Earth photographed from Apollo 17.

The third planet from the sun in the solar system. Other planets possess oceans or/and an atmosphere, but Earth is so far the only planet known to be inhabited by carbon based life.

The Earth revolves around the sun on an elliptical orbit. One orbit takes 365.2422 days to complete.


The shell within the earth, some tens of kilometres below the surface and of undefined thickness, which is a shell of weakness where plastic movements take place to permit pressure adjustments. It lies 70 to 100 km below the surface and may extend to a depth of 400 km. Corresponds to the seismic low-velocity zone.

Inner core

Central solid region of the Earth's core, probably mostly iron; radius about 1221 kilometers, discovered by Inge Lehmann in 1936.


The main bulk of the Earth, between the crust and the core, ranging from depths of about 40 to 3470 kilometers. It is composed of dense silicate rocks and divided into a number of concentric shells.


A zone in the Earth between 400 and 670 km below the surface separating the upper mantle from the lower mantle.

Outer core

Outer liquid shell of the Earth's core, probably iron with some oxygen; inner radius, 1221 kilometers, outer radius, 3480 kilometers.

Click on an item to paste into clipboard or use clipboard symbol at end to clipboard all values
Equatorial diameter 12756000 mClip
Average Density 5520 kgm-3Clip
Axial tilt 23.4 degreesClip
Rotational period 23.93 hoursClip
Average surface temperature 15 CClip
Aphelion 152100000000 mClip
Perihelion 147100000000 mClip
Orbital tilt 0 degreesClip
Orbital period 365.26d y=Earth year, d=Earth dayClip
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Energy Balance

Averaged over the Earth and over long time periods, the energy budget of the climate system must be in balance.

The climate system derives all its energy from the Sun, this balance implies that on a global scale, the amount of incoming solar radiation must on average be equal to the sum of the outgoing reflected solar radiation and the outgoing infrared radiation emitted by the climate system.

The perturbation of this global radiation balance, be it human-induced or natural, is called radiative forcing.

Historical Notes

  1. 300BC Aristarchus proposed that the earth revolves around the sun; calculated diameter of the earth.
  2. 1650 Archbishop Usher estimated by reading the Bible that the earth was created on October 23, 4004 BC at 9:00am.
  3. 1785 Hutton proposed the idea of uniformitatianism in the geological history of the earth.
  4. 1851 Earth's rotation demonstrated by Leon Foucault.
  5. 1864 The distance of the Earth from the sum is calculated by astronomers as 147 million km - only 2.6 million km less than our estimate today.
  6. 1890 The English geologist Arthur Holmes uses radioactivity to date the Earth from rocks. He finds it is 4.6 billion years old.
  7. 1906 Proof of Earth's molten core proposed by Richard Oldham.
  8. 1916 Earth's molten core discovered by Albert Michelson.
  9. 1957 Sputnik becomes the first man-made object to orbit the Earth.
  10. 1963 Vine and Matthews discovered that rock layers with particular magnetic orientations, indicating reversals of the earth's magnetic field, are symmetrical about the mid-oceanic ridge, indicating that new crust is created at the ridge.

See also: Air, Albedo, Biosphere, Cryosphere, Earth Current, Earth Resonance, Earthlight, Escape Velocity, Extraterrestrial, Geocentric Model, Geocorona, Geodesy, Geomagnetic Field, Geostationary, Ionosphere, Jupiter, Lagrangian Points, Latitude, Lithosphere, Longitude, Magnetosphere, Mars, Mercury, Milankovich Theory, Moon, Neptune, Opposition, Perigee, Perihelic Opposition, Pluto, Saturn, Solar System, Standard Acceleration due to Gravity, Sun, Troposphere, Uranus, Van Allen Belts, Vernal Equinox, Zenith Attraction.

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Subjects: Astronomy Physics

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