Earth is the third planet from the sun and is the largest of the inner planets in the solar system in both diameter and mass. The only planet in the Universe on which life is known to exist, Earth is home to millions of species, including at least one sentient life form, Homo sapiens sapiens.

At the center of the Earth is a solid iron core, surrounded by magnetic liquid and a thick layer of mantle.

About 71% of Earth's surface is covered in liquid water, mostly in the form of oceans. These seas not only help drive the atmosphere, they lubricate the crust of the planet, enabling the motion of tectonic plates. The movement of the plates constantly recycles the mineral and biological material available on the planet and in turn, shapes climate and makes ecosystems possible.

Earth has one natural satellite, the moon, also known as Luna. This large satellite is believed to have been created from accreted orbital ejecta from the collision with Earth of a small planetoid early in the history of the solar system. The moon's gravity has attracted asteroids, comets, and meteors to its surface than may have otherwise struck Earth.

• EarthHour.org: Climate Change Activism

• ArcaMax Publishing: A Journey to the Center of the Earth - Jules Verne

• Amazon.com: Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan

• Amazon.com: Earth - James F Luhr

• Amazon.com: Planet Earth: As You've Never Seen It Before - Alastair Fothergill

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