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Earth (or the Earth) is the third planet from the Sun or Sol; it is the only planet known to be capable of supporting life.

Earth
Orbital characteristics
Mean radius149,600,000 km
Perihelion147,000,000 km
Aphelion152,000,000 km
Revolution period365.256 days
Inclination0°
Number of satellites1
Physical characteristics
Equatorial diameter12,756.3 km
Surface area510.072 million km2
Mass5.972×1024 kg
Mean density5.515 g/cm3
Surface gravity9.78 m/s2
Rotation period23.9345 hours
Axial tilt23.45°
Mean surface temp.288 K
Atmospheric pressure101 kPa
Atmospheric composition
Nitrogen78%
Oxygen21%
Argon1%
Carbon dioxide

Water vapor

trace

File:Earth-apollo17-thumb.jpg

View of Africa, Antarctica and Arabian Peninsula from Apollo 17. This picture was taken as Apollo 17 left Earth orbit en route to the Moon. Taken on December 7, 1972. click here for larger image

Water covers over 70% of Earth's surface and divides it into five oceans and seven continents. It has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water. The atmosphere acts as a buffer between Earth and the sun. The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes.

The interior of Earth is, like the other terrestrial planets, divided into an outer silicaceous solid crust, with a highly viscous mantle, an outer core that is less viscous than the mantle, and a solid inner nickel-iron core. The planet is big enough to have the core differentiated into an liquid outer core, which gives rise to a weak magnetic field due to the convection of its electrically conductive material, and a solid inner core.

The inner core of Earth was recently discovered to rotate slightly faster than the rest of the planet, completing one additional rotation every 600 years. It is not known exactly why this occurs, but it is thought to be a result of the circulation of the liquid outer core and interaction with Earth's magnetic field.

Earth is the only known place in the universe that supports life, notably humans. Scientists have not ruled out the possibility of the existence of life in other places in the universe, and some think it likely.

Some planetologists believe that the Earth is only marginally able to support life. The evidence is that life is practically nonexistent at the poles, and increases in density from the poles to the equator. The so-called "temperate" zones are unable to support life for a third to a quarter of an orbit. Life in the temperate zones exhibits extreme cold-adaptations including world-spanning migrations, anti-freeze blood, exothermic metabolisms, insulation and long-term estivation.

The Moon

Earth is unique in its solar system in having "the Moon", named Luna, a huge terrestrial planet-like satellite that is about 1/4 of Earth's diameter. The moons orbiting other planets are so called after Earth's moon.

By coincidence, the Moon is just far enough away to have, when seen from the Earth, the same apparent angular size as the Sun. This allows a total eclipse to occur on Earth.

Also, the Moon is tidally locked: its rotation period is the same as the time it takes to revolve around the Earth, meaning it always presents the same face to the planet, seeming to disappear and reappear as the solar terminator line moves around the moon.

The origin of the Moon is presently unknown, but one popular theory has it that it was formed from the collision of a Mars-sized protoplanet into the early Earth. This theory explains (among other things) the Moon's relative lack of iron and volatile elements. See Giant impact theory.

Paleontological evidence shows that Earth's axial tilt is stabilised by tidal interactions with its moon. Without this stabilization, the rotational axis might be chaotically unstable, as it is with a sphere. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result as one pole was continually heated and the other cooled. Planetologists 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 information.

Why Earth is Wet

Earth is the only planet in our solar system, or even the known universe, whose surface has liquid water. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen 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, the Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Archaea) had colonized the oceans, the greenhouse effect failed, and the Earth froze solid for 10 to 100 million years.

On other planets, such as Venus, gaseous water is cracked by solar ultraviolet, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. It is believed to be why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

On Earth, a shield of ozone absorbs most of this energetic ultraviolet high in the atmosphere, reducing the cracking effect. The magnetosphere also shields the ionosphere from direct scouring by the solar wind. Finally, vulcanism, aided by tidal effects, continuously emits water(vapor) from the interior.

It is known that lunar tides keep the crust broken, and therefore aid the recycling of carbon and water as limestone fields are subducted into magma and volcanically emitted as gaseous carbon dioxide and steam.

Without lunar tides, some planetologists speculate that Earth's crust would have solidified. Its crust would have accreted all available carbon as limestone. This would remove the carbon needed by the biosphere. As the greenhouse effect failed, the result would be an ice planet similar to Mars. The oxygen would all combine with surface minerals, and then as the water sublimated, the hydrogen would be cracked from the water and scoured away by the solar wind, creating a waterless desert.

Geography

Map references:

World, Time Zones

Area:

  • total: 510.072 million sq km
  • land: 148.94 million sq km
  • water: 361.132 million sq km
  • 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 251,480.24 km (not counting shared boundaries twice)

Coastline: 356,000 km

Maritime claims:

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 meters of water per year to less than a millimeter.

Terrain:

The greatest ocean depth is the Mariana Trench at 10,924 m in the Pacific Ocean

Elevation extremes: (measured relative to sea level)

Natural resources

Some of these resources, such as fossil 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%
  • other: 31% (1993 est.)

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

Natural 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, tornados, sinkholes, floods, droughts, and other calamities and disasters.

Environment - current issues:

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, soil degradation, soil depletion, erosion

Human population:

6,080,671,215 (July 2000 est.)

Age structure:

  • 0-14 years: 29.92% (male 932,832,913; female 885,970,165)
  • 15-64 years: 63.17% (male 1,942,402,264; female 1,898,479,062)
  • 65 years and over: 6.91% (male 184,072,470; female 235,017,660) (2000 est.)

Population growth rate: 1.3% (2000 est.)

Birth rate: 22 births/1,000 population (2000 est.)

Death rate: 9 deaths/1,000 population (2000 est.)

Sex ratio:

  • at birth: 1.05 male(s)/female
  • under 15 years: 1.05 male(s)/female
  • 15-64 years: 1.02 male(s)/female
  • 65 years and over: 0.78 male(s)/female
  • total population: 1.01 male(s)/female (2000 est.)

Infant mortality rate: 54 deaths/1,000 live births (2000 est.)

Life expectancy at birth:

  • total population: 64 years
  • male: 62 years
  • female: 65 years (2000 est.)

Total fertility rate: 2.8 children born/woman (2000 est.)

Government:

Data code: None; there is no FIPS 10-4 country code for the World, so the CIA Factbook uses the "W" data code from DIAM 65-18 "Geopolitical Data Elements and Related Features," Data Standard No. 3, March 1984, published by the Defense Intelligence Agency; see the Cross-Reference List of Country Data Codes appendix

Administrative divisions: 267 nations, dependent areas, other, and miscellaneous entries

Legal system: See international law

Economy - overview: Growth in global output (gross world product, GWP) rose to 3% in 1999 from 2% in 1998 despite continued recession in Japan, severe financial difficulties in other East Asian countries, and widespread dislocations in several transition economies, notably Russia. The US economy continued its remarkable sustained prosperity, growing at 4.1% in 1999, and accounted for 23% of GWP. Western Europe's economies grew at roughly 2%, not enough to cut deeply into the region's high unemployment; the EU economies produced 20% of GWP. China, the second largest economy in the world, continued its strong growth and accounted for 12% of GWP. Japan grew at only 0.3% in 1999; its share in GWP is 7%. As usual, the 15 successor nations of the USSR and the other old Warsaw Pact nations experienced widely different rates of growth. The developing nations varied widely in their growth results, with many countries facing population increases that eat up gains in outp

GDP: GWP (gross world product) - purchasing power parity - $40.7 trillion (1999 est.)

GDP - real growth rate: 3% (1999 est.)

GDP - per capita: purchasing power parity - $6,800 (1999 est.)

Inflation rate (consumer prices): all countries 25%; developed countries 1% to 3% typically; developing countries 5% to 60% typically (1999 est.)
note: National inflation rates vary widely in individual cases, from stable prices in Japan to hyperinflation in a number of Third World countries

Unemployment rate: 30% combined unemployment and underemployment in many non-industrialized countries; developed countries typically 4%-12% unemployment (1999 est.)

Industries: dominated by the onrush of technology, especially in computers, robotics, telecommunications, and medicines and medical equipment; most of these advances take place in OECD nations; only a small portion of non-OECD countries have succeeded in rapidly adjusting to these technological forces; the accelerated deployment of new industrial (and agricultural) technology is complicating already grim environmental problems in some regions.

Yearly electricity - production: 12,342.7 billion kWh(1994)

Yearly electricity - consumption: 12,342.7 billion kWh (1994)

Yearly exports: $5.6 trillion (f.o.b., 1999 est.)

Exports - commodities: the whole range of industrial and agricultural goods and services

Exports - partners: in value, about 75% of exports from the developed countries

Yearly imports: $5.6 trillion (f.o.b., 1999 est.)

Imports - commodities: the whole range of industrial and agricultural goods and services

Imports - partners: in value, about 75% of imports by the developed countries

Debt - external: $2 trillion for less developed countries (1999 est.)

Yearly economic aid - recipient: traditional worldwide foreign aid $50 billion (1997 est.)

(Note: All exports, imports, debts and economic aid listed are between nations on Earth. There are currently no significant extraterrestrial imports or exports.)

Internet Service Providers (ISPs): 13,119 (1999)

Transportation:

Railways:

Ports and harbors: Chiba, Houston, Kawasaki, Kobe, Marseille, Mina' al Ahmadi (Kuwait), New Orleans, New York, Rotterdam, Yokohama

Military: Military expenditures - dollar figure: aggregate real expenditure on arms worldwide in 1999 remained at approximately the 1998 level, about three-quarters of a trillion dollars (1999 est.)

Yearly military expenditures - percent of GDP: roughly 2% of gross world product (1999 est.)

Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets

In addition to being a planet, earth is also one of the Chinese five elements. In this sense, it means soil.

Some other names for 'Earth' are;

The Earth has often been personified as a deity, often a goddess. See Gaea and Mother Earth. In Norse mythology, Earth was the son of Nott and Annar.

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