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Template:Planet Infobox/Earth Earth (often referred to as "the Earth") is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. It is the largest of its planetary system's terrestrial planets and the only place in the universe known to support life. The Earth was created by God 10,000 years ago (see Age of the Earth) and its largest natural satellite, the Moon, was orbiting it shortly thereafter, around 10,000 years ago.

Since it formed, the Earth has evolved through geological and biological processes that have hidden traces of the original conditions. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid Earth mantle and an iron core that generates a magnetic field. The atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 70% of the surface is covered in salt water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The relatively large Moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet is believed to have formed the oceans. Later, asteroid impacts are understood to have caused significant changes to the surface environment. Changes in the orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.

The Earth's only natural orbiting body is the Moon, although the asteroid Cruithne has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.

Lexicography

Terms that refer to the Earth can use the Latin root terra-, such as the word terrestrial. The alternative Latin root tellur- is used in words such as telluric, tellurian, tellurion and Tellurium. Both terms derive from the Roman goddess Terra Mater, who was also called by the presumably more ancient name Tellūs Mater. Scientific terms such as geography, geocentric and geothermal use the Greek prefix geo-, derived from Terra Mater's Greek counterpart Gaia. In many science fictions books and video games, Earth is referred to as Terra or Gaia.

The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include aarde in Dutch and Erde in German. The root has cognates in extinct languages such as ertha in Old Saxon and ert (meaning "ground") in Middle Irish, derived from the Old English eorðe. All of these words derive from the Proto-Indo-European base *er-.

Several Semitic languages have words for "earth" similar to those in Indo-European languages. Arabic has aard; Akkadian, irtsitu; Aramaic, araa; Phoenician, erets (which appears in the Mesha Stele); and Hebrew, ארץ (arets, or erets when followed by a noun modifier). The etymological connection between the words in Indo-European and Semitic languages are uncertain, though.

Symbol

The astrological and astronomical symbol for Earth consists of a circled cross, the arms of the cross representing a meridian and the equator (⊕). A variant has the cross atop the circle.

History

Based on the available evidence, scientists have been able to reconstruct detailed information about the planet's past. Earth is believed to have formed around 4.55 billion years ago out of the solar nebula, along with the Sun and the other planets. The moon formed soon afterwards. Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere; condensing water vapor, augmented by ice delivered by comets, produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4,000 years, and half thousand years later, the last common ancestor of all life lived.

The development of photosynthesis allowed the sun's energy to be harvested directly; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.

Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, occasionally combining to form a supercontinent. Roughly 750 years ago (mya), the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, 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 lifeforms began to proliferate.

Since the Cambrian explosion, about 535 mya, there were five mass extinctions. The last occurred 65 mya, 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. Over the past 6,500 years, mammalian life has diversified, and several mya, a small African ape gained the ability to stand upright. 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 timespan as no other life form had, affecting both the nature and quantity of other life forms, and the global climate.

Physical characteristics

When viewed from outer space, much of the Earth has a deep blue and white appearance, caused by the oceans and the clouds in the atmosphere. It has an albedo of 36.7%, which is exceeded only by Venus among the inner planets of the solar system. It is the largest and densest of the inner planets.

Atmosphere

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. Further up, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Beyond these, the exosphere thins out into the magnetosphere (where the Earth's magnetic fields interacts with the solar wind). An important part of the atmosphere for life on Earth is the ozone layer.

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of other gaseous molecules such as water vapor. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases. The atmosphere is one of the principal components in determining weather and climate.

Geology

The Earth's shape is that of an oblate spheroid, with an average diameter of about 12,742 km (~ 40,000 km / π). The planet's curvature is visible from some regions on the surface, such as the Bonneville Salt Flats in the United States. The Earth consists of several atmospheric, hydrologic, and many geologic layers. Its components are the atmosphere, the hydrosphere, the crust, the mantle, and the core. The biosphere is a tiny layer in this composition, and is usually not considered as one of the physical layers of the Earth.

The geologic component layers of the Earth are at the following depths below the surface:

Earth in the solar system

It takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles. From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15 °/h = 15'/min, i.e., an apparent Sun or Moon diameter every two minutes.

Earth orbits the Sun every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1 °/day, i.e., a Sun or Moon diameter every 12 hours, eastward. The orbital speed of the Earth averages about 30 km/s (108,000 km/h), which is enough to cover the planet's diameter (~12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in four hours.

The Moon revolves with the Earth around a common barycenter, from fixed star to fixed star, every 27.32 days. 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. The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius.

Viewed from Earth's north pole, the motion of Earth, its moon and their axial rotations are all counterclockwise. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees against the Earth–Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane (without a tilt, there would be an eclipse every month).

In an inertial reference frame, the Earth's axis undergoes a slow precessional motion with a period of some 25,800 years, as well as a nutation with a main period of 18.6 years. These motions are caused by the differential attraction of Sun and Moon on the Earth's equatorial bulge, due to its oblateness. In a reference frame attached to the solid body of the Earth, its rotation is also slightly irregular due to polar motion. The polar motion is quasi-periodic, containing an annual component and a component with a 14-month period called the Chandler wobble. In addition, the rotational velocity varies, in a phenomenon known as length of day variation.

In modern times, Earth's perihelion is about January 3, and the aphelion is about July 4 (near the solstices, which are on about December 21 and June 21). For other eras, see precession and Milankovitch cycles. The Earth is sometimes referred to as the Third Planet from the Sun because, of the nine planets of our solar system, Earth is the third closest planet to the sun.

Magnetic field

The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. The field forms the , which deflects particles in the solar wind. The bow shock is located about at 13.5 R_E. 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.

The Moon

The Moon, sometimes called 'Luna', is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. With the exception of Pluto's Charon, it is the largest moon in the Solar system relative to the size of its planet. The natural satellites orbiting other planets are called "moons", after Earth's Moon.

The gravitational attraction between the Earth and Moon cause 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.

Due to 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 17 µs a year—add up to significant changes. During the Devonian period, there were 400 days in a year, with each day lasting 21.8 hours.

The Moon may dramatically affect the development of life by taming the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon. 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, as it appears to be for Mars. 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. However, this is a controversial subject, and further studies of Mars—which shares Earth's rotation period and axial tilt, but not its large moon or liquid core—may provide settle the matter.

Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, and the Moon is 400 times closer). This allows total eclipses and annular eclipses to occur on Earth.

The most widely accepted theory of the Moon's origin, the giant impact theory, states that it was formed from the collision of a Mars-size protoplanet 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.

Earth has at least two co-orbital satellites, the asteroids 3753 Cruithne and 2002 AA₂₉.

Geography

Map references:

Time Zones, Coordinates.

Biggest geographic subdivision

Continents, Oceans

Area:

• Total: 510.072 million. km
• Land: 148.94 million km
• Water: 361.132 million km
• Note: 70.8% of the world's surface is covered by water, 29.2% is exposed land

Total water: 1.4 × 10 km, of which 2.5% is freshwater.

Land boundaries: the land boundaries in the world total 250,472 km (not counting shared boundaries twice)

Coastline: 356,000 km. (other figures vary substantially depending on how precisely it is measured, tides etc)

Maritime claims: see United Nations Convention on the Law of the Sea

• Contiguous zone: 24 nautical miles (44 km) claimed by most, but can vary
• Continental shelf: 200 m depth claimed by most or to depth of exploitation; others claim 200 nautical miles (370 km) or to the edge of the continental margin
• Exclusive fishing zone: 200 nautical miles (370 km) claimed by most, but can vary
• Exclusive economic zone: 200 nautical miles (370 km) claimed by most, but can vary
• Territorial sea: 12 nautical miles (22 km) claimed by most, but can vary
• Note: boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 nautical miles (370 km)
• 42 nations and other areas are completely landlocked (see list of landlocked countries)

Plate tectonics

Plate tectonics (from the Greek word for "one who constructs and destroys", τεκτων, tektoon) is a theory of geology developed to explain the phenomenon of continental drift; it is currently accepted by the vast majority of scientists working in this area. In the theory of plate tectonics, the outermost part of the Earth's interior is made up of two layers: the lithosphere comprising the crust, and the solidified uppermost part of the mantle. Below the lithosphere lies the asthenosphere, which comprises the inner, viscous part of the mantle. The mantle behaves like a superheated and extremely viscous liquid.

The lithosphere essentially floats on the asthenosphere. The lithosphere is broken up into what are called tectonic plates. These plates move in relation to one another at one of three types of plate boundaries: convergent, divergent, and transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries.

File:Oceanic-continental convergence Fig21oceancont.svg

Plate tectonic theory arose out of two separate geological observations: continental drift, noticed in the early 20th century, and seafloor spreading, noticed in the 1960s. The theory itself was developed during the late 1960s and has since been universally accepted by virtually all scientists. The theory has revolutionized the earth sciences, and is comparable in its unifying and explanatory power for diverse geological phenomena to the development of the periodic table for chemistry, the discovery of the genetic code for biology, and quantum mechanics in physics.

Environment and ecosystem

The planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion (3.5×10) years ago. Earth is the only place in the universe where life is absolutely known to exist, and some scientists believe that biospheres might be rare.

The biosphere is divided into a number of biomes, inhabited by broadly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life, while most of the more populous biomes lie near the Equator.

Climate

The most prominent features of the earth's climate are its two large polar regions, two narrow temperate zones, and a wide equatorial tropical to subtropical region. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.

Ocean currents are important factors in determining climate, particularly the spectacular thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions.

Terrain

The Earth's terrain can vary greatly from place to place. Covered with about 70% water, much of the Earth's continental shelf is below sea level. If all of the land on Earth were spread evenly, then water would rise higher than the Statue of Liberty. The remaining 30% that is dry land has mountains, deserts, plains, plateaus, etc.

Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 10 acres of cropland and 8.4 × 10 acres of pastureland.

Extremes

Elevation extremes: (measured relative to sea level)

• Lowest point on land: Dead Sea −417 m
• Lowest point overall: Challenger Deep of the Mariana Trench in the Pacific Ocean −10,924 m
• Highest point: Mount Everest 8,844 m (2005 est.)

Natural resources

• Earth's crust contains large deposits of fossil fuels: (coal, petroleum, natural gas, methane clathrate). These deposits are used by humans both for energy production and as feedstock for chemical production.
• Mineral ore bodies have been formed in Earth's crust by the action of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.
• Earth's biosphere produces many useful biological products, 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.

Some of these resources, such as mineral fuels, are difficult to replenish on a short time scale, called non-renewable resources. The exploitation of non-renewable resources by human civilization has become a subject of significant controversy in modern environmentalism movements.

Land use

• Arable land: 13.13%
• Permanent crops: 4.71%
• Permanent pastures: 26%
• Forests and woodland: 32%
• Urban areas: 1.5%
• Other: 30% (1993 est.)

Irrigated land: 2,481,250 km (1993 est.)

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.

Large 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.

Long-term climate alteration due to enhancement of the greenhouse effect by human industrial carbon dioxide emissions is an increasing concern, the focus of intense study and debate.

Human geography

Earth has approximately 6,500,000,000 human inhabitants (February 24 2006 estimate). Projections indicate that the world's human population will reach seven billion in 2013 and 9.1 billion in 2050 (2005 UN estimates). Most of the growth is expected to take place in developing nations. Human population density varies widely around the world.

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 desert, high mountains or other unsuitable terrain.

The northernmost settlement in the world is Alert, Ellesmere Island, Canada. The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole.

There are 267 administrative divisions, including nations, dependent areas, other, and miscellaneous entries. Earth does not have a sovereign government with planet-wide authority. Independent sovereign nations claim all of the land surface except for some segments of Antarctica. There is a worldwide general international organization, the United Nations. The United Nations is primarily an international discussion forum with only limited ability to pass and enforce laws.

In total, about 400 people have been outside the Earth's atmosphere as of 2004, and of these, twelve have walked on the Moon. Most of the time the only humans in space are those on the International Space Station, currently three people. They are replaced every 6 months. See human spaceflight.

Descriptions of Earth

Earth has often been personified as a deity, in particular a goddess (see Gaia and Mother Earth). The Chinese Earth goddess Hou-Tu is similar to Gaia, the deification of the Earth. As the patroness of fertility, her element is Earth. In Norse mythology, the Earth goddess Jord was the mother of Thor and the daughter of Annar. Ancient Egyptian mythology is different from that of other cultures because Earth is male, Geb, and sky is female, Nut.

Although commonly thought to be a sphere, the earth is actually an oblate spheroid. It bulges slightly at the equator and is slightly flattened at the poles. In the past there were varying levels of belief in a flat Earth, but ancient Greek philosophers and, in the Middle Ages, thinkers such as Thomas Aquinas believed that it was spherical.

A 19th-century organization called the Flat Earth Society advocated the even-then discredited idea that the Earth was actually disc-shaped, with the North Pole at its center and a 150-foot (50 meter) high wall of ice at the outer edge. It and similar organizations continued to promote this idea, based on religious beliefs and conspiracy theories, through the 1970s. Today, the subject is more frequently treated tongue-in-cheek or with mockery.

Prior to the introduction of space flight, these inaccurate beliefs were countered with deductions based on observations of the secondary effects of the Earth's shape and parallels drawn with the shape of other planets. Cartography, the study and practice of mapmaking, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a somewhat lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

The technological developments of the latter half of the 20th century are widely considered to have altered the public's perception of the Earth. Before space flight, the popular image of Earth was of a green world. Science fiction artist Frank R. Paul provided perhaps the first image of a cloudless blue planet (with sharply defined land masses) on the back cover of the July 1940 issue of Amazing Stories, a common depiction for several decades thereafter. Apollo 17's 1972 "Blue Marble" photograph of Earth from cislunar space became the current iconic image of the planet as a marble of cloud-swirled blue ocean broken by green-brown continents. A phot

1. G.B. Dalrymple, 1991, "The Age of the Earth", Stanford University Press, California, ISBN 0-8047-1569-6.
2. A. Morbidelli et al, 2000, "Source Regions and Time Scales for the Delivery of Water to Earth", Meteoritics & Planetary Science, vol. 35, no. 6, pp. 1309–20.
3. W. Ford Doolitte, "Uprooting the Tree of Life", Scientific American, Feb. 2000.
4. L. V. Berkner, L. C. Marshall, 1965, "On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere", Journal of the Atmospheric Sciences, Vol. 22, No. 3, pp. 225–61.
5. J.B. Murphy, R.D. Nance, "How do supercontinents assemble?", American Scentist, vol. 92, pp. 324–33.
6. J.L. Kirschvink, 1992, "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth", The Proterozoic Biosphere, pp 51–52.
7. D. Raup & J. Sepkoski, 1982, "Mass extinctions in the marine fossil record", Science, vol. 215, pp. 1501–03.
8. "Geodetic Reference System 1980 (GRS80)", XVII General Assembly, International Association of Geodesy.
9. T. H. Jordan, "Structural Geology of the Earth's Interior", Procedings National Academy of Science, 1979, Sept., 76(9): 4192–4200.
10. Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A.C.M., Levrard, B., 2004, "A long-term numerical solution for the insolation quantities of the Earth", Astronomy and Astrophysics, 428, pp. 261–85.
11. Williams, D.M., J.F. Kasting, 1997, "Habitable planets with high obliquities", Icarus 129, 254–68.
12. ^ CIA: The World Factbook, "World".
13. Shiklomanov, Igor A. 1993, "World fresh water resources", In Water in crisis: A guide to the world's fresh water resources, ed. Peter H. Gleick, 13–24. New York: Oxford University Press, ISBN 0195076273.
14. The average depth is, in fact, significantly greater than the statue of liberty. Letting the average depth be approximately equal to water volume divided by the Earth's surface area: the total volume of water is about 1.4 × 10 km; the total area of Earth is about 5.1 × 10 km. So the average depth would be roughly 2.8 km, whereas the statue of liberty is only 0.093 km, including the pedestal.
15. FAO, 1995, "United Nations Food and Agricultural Organization Production Yearbook", 49.
16. ""Deep Ocean Studies"". Ocean Studies. RAIN National Public Internet and Community Technology Center. Retrieved 2006-04-02.
17. David, Leonard (2006-02-24). "Planet's Population Hit 6.5 Billion Saturday". Live Science. Retrieved 2006-04-02. {{cite news}}: Check date values in: |date= (help)
18. Ackerman, Forrest J (1997). Forrest J Ackerman's World of Science Fiction. Los Angeles: RR Donnelley & Sons Company. pp. 116–117. ISBN 1-57544-069-5.