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 formed around 4.57 billion years agoG.B. Dalrymple, 1991, "The Age of the Earth", Stanford University Press, California, ISBN 0-8047-1569-6. (see Age of the Earth) and its largest natural satellite, the Moon, was orbiting it shortly thereafter, around 4.533 billion 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 molten 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

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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-. Given metathesis, there are cognates of "earth" between terra and the modern Romance languages, for instance tierra in Spanish and terra in Portuguese.

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).

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

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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.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. The 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 lived.W. Ford Doolitte, "Uprooting the Tree of Life", Scientific American, Feb. 2000.

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.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. 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 million 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.J.B. Murphy, R.D. Nance, "How do supercontinents assemble?", American Scentist, vol. 92, pp. 324–33.

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. J.L. Kirschvink, 1992, "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth", The Proterozoic Biosphere, pp 51–52.

Since the Cambrian explosion, about 535 mya, there were five mass extinctions.D. Raup & J. Sepkoski, 1982, "Mass extinctions in the marine fossil record", Science, vol. 215, pp. 1501–03. 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 65 million 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

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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)."Geodetic Reference System 1980 (GRS80)", XVII General Assembly, International Association of Geodesy. 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 EarthT. H. Jordan, "Structural Geology of the Earth's Interior", Procedings National Academy of Science, 1979, Sept., 76(9): 4192–4200. are at the following depths below the surface:

Depth		Layer
Kilometres	Miles
0–60	0–37	Lithosphere (locally varies between 5 and 200 km)
0–35	0–22	Crust (locally varies between 5 and 70 km)
35–60	22–37	Uppermost part of mantle
35–2890	22–1790	Mantle
100–700	62–435	Asthenosphere
2890–5100	1790–3160	Outer core
5100–6378	3160–3954	Inner core

Earth in the solar system

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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 Magnetosphere, 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

Name	Diameter (km)	Mass (kg)	Semi-major axis (km)	Orbital period
Moon	3,474.8	7.349	384,400	27 Days, 7 hours, 43.7 minutes

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.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. 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.Williams, D.M., J.F. Kasting, 1997, "Habitable planets with high obliquities", Icarus 129, 254–68. 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.