The Moon is a natural satellite of the Earth, and it is the fifth-largest satellite in the Solar System. It is about one-quarter the size of Earth and is located about 384,000 kilometers (239,000 miles) away from Earth. It takes about 27.3 days to orbit the Earth and has a synodic period (the period between successive identical phases, such as full Moon) of 29.5 days.
The Moon has a rocky surface that is covered with a layer of fine dust called regolith. The surface of the Moon is characterized by large impact craters, mountains, valleys, and a vast plain known as the Moon's "maria" (singular: "mare"). The Moon's surface is also marked by various types of volcanic features such as rilles, domes, and vents.
The Moon has no atmosphere, and the temperature on its surface varies greatly between the day and night. The day side can reach temperatures as high as 127°C (260°F) while the night side can drop as low as -173°C (-280°F). It also has no liquid water on its surface, but there is evidence of frozen water in some of the craters near the poles.
The Moon has been explored by humans and robots. The first manned mission to the Moon was the Apollo 11 mission in 1969, and a total of six manned missions were conducted between 1969 and 1972. The Moon has also been explored by a number of robotic spacecraft, including the Soviet Union's Luna program and NASA's Ranger, Surveyor, and Lunar Orbiter missions.
The Moon has a significant impact on life on Earth. It is responsible for the tides, and its gravitational pull affects the Earth's rotation. Its regular cycle of phases has been used for timekeeping for thousands of years and remains important for ritual, religious and calendrical purposes.
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