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Template:Planet Infobox/Earth Earth, also known as 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. Scientific evidence indicates that 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: ⊕ or ♁). Besides words derived from Terra, such as terrestrial, terms that refer to the Earth include tellur- (telluric, tellurian, from the Roman goddess Tellūs) and geo- (geocentric, geothermal; from the Greek goddess Gaia).

The word Earth has cognates in many modern as well as defunct - including ancient - languages. Examples in modern tongues include aarde in Dutch and Erde in German. The root also has cognates in extinct languages such as ertha in Old Saxon and ert (meaning 'ground') in Middle Irish. It is derived from Old English eorðe. Taking into account metathesis, we can find cognates of the word Earth in the Latin terra and in the modern Romance Languages (i.e. tierra in Spanish).

Although a link to such Indo-European languages has not been proved, several Semitic languages have similar-sounding words for Earth: aard in Arabic, irtsitu in Assyrian, araa in Aramaic, erets in Phoenician (which appears in the Mesha Stele), and ארץ (arets, or erets when followed by a noun modifier) in Hebrew.

Physical characteristics

, see also Geology (science)

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

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 ca. 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 orbits around Earth every 27 1/3 days. From Earth this gives an apparent movement of the Moon with respect to the Sun and the stars at a rate of roughly 12 °/day, i.e., a Moon diameter every hour eastward.

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 thousand 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 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.

In modern times, Earth's perihelion is always about January 3, and aphelion is about July 4. For other eras, see precession and Milankovitch cycles.

The Earth is sometimes referred to as the Third Rock from the Sun because, of the nine planets of our solar system, Earth is the third closest to the sun.

The 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 moderating 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.

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 known co-orbital asteroid, 3753 Cruithne.

Geography

Map references:

Time Zones, Coordinates.

Biggest geographic subdivision

Continents, Oceans

Area:

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 km

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

Contiguous zone: 24 nautical miles (44.4 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 (370.4 km) or to the edge of the continental margin
Exclusive fishing zone: 200 nautical miles (370.4 km) claimed by most, but can vary
Exclusive economic zone: 200 nautical miles (370.4 km) claimed by most, but can vary
Territorial sea: 12 nautical miles (22.2 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 (370.4 km)
42 nations and other areas are completely landlocked (see list of landlocked countries)

Environment and Ecosystem

, see also Life.

Earth is the only place in the universe 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 (3.5×10⁹) 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.

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)

Lowest point on land: Dead Sea −417 m
Lowest point overall: Mariana Trench in the Pacific Ocean −10,924 m [1]
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 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 overpopulation, industrial disasters such as 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

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.). It is estimated that only 1/8th of the surface of the earth is suitable for humans to live on - 3/4 is covered by oceans, and half of the landmass is unsuitable, being desert, high mountain, etc. Coastal areas constitute the highest density.

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. Out of those only twelve humans have ever walked on a world other than Earth, the men of Apollo 11, 12, 14, 15, 16 and 17 who walked on the Moon between 1969 and 1972.

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, but ancient Greek philosophers and, in the Middle Ages, thinkers 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.

References

NASA's Earth fact sheet
Discovering the Essential Universe (Second Edition) by Neil F. Comins (2001)
space.about.com - Earth - Pictures and Astronomy Facts

External links

USGS Geomagnetism Program
[2] (pdf) - density, pressure, gravity, P-wave and S-wave seismic wave velocities, and Poisson's ratio as a function of depth
Earth 3D Map from NASA free downloadable (184.3 MB).
Google Earth - google earth search system.
[3] Nasa Earth Observatory - the Blue Marble (very high resolution images)

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