The Moon is Earth’s only natural satellite and the fifth-largest moon in the Solar System. It orbits Earth at an average distance of about 384,400 km, completing a revolution in roughly 27.3 days and showing phases over a synodic cycle of about 29.5 days. Because it is tidally locked to Earth, the same near side always faces us.
In physical terms, the Moon is a differentiated, terrestrial body with a diameter of about 3,475 km and a mass about 1.2% that of Earth. Its surface gravity is about one-sixth of Earth’s. The Moon has no dense atmosphere, only a very tenuous exosphere, and lacks a global magnetic field. Its surface is covered by regolith and marked by impact craters, while the dark lunar maria are basaltic plains formed by ancient volcanic activity. The far side is more heavily cratered than the near side, and libration allows observers on Earth to see about 59% of the lunar surface over time.
The Moon plays a major role in Earth’s tides through gravitational attraction, and its apparent size in the sky is nearly the same as the Sun’s, making total solar eclipses possible. The leading explanation for its origin is the giant-impact hypothesis: the Moon formed about 4.51 billion years ago from debris produced when a Mars-sized body, Theia, struck the early Earth.
The Moon has been central to human culture, calendars, mythology, and science. It was the first extraterrestrial body reached by spacecraft, beginning with Luna 1 in 1959, and the first visited by humans with Apollo 11 in 1969. Six Apollo program landings placed twelve people on its surface between 1969 and 1972, and renewed exploration continues with plans for future human return.