Earth

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 only planet in the universe known to man to harbour life of any kind. It's dominant species is human which for the most part lives in harmony with the abundant animal lifeforms. In addition, innumerable plant lifeforms are utilized for diverse purposes such as food and shelter.

It is the largest of the local planetary system's terrestrial planets and is uniquely situated in a zone around the Sun to allow life to flourish. The Earth is believed to have formed around 4.57 billion years ago (see Age of the Earth) and its largest natural satellite, the Moon, is thought to have been orbiting it shortly thereafter, around 4.533 billion years ago. Since it formed, the earth has apparently changed through geological and biological processes that have hidden any trace 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 convecting yet solid 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.

The Earth's only natural orbiting body is the moon, although the asteroid Cruithne has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.