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The Moon, although smaller than the Earth, is much closer to the Earth in size than any other satellite is to its parent planet in the entire Solar System. For this reason, and because the Moon is so intimately connected with the Earth, the Earth-Moon system is often regarded as a 'double planet'. The Moon is 3,476 kilometers in diameter. It orbits Earth at an average distance of 384,400 kilometers. It was formed, like the rest of the Solar System, 4,600,000,000 years ago. Most probably, it followed the same processes of accretion - attracting and mopping up smaller pieces of material - as the Earth and the other terrestrial planets. Its composition is different from that of the Earth. So the theory that it broke away from the Earth early in the lift of the Solar System is unlikely. Perhaps it was formed close to Earth. Perhaps it formed far away, to be 'snared' by Earth's gravitational pull later. However it developed, the Moon is currently moving away from the Earth at the rate of about 3 centimeters every year.
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Exploreuniverse.com
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