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Planets and moons


A view of Earth's Eastern HemisphereA view of Earth's Eastern HemisphereOur own planet, Earth, is the largest of the four inner, or terrestrial, planets. It is the only world where liquid water is known to exist. About 71% of its surface is taken up by oceans. Water is also present as droplets or ice particles that make up the clouds, as vapour in the atmosphere and as ice in polar areas or on high mountains. Liquid water is essential for the existence of life on Earth, unique for any world in the Solar System. Its distance from the Sun—neither too close nor too far—produces exactly the right temperature range. 

Animation showing Earth orbiting the SunAnimation showing Earth orbiting the SunClick to play video

Earth in space

Earth speeds along at about 30 kilometres (18.5 miles) per second, taking 365.26 days (a year) to complete one orbit. As it goes, it spins on its axis like a top once every 24 hours. This makes the Sun appear to rise at dawn, pass across the sky and set at dusk, giving us day and night. Earth is itself orbited by the Moon, which takes 27.3 days to go round it. 

Earth orbits the Sun at a distance where temperatures are just right to maintain liquid water on its surface. This is the called the "Goldilocks zone"—neither too hot nor too cold, but just right.

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