In the mid-17th century, Galileo and other astronomers made observations of the Moon through the telescope and discovered many craters. Since then, and given its proximity, it has been the most studied space object. The current knowledge of the Moon is greater than that of the rest of the Solar System objects except Earth.
The Moon is 384,403 kilometers from Earth. Its diameter is 3,476 kilometers. Both the rotation of the Moon and its orbit around the Earth last 27 days, 7 hours and 43 minutes. This synchronous rotation is caused by the asymmetric distribution of the moon's mass, which has allowed Earth's gravity to keep a lunar hemisphere permanently turned towards Earth.
The Moon has been heavily bombarded by meteorites, which causes many of the rocks in the old crust to have mixed, melted or buried. The dark seas, with relatively few craters, cover approximately 16% of the lunar surface and are concentrated on the near side of the Moon, mainly within the impact basins. The hidden face of the moon has only been observed from space travel.
This photo of the Moon was taken by the Apollo 17 crew during their passage behind the Earth on the trip back home, after a successful moon landing in December 1972.
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