The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period (lunar month) with its rotation period (lunar day) at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides. In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the United States from coast to coast). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet, the fifth largest and fifth most massive moon overall, and larger and more massive than all known dwarf planets.[17] Its surface gravity is about one sixth of Earth's, about half of that of Mars, and the second highest among all Solar System moons, after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia. The lunar surface is covered in lunar dust and marked by mountains, impact craters, their ejecta, ray-like streaks, rilles and, mostly on the near side of the Moon, by dark maria ("seas"), which are plains of cooled lava. These maria were formed when molten lava flowed into ancient impact basins. The Moon is, except when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases.[18] The Moon is the brightest celestial object in Earth's night sky. This is mainly due to its large angular diameter, while the reflectance of the lunar surface is comparable to that of asphalt. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun completely during a total solar eclipse. From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective (libration), making parts of the far side of the Moon visible. The Moon has been an important source of inspiration and knowledge for humans, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. The first human-made objects to fly to an extraterrestrial body were sent to the Moon, starting in 1959 with the flyby of the Soviet Union's Luna 1 and the intentional impact of Luna 2. In 1966, the first soft landing (by Luna 9) and orbital insertion (by Luna 10) followed. On July 20, 1969, humans for the first time stepped on an extraterrestrial body, landing on the Moon at Mare Tranquillitatis with the lander Eagle of the United States' Apollo 11 mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Since then, exploration of the Moon has continued robotically, and crewed missions are being planned to return beginning in the late 2020s.
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