The Moon is Earth's only natural satellite. It orbits around Earth at an average distance of 384,399 kilometres (238,854 mi), about 30 times Earth's diameter. Its orbital period (lunar month) and its rotation period (lunar day) are synchronized at 29.5 days by the pull of Earth's gravity. This makes the Moon tidally locked to Earth, always facing it with the same side. The Moon's gravitational pull produces tidal forces on Earth which are the main driver 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 contiguous United States). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet. It is the fifth-largest and fifth-most massive moon overall, and is larger and more massive than all known dwarf planets. Its surface gravity is about one-sixth of Earth's, about half that of Mars, and the second-highest among all moons in the Solar System after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with only a minuscule hydrosphere, atmosphere, and magnetic field. The lunar surface is covered in regolith dust, which mainly consists of the fine material ejected from the lunar crust by impact events. The lunar crust is marked by impact craters, with some younger ones featuring bright ray-like streaks. The Moon was until 1.2 billion years ago volcanically active, filling mostly on the thinner near side of the Moon ancient craters with lava, which through cooling formed the prominently visible dark plains of basalt called maria ('seas'). 4.51 billion years ago, not long after Earth's formation, the Moon formed out of the debris from a giant impact between Earth and a hypothesized Mars-sized body named Theia.
From a distance, the day and night phases of the lunar day are visible as the lunar phases, and when the Moon passes through Earth's shadow a lunar eclipse is observable. The Moon's apparent size in Earth's sky is about the same as that of the Sun, which causes it to cover the Sun completely during a total solar eclipse. The Moon is the brightest celestial object in Earth's night sky because of its large apparent size, while the reflectance (albedo) of its surface is comparable to that of asphalt. About 59% of the surface of the Moon is visible from Earth owing to the different angles at which the Moon can appear in Earth's sky (libration), making parts of the far side of the Moon visible.
The Moon has been an important source of inspiration and knowledge in human history, 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 probe and the intentional impact of Luna 2. In 1966, the first soft landing (by Luna 9) and orbital insertion (by Luna 10) followed. Humans arrived for the first time at the Moon, or any extraterrestrial body, in orbit on December 24, 1968, with Apollo 8 of the United States, and on the surface at Mare Tranquillitatis on July 20, 1969, with the lander Eagle of Apollo 11. By 1972, six Apollo missions had landed twelve humans on the Moon and stayed up to three days. Renewed robotic exploration of the Moon, in particular to confirm the presence of water on the Moon, has fueled plans to return humans to the Moon, starting with the Artemis program in the late 2020s.
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