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


Larger image
Orbital characteristics
Mean radius149,597,870 km
Perihelion0.983 AU
Aphelion1.017 AU
Orbital period365.25636 days
Avg. Orbital Speed29.79 km/s
Satellites1 (the Moon)
Satellite ofSun
Physical characteristics
Equatorial diameter12,756.3 km
Surface area5.10072×108 km2
Mass5.974×1024 kg
Mean density5.515 g/cm3
Surface gravity9.78 m/s2
Escape velocity11.2 km/s
Rotation period23.9345 hours
Axial tilt23.45°
Surface temperature
182 K282 K333 K
Atmospheric characteristics
Pressure101.325 kPa
carbon dioxide
water vapor

Earth (or "the Earth" or "Terra") is the third planet of the Sol system and the only one in the universe known by us to be inhabited by intelligent[?] life (there is continuing controversy about the existence and nature of extraterrestrial life). The planet has one moon, Luna.

The image to the right is of Africa, Antarctica, and the Arabian Peninsula as taken en route to the Moon by Apollo 17 on December 7, 1972. Click here for a larger image.

Table of contents

Physical characteristics


The interior of Earth is, like the other terrestrial planets, chemically divided into an outer silicaceous solid crust, with a highly viscous mantle, an outer core that is less viscous than the mantle, and an inner 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.

New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see seafloor spreading). Much of the Earth's surface is less than 100,000,000 years old.


The interior of the Earth reaches temperatures of 5270 K. The planet's internal heat was originally released 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.

  • 0-60 km -- Lithosphere
    • 0-30/35 km -- Crust
  • 30/35-2900 km -- Mantle
  • 2900-5100 km -- Outer Core
  • 5100-~6375 km -- Inner Core

The Core

The outer core has a radius of ~3500km. The inner core has a radius of ~1250km.

The average density of Earth is 5,515 kg/m3. Since, the average density of surface material is around 3000 kg/m3, which indicates that denser materials exist within the core. (see: planetary differentiation) It is thought that the core is largely composed of iron (80%), along with nickel and silicon; with other dense elements such as lead and uranium either being too rare to be significant or being felsic-seeking in nature (and thus concentrated in the crust rather than the core).

The Earth was entirely molten about 4.6 billion years ago. Gravity would have caused denser substances to sink towards the center in a process called chemical differentiation[?], while less dense substances would have migrated to the crust.

The inner core is generally believed to be solid and to be composed entirely of iron and some nickel. Some believe it may be entirely composed of a single iron crystal. The inner core is surrounded by the outer core, which is believed to be liquid iron mixed with liquid nickel.

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet, by ~2° per year (Comins[?] DEU-p.82). It is generally believed that the rotation of the inner core (which is primarily composed of iron) creates the Earth's magnetic field. It is not known, exactly, why this occurs. (See also: dynamo theory)


The Earth's mantle extends to a depth of 2,900 km. The pressure, at the bottom of the mantle, is ~1.4Matm (140 GPa). 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).

Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting point of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-melted (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.


The crust ranges from 5-35 km in depth. It is composed of silicon-based rocks. The crust-mantle boundary occurs as two physically different events. Firstly, there is a discontinuity in the seismic velocity which is known as the Mohorovicic 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 none (below). The second event is a chemical discontinuity between ultramafic cumulates and tectonized hartzburgites which has been observed from parts of the oceanic crust that have been obducted.


Earth is the only place in the universe where we, humans, have reliably observed life. Every part of the planet supports life, from the polar ice caps to the equator. Recent advances in microbiology have proven that microscopic life lives inside rocks under the Earth's surface, and that the total mass of microbial life in so-called "uninhabitable zones" may, in terms of sheer biomass, outweight all animal and plant life combined on the surface of the Earth.

Large scale plant and life is practically nonexistent at the poles, and increases in density from the poles to the equator. Life in the temperate zones exhibits extreme cold-adaptations including world-spanning migrations, anti-freeze[?] blood, exothermic metabolisms, insulation and long-term estivation.

Most successful animal of the planet: Antarctic krill, Euphausia superba, with a biomass probably over 500 million tonnes.


Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water [1] (http://earthobservatory.nasa.gov/Library/Water/)) 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. This is sometimes described as the "third atmosphere" to distinguish it from earlier atmospheric compositions. See also: Earth's atmosphere.


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-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, 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 the moon's tidal effects, continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone fields are subducted into magma and volcanically emitted as gaseous carbon dioxide and steam.

The Earth also suffers from the Chandler wobble.

Luna, Earth's moon Earth is unique in its solar system in having a moon, named Luna (mostly known as "The Moon"), which is a relatively large terrestrial planet-like satellite, about one quarter of Earth's diameter. The natural satellites (or "moons") orbiting other planets are called after Earth's moon.

The moon may enable life by moderating the weather. 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.

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.

Earth also has at least one known co-orbital asteroid, 3753 Cruithne.

Geography Map references:

World, Time Zones


  • total: 510.072 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 251,480.24 km (not counting shared boundaries twice)

Coastline: 356,000 km

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

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: 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 km2 (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, tornadoes, 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.)

As of 2003, there is a permanent human presence in space, in the International Space Station, of three people, who are from time to time replaced.

Age structure:

  • 0-14 years: 1,818,803,078 (29.92%)
    • male: 932,832,913 (15.35%)
    • female: 885,970,165 (14.57%)
  • 15-64 years: 3,840,881,326 (63.19%)
    • male: 1,942,402,264 (31.95%)
    • female: 1,898,479,062 (31.23%)
  • 65 years and over: 419,090,130 (6.89%)
    • male: 184,072,470 (3.03%)
    • female: 235,017,660 (3.87%) (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 Administrative divisions: 267 nations, dependent areas, other, and miscellaneous entries

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.

See also


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