Earth (often referred to as "the Earth") is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. It is the largest of its planetary system's terrestrial planets and the only place in the universe known to support life. The Earth was formed around 4.57 billion years agoG.B. Dalrymple, 1991, "The Age of the Earth", Stanford University Press, California, ISBN 0-8047-1569-6. (see Age of the Earth) and its largest natural satellite, the Moon, was orbiting it shortly thereafter, around 4.533 billion years ago.

Since it formed, the Earth has evolved through geological and biological processes that have hidden traces of the original conditions. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of molten Earth mantle and an iron core that generates a magnetic field. The atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 70% of the surface is covered in salt water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The relatively large Moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet is believed to have formed the oceans. Later, asteroid impacts are understood to have caused significant changes to the surface environment. Changes in the orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.

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iyongyut (yongyut) Fri, 01 Jan 2010 16:23:07 -0000
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