The Moon is Earth’s only natural satellite and the fifth largest natural satellite in the Solar System.
The average centre-to-centre distance from the Earth to the Moon is 384,403 km, about thirty times the diameter of the Earth. The Moon’s diameter is 3,474 km, a little more than a quarter that of the Earth. This means that the Moon’s volume is about 2 percent that of Earth and the pull of gravity at its surface about 17 percent that of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the Earth-Moon system are responsible for the lunar phases that repeat every 29.5 days (the synodic period).
The Moon is the only celestial body to which humans have travelled and upon which humans have landed. The first artificial object to escape Earth’s gravity and pass near the Moon was the Soviet Union’s Luna 1, the first artificial object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9, and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966. The United States Apollo program achieved the only manned missions to date, resulting in six landings between 1969 and 1972. Human exploration of the Moon ceased with the conclusion of the Apollo program, although several countries have announced plans to send people or robotic spacecraft to the Moon.
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