It is generally accepted that the Moon formed when a Mars-sized object knicked the Earth over four billion years ago. The blasted material from the Earth formed a ring, and eventually collided and clumped together to form the Moon. The orbital period of the Moon is 27.3 days long, but it takes 29.5 days for the Moon to complete a cycle of phases. The Moon's axil rotation is exactly equal to its orbital period, so the same face of the Moon always faces the Earth. The farside was not seen by mankind until the Soviet probe Lunik 3 photographed it in 1959. The Moon used to be much closer to the Earth; possibly as close as 14,000 kilometers. However, tidal friction forces have pushed the Moon away from the Earth, and it continues to move away from us at the rate of 4 centemeters per year.
The most obvious feature on the Moon are the dark, lowland maria (mare is singular). The maria are lowland valleys that were flooded with lava during a time when volcanos were active on the Moon. All volcanic activity ended over 3.5 billion years ago, and the lava in the maria cooled and darkened. Next are the countless craters that speckle the surface. These craters range in size from pits to hundred of kilometers across. The largest is Bailly Crater, which is 293 kilometers across. Most of the craters were formed by bombardments from Solar System debris early in its history. The Moon also has impressive mountain ranges, with the largest being the Alps. Smaller mountain ranges border the larger craters and maria. The Moon also has valleys and rilles, which are collapsed crack like features.
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