the Moon

The Moon is Earth's only natural satellite, and the fifth largest moon in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 km which is about thirty times the diameter of the Earth. The Moon has a diameter of 3,474 km —slightly more than a quarter that of the Earth and a little bit smaller than the length across the United States. This means that the volume of the Moon is close to 1/50th that of Earth. The gravitational pull at its surface is about 1/6th of Earth's. The Moon makes a complete orbit around the Earth every 27.3 days, and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the lunar phases that repeat every 29.5 days.
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The Earth's only natural satellite. It is a heavily cratered body. Its composition is deficient in the heavier, metallic materials. This is thought to be explained by the fact that the Moon formed from some of the ejecta produced by the collision of a massive body with the forming Earth. The heavier ejecta did not reach Earth's orbit: it fell back to the planet under the pull of gravity. The lighter material did escape and eventually formed the Moon. The Moon has no permanent atmosphere or running water and, because of this, it has never suffered erosion or weathering. This means that it presents scientists with a cratering record that dates back to when the solar system was still forming. Hence, its study is invaluable in the understanding of how the planets came into being. It is 384,400 km (mean) centre to centre from the Earth. It rotates with a period of 27.32 days and so always presents one face to the Earth; at one time it spun much faster but continues to slow due to the friction of the gravitational wave in its crust. It is 1/81 the mass of the Earth and the gravitational acceleration on its surface 0.165 that on Earth.