as Jupiter. It is the largest out of the four terrestrial planets in mass and diameter. Earth also has the strongest gravitational and magnetic field.


The Earth's shape is an oblate spheroid. This means it is basically a sphere but it bulges around the middle. The circumference of the Earth is about 40,000 kilometers; the average width of the Earth is about 12,700 km. The highest point on Earth is the peak of Mount Everest at 8,848 m above sea level. The lowest natural point is the bottom of the Mariana Trench at 10,911 meters below sea level. Because of the bulge at the middle or the equator, the farthest point from the Earth's center is the top of Mount Chimborazo in Ecuador.[7][8][9]

[[File:|thumb|left|A diagram of the inside of the Earth, showing the different layers.]]

Internal structure

Inside, the Earth is similar to the other terrestrial planets. It has an outer, solid rock layer called the crust. Everything that lives on Earth is on top of the crust. Below that is a layer of thick, semi-liquid rock called the mantle. Under that is a thin liquid layer called the outer core and then the solid iron inner core. The thickness of the crust changes. On land the average is between 30–50 kilometers thick. Under the oceans in some places it is only 6 kilometers thick.[10] The inside of the Earth is very hot, the temperature of the outer core may be as high as 7,000 degrees Celsius.[11]

Tectonic plates

File:Plates tect2
A map showing the Earth's major tectonic plates.

The Earth is the only terrestrial planet with active plate tectonics.[12] Due to plate tectonics the Earth's crust basically floats on the thick liquid rock of the mantle below.[13] The crust is split up into parts called plates. These plates interact as they move about causing earthquakes and creating volcanoes and mountain ranges. The place where plates meet are called plate boundaries. There are three types of plate boundary: constructive, destructive and transform.[12][14][15]

  • At a constructive plate boundary two plates move away from each other, and hot magma (liquid rock) is pushed upwards through the cracks. These kinds of boundaries make ocean rifts, undersea volcanoes or ridges. An example of this kind of plate boundary is the island of Hawaii.[14]
  • At a destructive plate boundary, two plates move towards each other. This forms islands, volcanoes and high mountain ranges. The Himalayas were made by this process.[14]
  • At a transform plate boundary, two plates move parallel to each other. As they move the grind against each other. This kind of plate boundary causes earthquakes.[15]


The Earth changes greatly from place to place. Over 70% of the Earth surface is covered by water.[16] The underwater surface has many of the same features as the above sea with volcanos, mountains and trenches or canyons. The 30% not covered by water is mostly forests, deserts, plains, mountains and plateaux. Human civilisation has led to increasing urbanisation — the growth of cities.

Many things can change the surface of the Earth. Plate tectonics is main cause of change but there are others such as erosion from wind and rain, erosion by the oceans or meteorite impacts. There are three main types of rock that make up the Earth's surface:

  • Igneous rock is made when magma or lava from the mantle reaches the surface and cools. As it gets colder it turns into rock or solidifies.
  • Sedimentary rock is made from sediment, like sand or small bits of other rock, that has been crushed and packed tightly together.


All the water on Earth, on land or in the atmosphere, is part of the hydrosphere. About 97.5% of all water is salt water. About half the fresh water is currently ice.[17] The oceans absorb or soak up carbon dioxide, a gas that adds to the greenhouse effect and global warming.


A planet's atmosphere is a layer of different gases surrounding it. It is kept there by gravity. The Earth's atmosphere is made of about 78% nitrogen, 21% oxygen and small amounts of other gases.[18] This mixture is often called air. Further up there is a layer of ozone gas called the Ozone layer. Ozone absorbs ultraviolet radiation from the Sun. Ultraviolet radiation is dangerous to people, so without the Ozone layer life would not be possible. The atmosphere also protects the earth from crashes with meteors and small asteroids. This is because they burn up due to all the friction as they fly through it. It also helps to keep Earth warm. Some gases including carbon dioxide and methane act like a blanket around the Earth, they trap heat under them, keeping the Earth warm. This is called the natural greenhouse effect.[19] When humans build factories and power plants to make electricity, combustion is involved. Combustion lets out a lot of carbon dioxide. This carbon dioxide goes up into the atmosphere and traps more heat. This is called global warming.

Weather, climate, and water cycle

Hot air rises. As it rises it gets colder again and falls. This is called convection currents.[20] When hot air meets cold air different weather effects happen. Convection currents are the cause of almost all weather on Earth. When it gets hot on the surface water evaporates and becomes steam or water vapour. This hot water vapour rises. As it rises it gets colder. When it gets cold enough it turns back into water again. This causes the clouds and rain. It is called the water cycle.[20]

Orbit and rotation

[[File:|thumb|right|200px|Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit.]] The Earth takes about 24 hours to complete one day and 365 days to complete a year. The Earth is, on average, 150 million kilometers away from the Sun, and moving at a speed of 30 kilometers a second or 108,000 miles an hour.[21]

The Moon orbits the Earth at an average distance of 250,000 miles. It is tidally locked to Earth, which means it always has the same side facing the Earth. It takes roughly one month to complete one orbit.

The Earth is part of the Solar System and orbits the sun along with thousands of small objects and eight planets. The Sun, and therefore the Solar System, are currently traveling through the Orion Arm of the Milky Way Galaxy and will be for approximately the next 10,000 years.[22]


  1. Blue Planet is used as the title of several films Blue Planet and The Blue Planet, in the Life issue The Incredible Year '68 featuring the Earthrise photograph with lines from poet James Dickey Behold/The blue planet steeped in its dream/Of reality [1] pp. 7–8 [2], and in the title of the European Space Agency bulletin report Exploring the water cycle of the 'Blue Planet' [3]
  2. Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. Cf. Blue, Jennifer (July 5, 2007). "Descriptor Terms (Feature Types)". Gazetteer of Planetary Nomenclature. USGS. Retrieved 2007-07-05. 


  1. "How Many Species are There on Earth". Harvard University. Retrieved 2009-07-28. 
  2. "BBC : The Earth". BBC. Retrieved 2009-07-28. 
  3. "The age of the Earth in the twentieth century- a problem (mostly) solved". Geological Society, London, Special Publications. Retrieved 2009-07-28. 
  4. "History of earth". Microsoft. Retrieved 2009-07-28. 
  5. "Origins of life on Earth". Retrieved 2009-07-28. 
  6. "Rover reveals Mars was once wet enough for life". Microsoft. Retrieved 2009-07-28. 
  7. Senne, Joseph H. (2000). [Expression error: Unexpected < operator "Did Edmund Hillary Climb the Wrong Mountain"]. Professional Surveyor 20 (5): 16–21. 
  8. Sharp, David (2005-03-05). [Expression error: Unexpected < operator "Chimborazo and the old kilogram"]. The Lancet 365 (9462): 831–832. doi:10.1016/S0140-6736(05)71021-7. 
  9. "Tall Tales about Highest Peaks". Australian Broadcasting Corporation. Retrieved 2008-12-29. 
  10. Toshiro Tanimoto. "Crustal Surface of the Earth". American Geophysical Union. Retrieved 2009-08-02. 
  11. D. Alfé; M. J. Gillan, L. Vočadlo, J. Brodholt, G. D. Price (2002-04-25). "The ab initio simulation of the Earth’s core". D. Alfé. Retrieved 2009-08-02. 
  12. 12.0 12.1 Tackley, Paul J. (2000-06-16). [Expression error: Unexpected < operator "Mantle Convection and Plate Tectonics: Toward an Integrated Physical and Chemical Theory"]. Science 288 (5473): 2002–2007. doi:10.1126/science.288.5473.2002. PMID 10856206. 
  13. "The Crust". Oregon State University. Retrieved 2009-07-03. 
  14. 14.0 14.1 14.2 Seyfert, Carl K. (1987). The Encyclopedia of Structural Geology and Plate Tectonics. ISBN 9780442281250. 
  15. 15.0 15.1 Oreskes (2003). Plate Tectonics : An Insider's History of the Modern Theory of the Earth. Westview Press. ISBN 9780813341329. 
  16. "CHAPTER 8: Introduction to the Hydrosphere". Physical Geography. Retrieved 2009-08-06. 
  17. "WORLD WATER RESOURCES AND THEIR USES". UNESCO. Retrieved 2009-08-06. 
  18. "NASA - Earth's atmosphere". NASA. Retrieved 2009-08-06. 
  19. "Fundamentals of physical geography - the greenhouse effect". Physical Geography. Retrieved 2009-08-06. 
  20. 20.0 20.1 "What causes weather?". NASA. Retrieved 2009-08-06. 
  21. "NASA- an Earth fact sheet". NASA. Retrieved 2009-08-06. 
  22. "Earth's location in the Milky Way". NASA. Retrieved 2009-08-06. 

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