The Earth's Moon is Earth's sole natural satellite and the fifth largest moon in the Solar System. It is the largest satellite relative to the size of its host planet, with a diameter of approximately 3,474 kilometers, about one-quarter that of Earth. The Moon orbits Earth at an average distance of 384,400 kilometers, completing one orbit every 27.3 days. This orbit is elliptical, causing variations in distance and apparent size from Earth.
The Moon formed approximately 4.5 billion years ago, shortly after the Solar System's origin. The prevailing theory, known as the giant impact hypothesis, posits that a Mars-sized protoplanet named Theia collided with the early Earth, which was then a molten body covered in magma oceans. The immense impact ejected debris into orbit around Earth, which coalesced over time to form the Moon. This event occurred about 30 to 50 million years after the Solar System began forming.
Early in its history, the Moon was a molten world with widespread volcanic activity. Basaltic lava flows created vast plains called maria (Latin for "seas"), which filled impact basins. Volcanism persisted for billions of years, with evidence from returned samples indicating activity as recent as 120 million years ago during the Mesozoic era on Earth. The Moon's surface records the Solar System's violent history through impact craters formed by asteroids and comets. The Moon lacks an atmosphere and magnetic field, preserving these features without erosion.
Isotopic analysis of lunar rocks, particularly potassium isotopes, reveals that the Moon contains material from proto-Earth, supporting the giant impact theory. Pockets of this ancient material may have sunk into the Moon's mantle and resurfaced through volcanism.
The Moon has a mass of 7.34 × 10²² kilograms, about 1/81 that of Earth. Its density is 3.34 g/cm³, lower than Earth's due to a smaller iron core, estimated at 1-2% of its mass. The Moon is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminum, similar to Earth's crust and mantle.
Surface temperatures vary dramatically: up to 127°C (260°F) in sunlight and down to -173°C (-280°F) in shadow. The Moon experiences moonquakes, detected by seismometers, caused by tidal forces from Earth, thermal expansion, and meteor impacts. Ancient ice deposits exist in permanently shadowed craters at the poles, confirmed by missions like Lunar Prospector.
The Moon influences Earth by stabilizing its axial tilt, which maintains consistent seasons, and driving ocean tides through gravitational pull. Its apparent size in the sky is nearly identical to the Sun's, enabling total solar eclipses.
The Moon's orbit is tidally locked, meaning its rotation period matches its orbital period of 27.3 days (sidereal month). This synchronous rotation results in the same hemisphere always facing Earth, known as the near side, while the far side (often miscalled the "dark side") faces away. The far side receives equal sunlight but was unseen until 1959.
The Moon recedes from Earth at 3.8 cm per year due to tidal interactions, lengthening Earth's day over geological time. Phases of the Moon—new, crescent, quarter, gibbous, and full—result from changing angles of sunlight as seen from Earth, completing a cycle every 29.5 days (synodic month).
The Moon's surface consists of highlands (bright, cratered terrain) and maria (dark, basaltic plains). Notable features include the South Pole-Aitken basin, the solar system's largest impact crater (2,500 km diameter), and craters like Tycho and Copernicus. The near side has more maria due to thinner crust, while the far side is more cratered.
No active volcanoes exist today, but cooled lava landforms and rilles (lava channels) remain. The Moon has no weather, liquid water, or life, but subsurface water ice and potential volatiles are targets for future exploration.
Human observation of the Moon dates to prehistoric times, influencing calendars, mythology, and navigation. Telescopic studies began with Galileo Galilei in 1609, revealing mountains and craters.
The Space Age began with the Soviet Union's Luna program in 1959, achieving the first flyby and soft landing. The U.S. Apollo Program landed humans on the Moon from 1969 to 1972, with Apollo 11's Neil Armstrong and Buzz Aldrin as the first. They deployed seismometers, revealing the Moon's internal structure: a crust, mantle, and small core.
Recent missions include China's Chang'e program, India's Chandrayaan, and NASA's Artemis, aiming for sustainable presence. In 1958, the U.S. considered a nuclear detonation on the Moon for Cold War prestige, but it was rejected. Hypotheses of artificial origins persist in fringe theories but lack scientific support.