Talk:Earth/rewrite

Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third-closest planet to the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harboring life. The planet formed around 4.57 billion (4.57×109) years ago, and shortly thereafter (4.533 billion years ago) acquired its single natural satellite, the Moon.

Its astronomical symbol consists of a circled cross, representing a meridian and the equator; a variant puts the cross atop the circle (Unicode: &#x2295; or &#x2641;). Besides words derived from Terra, such as terrestrial, terms that refer to the Earth include tellur- (telluric, tellurian, from the Roman goddess Tellus) and geo- (geocentric, geothermal (from the Greek goddess Gaia).

Shape and structure
The Earth's shape is that of an oblate spheroid, with an average diameter of approximately 12,742 km. The rotation of the Earth causes the equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,850 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%. For perspective, this is less than the 0.22% tolerance allowed in pool balls. Due to the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador. The mass of the Earth is approximately 5,980 yottagrams (5.98 x 1024 kg).

Calculated by mass, the Earth is mainly composed by iron (34.1%), oxygen (28.2%), and silicon (17.2%). There are also large amounts of nickel (1.6%), calcium (1.5%), aluminium (1.5%), sulfur (0.7%), sodium (0.25%), titanium (0.071%), and potassium (0.019%). Other elements make up only 0.53% combined.



The interior of the Earth, like that of the other terrestrial planets, is chemically divided into an outer siliceous solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The liquid outer core gives rise to a weak magnetic field due to the convection of its electrically conductive material.

Earth's composition (by depth below surface):
 * 0 to 60 km - Lithosphere (locally varies 5-200 km)
 * 0 to 35 km - Crust (locally varies 5-70 km)
 * 35 to 60 km - Uppermost part of mantle
 * 35 to 2890 km - Mantle
 * 100 to 700 km - Asthenosphere
 * 2890 to 5100 km - Outer core
 * 5100 to 6378 km - Inner core

The interior of the Earth reaches temperatures of 5650 +/- 600 kelvins. The planet's internal heat was originally generated during its accretion (see gravitational binding energy), and since then additional heat has continued to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.

Core
The average density of the Earth is 5515 kg/m3, making it the densest planet in the Solar system. Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within the core of the Earth. In its earliest stages, about 4.5 billion (4.5&times;109) years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation, while less-dense materials would have migrated to the crust. As a result, the core is largely composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).

The core is divided into two parts, a solid inner core with a radius of ~1250 km and a liquid outer core extending beyond it to a radius of ~3500 km. The inner core is generally believed to be solid and composed primarily of iron and some nickel. Some have argued that the inner core may be in the form of a single iron crystal. The outer core surrounds the inner core and is believed to be composed of liquid iron mixed with liquid nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation (see: Coriolis effect), gives rise to the Earth's magnetic field through a process described by the dynamo theory. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilise the magnetic field generated by the liquid outer core. Recent evidence suggests that the inner core of Earth may rotate slightly faster than the rest of the planet.

Mantle
Earth's mantle extends to a depth of 2890 km. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 1021 and 1024 Pa·s, depending on depth. Thus, the upper mantle can only flow very slowly.

Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting points of iron-rich substances are higher than that of pure iron. The core is composed almost entirely of pure iron, whereas iron-rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.

Crust
The crust ranges from 5 to 70 km in depth. The thin parts are oceanic crust composed of dense (mafic) iron magnesium silicate rocks and underlie the ocean basins. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is known as the Mohorovi&#269;i&#263; discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted into the continental crust and preseved as ophiolite sequences.

Orbit and rotation
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 ca. 1 &deg;/day, i.e. a Sun or Moon diameter every 12 hours eastward. The orbital speed of the Earth averages about 30 km/s, which is enough to cover one Earth diameter (~12,700 km) in 7 minutes, and one distance to the Moon (384,000 km) in 4 hours.

It takes Earth 23 hours, 56 minutes and 4.09 seconds (1 sidereal day) to rotate around the axis connecting the north pole and the south pole. Thus from Earth the main apparent motion of celestial bodies in the sky (except meteors which are within the atmosphere and low orbiting satellites) is the movement to the west at a rate of 15 &deg;/h = 15'/min, i.e. a Sun or Moon diameter every two minutes. 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 (otherwise 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 equatorial bulge due to the Earth's 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. Also the rotational velocity varies, a phenomenon known as length of day variation.

The Hill sphere (sphere of gravitational influence) of the Earth is about 1.5 Gm (930 thousand miles) in radius, within which one natural satellite (the Moon) comfortably orbits. Earth has at least one known co-orbital asteroid, 3753 Cruithne.

Moon
Earth has one natural satellite, the Moon. The Moon is a relatively large terrestrial planet-like satellite, about one quarter of Earth's diameter. The Moon's origin is unknown, but one popular hypothesis, the giant impact theory, states that it was formed from the collision of a Mars-sized protoplanet with the early Earth. Among other things, this hypothesis explains the Moon's relative lack of iron and volatile elements.

The Moon orbits around Earth every 27 1/3 days. Thus from Earth this gives an apparent movement of the Moon with respect to the Sun and the stars at a rate of roughly 12 &deg;/day, i.e. a Moon diameter every hour eastward. The gravitational attraction between the Earth and Moon cause the 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. The Moon is just far enough away to have, when seen from Earth, very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, but the Moon is 400 times closer). This allows total eclipses as well as annular eclipses to occur on Earth.



The Moon may enable life by moderating the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilised by tidal interactions with the Moon. Without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, some theorists believe that the rotational axis might be chaotically unstable, as it appears to be with Mars. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result as this would make seaonal differences extreme. 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. This remains a controversial subject, however, and further studies of Mars &mdash;which shares Earth's rotation period and axial tilt, but not its large moon or liquid core&mdash; may provide additional information.

Atmosphere
Earth has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water vapor. The atmosphere acts as a buffer between Earth and the Sun. The Earth's atmospheric composition is unstable and is maintained by the biosphere. Namely, the large amount of free diatomic oxygen is maintained through solar energy by the Earth's plants, and without the plants supplying it, the oxygen in the atmosphere will over geological timescales combine with material from the surface of the Earth. Free oxygen in the atmosphere is a signature of life.

The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes.

The total mass of the atmosphere is about 5.1 &times; 1018 kg, ca. 0.9 ppm of the Earth's total mass.

Hydrosphere


Earth is the only planet in the Solar System whose surface has liquid water. Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water ) and divides it into five oceans and seven continents. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.

Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (cyanobacteria) had colonized the oceans, the greenhouse effect failed, and Earth's oceans may have completely frozen over for 10 to 100 million years in a snowball Earth event.

On other planets, such as Venus, gaseous water is destroyed (cracked) by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

In the Earth's atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere (plants). The magnetosphere also shields the ionosphere from direct scouring by the solar wind.

Finally, vulcanism continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone rocks are subducted into the mantle and volcanically released as gaseous carbon dioxide and steam. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.

The total mass of the hydrosphere is about 1.4 &times; 1021 kg, ca. 0.023 % of the Earth's total mass.



Biosphere
Earth is the only place where life is known to exist. 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&times;109) years ago. 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.

Human geography
On 25 February 2005 the United Nations Population Division issued revised estimates and projected that the world's population will reach 7 billion by 2013 and swell to 9.1 billion in 2050. Most of the growth is expected to take place in developing nations.

Nearly all humans currently reside on Earth: 6,411,000,000 inhabitants (January 5 2005 est.)

Two humans are presently in orbit around Earth on board the International Space Station. The station crew is replaced with new personnel every six months. During the exchange there are more, and sometimes others are also traveling briefly above the atmosphere.

In total, about 400 people have been outside Earth (in space) as of 2004.

See also space colonization.

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 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.

Descriptions of Earth
Earth has often been personified as a deity, in particular a goddess (see Gaia and Mother Earth). The Chinese earth goddess Hu-Tu, is similar to Gaia, the deification of the earth. 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.

Since Earth is rather large, it is not immediately obvious to the naked eye viewing from the surface that it is an oblate spheroid, bulging slightly at the equator and slightly flattened at the poles. In the past there were varying levels of belief in a flat Earth because of this. Prior to the introduction of space flight, this belief was 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 requsite 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. A photo taken of Earth by Voyager 1 inspired Carl Sagan to describe the planet as a "Pale Blue Dot". Earth has also been described as a massive spaceship, with a life support system that requires maintenance. See Spaceship Earth.

For descriptions of the Earth in (science) fiction, see Earth in fiction.