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The Moon or Moon is the Earth's natural satellite, the fifth largest satellite in the solar system. The distance from the center of the earth to the center of the moon is 384,400 kilometers on average, about 30 times the diameter of the Earth and one-fourth of the distance from Earth to the Sun. The diameter of the moon is about 3474 km, which is about a quarter of the Earth and about a quarter of the sun. The volume of the moon is about 1/50 of the Earth's surface, and the gravity on the surface is about 17% of the earth's surface. The moon circles the earth about 27.3 days, and the change of the position of the earth-moon-sun makes the image of the moon that changes by 29.5 days. As the closest celestial body to Earth, the moon is the only extraterrestrial object that humans have ever explored. In 1969, the first manned spacecraft was sent to the moon, and by 1972, the moon had been explored 6 times directly. Lunar exploration stopped, but the lunar exploration resumed in the 2000s, and exploration is planned to send astronauts by 2020. The lunar probe Chandrayaan 1, launched in India, found water on the Moon on September 26, 2009.