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A colour image of Earth as seen from Apollo 17.
The Blue Marble, taken from Apollo 17
Orbital characteristics ( Epoch J2000)
Aphelion 152,097,701 km
(1.016 710 333 5 AU)
Perihelion 147,098,074 km
(0.983 289 891 2 AU)
Semi-major axis 149,597,887.5 km
(1.000 000 112 4 AU)
Semi-minor axis 149,576,999.826 km
(0.999 860 486 9 AU)
Orbital circumference 924,375,700 km
(6.179 069 900 7 AU)
Orbital eccentricity 0.016 710 219
Sidereal orbit period 365.256 366 d
(1.000 017 5 a)
Synodic period n/a
Max. orbital speed 30.287 km/s
(109,033 km/h)
Average orbital speed 29.783 km/s
(107,218 km/h)
Min. orbital speed 29.291 km/s
(105,448 km/h)
Orbital inclination to ecliptic 0.000 05 °
(7.25° to Sun's equator)
Longitude of the ascending node 348.739 36°
Argument of the perihelion 114.207 83°
Satellites 1 (the Moon)
(see also 3753 Cruithne)
Physical characteristics
Aspect Ratio 0.996 647 139
Ellipticity 0.003 352 861
Radius:
Equatorial 6,378.135 km
Polar 6,356.750 km
Mean 6,372.795 km
Diameter:
Equatorial 12,756.270 km
Polar 12,713.500 km
Mean 12,745.591 km
Circumference:
Equatorial 40,075.004 km
Meridional 40,007.849 km
Mean 40,041.455 km
Surface Area 510,065,284.702 km²
Land 148,939,063.133 km² (29.2 %)
Water 361,126,221.569 km² (70.8 %)
Volume 1.0832×1012 km³
Mass 5.9736×1024 kg
Density 5,515 kg/m³
Equatorial surface gravity 9.7801 m/s²
(0.997 32 g)
Escape velocity 11.186 km/s
Sidereal rotation period 0.997 258 d (23.934 h)
Rotational velocity
(at the equator)
465.11 m/s
Axial tilt 23.439 281°
Right ascension
of North pole
0° (0 h 0 min 0 s)
Declination 90°
Albedo 0.367
Surface temperature
- min
- mean
- max

185 K (-88 °C)
287 K (14 °C)
331 K (58 °C)
Surface pressure 100 kPa
Adjective Earthling or Terrestrial
Atmospheric constituents
nitrogen 77 %
oxygen 21 %
argon 1 %
carbon dioxide 0.038%
water vapor trace (varies with climate)
Earth is the third planet from the Sun. It is the largest of the solar system's terrestrial planets and the only place in the universe known to harbor life. Scientific evidence indicates that the Earth was formed around 4.57 billion (4.57×109) years ago (see Age of the Earth) and that its single natural satellite, the Moon, was orbiting it shortly thereafter, around 4.533 billion years ago.

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Contents

Lexicography
History
Physical characteristics
Earth in the solar system
Geography
Environment and ecosystem
Descriptions of Earth



Lexicography - Contents

The official name for the planet Earth, as set by the International Astronomical Union, is Terra, after the Roman goddess Terra Mater. With this name, all the planets in our solar system known from ancient times are now named for Roman gods or goddesses. Terra is also the Latin word for "earth" (soil).Terms that refer to the Earth can use the Latin root terra-, such as the word terrestrial. There is also the alternative Latin root tellur-, as used in words such as telluric, tellurian, tellurion and Tellurium, which derives the goddess Terra Mater's 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.The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include aarde in Dutch, Erde in German and arde in Arabic. The root also has cognates in extinct languages such as ertha in Old Saxon and ert (meaning "ground") in Middle Irish, derived from the Old English eorðe. Given metathesis, we can find cognates of "earth" between terra and the modern Romance languages, for instance tierra in Spanish or terra in Portuguese.Several Semitic languages have words for "earth" similar to those in Indo-European languages, although evidence of a link is not overwhelming. 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 puts the cross atop the circle ().


History - Contents

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 4.55×109 years ago out of the solar nebula, along with the Sun and 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 4×109 years ago, and half a billion 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. With the ozone layer absorbing harmful ultraviolet radiation, life colonized the surface of Earth. Continents formed and broke up as the surface of Earth continually reshaped itself. 65 million years ago, a meteorite collision probably triggered the extinction of the dinosaurs and other large reptiles, but spared small animals such as the mammals, then resembling shrews. Life diversified, and several million years ago, a small African ape gained the ability to stand upright. As brain size increased, these hominids developed the use of tools and language. The development of agriculture, and then civilization allowed humans to affect the Earth in a short timespan like no other life form had before, affecting both the nature and quantity of other life forms as well as global climate.


Physical characteristics - Contents

Earth cutaway from core to exosphere. Partially to scale
Earth cutaway from core to exosphere. Partially to scale
The Earth consists of several atmospheric, hydrologic, and mainly geologic layers. Its components are the atmosphere, the hydrosphere, the crust, the mantle, and its core. The biosphere is a tiny layer in this composition and is usually not considered part of the physical layers of the Earth.The geologic component layers of the Earth are located at the following depths below surface:
  • 0 to 60 km - Lithosphere (locally varies between 5 and 200 km)
    • 0 to 35 km - Crust (locally varies between 5 and 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




Earth in the solar system - Contents

It takes the Earth 23 hours, 56 minutes and 4.091 seconds ( 1 sidereal day) to rotate around the axis connecting the north pole and the south pole. 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 °/h = 15'/min, i.e., a 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 one Earth diameter (~12,700 km) in 7 minutes, and one distance to the Moon (384,000 km) in 4 hours.Earth has one natural satellite, the Moon, which 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.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).The Hill sphere (sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius, within which one natural satellite (the Moon) comfortably orbits.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. Also, the rotational velocity varies, a phenomenon known as length of day variation.In modern times, Earth's perihelion is always about January 3, and aphelion is about July 4 (near the winter and summer solstices, which are on about December 21 and June 21, respectively). 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.

The Moon
Earthrise as seen from the Moon on Apollo 8, 24 December 1968. Due to tidal locking, from any point on the Moon's surface, the Earth does not rise or set, but is always located in the same position in the sky.
Earthrise as seen from the Moon on Apollo 8, 24 December 1968. Due to tidal locking, from any point on the Moon's surface, the Earth does not rise or set, but is always located in the same position in the sky.

Main article: Moon
Name Diameter (km) Mass (kg) Semi-major axis (km) Orbital period
Moon 3,474.8 7.349×1022 384,400 27 Days, 7 hours, 43.7 minutes
The Moon, sometimes called 'Luna', is a relatively large terrestrial planet-like satellite, whose diameter is 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 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 may dramatically affect the development of life by taming the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilised 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 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 seasonal 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—which shares Earth's rotation period and axial tilt, but not its large moon or liquid core—may provide additional insight.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 and annular eclipses to occur on Earth.
Earth and Moon to scale.
Earth and Moon to scale.
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 also has at least one co-orbital asteroid, 3753 Cruithne.


Geography - Contents

Main article: Geography
Physical map of the Earth (Medium) (Large 2 MB)
Physical map of the Earth ( Medium) ( Large 2 MB)
Map references: Time Zones, Coordinates.Biggest geographic subdivision Continents, OceansArea:
  • Total: 510.073 million km2
  • land: 148.94 million km2
  • Water: 361.132 million km2
  • Note: 70.8 % of the world's surface is covered by water, 29.2 % is exposed land
Land boundaries: the land boundaries in the world total 251,480 km (not counting shared boundaries twice)Coastline: 356,000 kmMaritime 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)



Environment and ecosystem - Contents

Main article: Biosphere
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 planet's lifeforms are sometimes said to form a "biosphere". This biosphere is generally believed to have begun evolving about 3.5 billion (3.5×109) 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.
A familiar scene on Earth which simultaneously shows the lithosphere, hydrosphere and atmosphere.
A familiar scene on Earth which simultaneously shows the lithosphere, hydrosphere and atmosphere.


Climate
Main article: Climate
Two large areas of polar climates separated by two rather narrow temperate zones from a wide equatorial band of tropical to subtropical climates. Precipitation patterns vary widely, ranging from several metres of water per year to less than a millimetre.Ocean currents, particularly the spectacular thermohaline circulation which distributes heat energy from the equatorial oceans to the polar regions, are important determinators of climate.

Terrain
Elevation extremes: (measured relative to sea level)

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 ore 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: 10%
  • Permanent crops: 1%
  • Permanent pastures: 26%
  • Forests and woodland: 32%
  • Urban areas: 1.5%
  • Other: 30% (1993 est.)
Irrigated land: 2,481,250 km2 ( 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, floods, droughts, and other calamities and disasters.Large areas are subject to man-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 at night, composite of pictures taken between October 1994 and March 1995.
Earth at night, composite of pictures taken between October 1994 and March 1995.
Main article: Human
Earth has approximately 6,500,000,000 human inhabitants ( February 25, 2006 estimate). [2] 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 four hundred people have been outside the Earth's atmosphere as of 2004, and of these twelve have walked on the Moon. See space exploration.


Descriptions of Earth - Contents

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. 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.Although commonly thought to be a sphere, the earth is actually an "oblate spheroid". It bulges 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, but ancient Greek philosophers and, in the Middle Ages, thinkers such as Thomas Aquinas knew that the Earth was a sphere.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 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. 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, or as having a biosphere that forms one large organism. See Spaceship Earth and Gaia theory.For descriptions of the Earth in (science) fiction, see Earth in fiction.
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