The Moon is Earth’s only natural satellite and the fifth-largest moon in the Solar System. A rocky, approximately spherical body, it has a mean diameter of about 3,475 kilometres and orbits at an average distance of 384,400 kilometres from Earth. Its surface preserves impact craters, ancient volcanic plains, and evidence of its early formation. Its gravitational interaction with Earth produces tides and gradually changes the rotation and orbit of the two bodies. (nssdc.gsfc.nasa.gov)
Physical characteristics
The Moon’s mass is approximately 7.35 × 10²² kilograms, about one eighty-first of Earth’s. Its average density is approximately 3,344 kilograms per cubic metre, substantially lower than Earth’s. Surface gravity is about 1.62 metres per second squared, roughly one-sixth of the terrestrial value, while its escape velocity is approximately 2.38 kilometres per second. These properties distinguish it from Earth despite their shared history. (nssdc.gsfc.nasa.gov)
The Moon shines primarily by reflecting sunlight, rather than producing visible light of its own. Although prominent in the night sky, its surface is relatively dark: its average albedo, or reflectivity, is about 0.11. Its angular diameter as viewed from Earth averages about half a degree, but varies as its distance changes along its elliptical orbit. (nssdc.gsfc.nasa.gov)
Orbit, rotation, and phases
The Moon completes an orbit relative to distant stars in approximately 27.32 days. Its rotation takes essentially the same time, a condition called tidal locking or synchronous rotation. Consequently, approximately the same hemisphere faces Earth. This does not mean the Moon does not rotate: it turns once on its axis during each orbit. The far side is not permanently dark; both hemispheres experience sunlight and darkness. (nssdc.gsfc.nasa.gov)
A complete cycle of lunar phases takes approximately 29.53 days. This interval exceeds the orbital period because Earth also moves around the Sun, so the Moon must travel farther to regain the same Sun–Earth–Moon alignment. New moon, crescent, quarter, gibbous, and full moon describe the changing fraction of the illuminated hemisphere visible from Earth. Ordinary phases are not caused by Earth’s shadow. (nssdc.gsfc.nasa.gov)
The lunar orbital plane is tilted about 5.1 degrees relative to Earth’s orbital plane. Therefore, solar eclipses and lunar eclipses do not occur every month. A solar eclipse occurs when the Moon passes between Earth and the Sun; a lunar eclipse occurs when the Moon passes through Earth’s shadow. Both require sufficiently close alignment near an intersection of the two orbital planes. (science.nasa.gov)
Origin and geological development
The leading explanation for lunar origin is the giant-impact hypothesis. Around 4.5 billion years ago, a large body, conventionally called Theia, collided with the young Earth. Material displaced by the collision formed the Moon. The impact’s precise configuration, the proportions contributed by Earth and the impactor, and the subsequent assembly process remain subjects of research. (science.nasa.gov)
Evidence includes the Moon’s relatively small metallic core, its chemical composition, and indications that its early surface was covered by a deep magma ocean. As this molten material cooled, dense components sank while lighter minerals accumulated near the surface, producing a differentiated interior and an early crust. Lunar samples and their isotopic compositions constrain models of this development. (science.nasa.gov)
The visible surface contrasts bright, heavily cratered highlands with darker maria, a Latin name meaning “seas.” The maria are not bodies of water but plains formed by ancient lava flows, commonly within large impact basins. They are much more extensive on the near side than on the far side. Cratering and volcanism have therefore produced different landscapes across the two hemispheres. (science.nasa.gov)
Interior and surface environment
The Moon has a crust, a rocky mantle, and a small iron-rich core containing solid and molten regions. Its crust is generally thicker on the far side. Seismic observations, orbital measurements, and laser-ranging experiments constrain the interior, although details of its deep structure remain uncertain. (science.nasa.gov)
Most of the surface is covered by regolith: loose dust, mineral fragments, broken rocks, and glass produced by repeated impacts. This material differs from biologically developed terrestrial soil. Its finest particles are abrasive, while repeated bombardment continually fragments and mixes the surface. (science.nasa.gov)
The Moon’s extremely tenuous atmosphere provides little protection from radiation or meteoroids. Exposed surface temperatures can reach approximately 127°C in sunlight and fall to about −173°C in darkness. Water occurs in several forms, including ice in permanently shadowed polar regions and small quantities of molecules associated with sunlit surface material. These discoveries do not imply lakes or a terrestrial water cycle. (science.nasa.gov)
Interaction with Earth and exploration
The Moon is a major driver of Earth’s tides. Tidal interactions dissipate rotational energy, gradually slow Earth’s rotation, and transfer angular momentum to the lunar orbit. The Moon consequently recedes at approximately four centimetres per year; this present rate should not be assumed constant throughout its history. (science.nasa.gov)
Robotic exploration began in the 1950s. Soviet Luna 3 photographed the far side in 1959, and Luna 9 achieved the first soft landing in 1966. Apollo 11 made the first human lunar landing on July 20, 1969. Across six successful surface missions from 1969 to 1972, twelve astronauts collected approximately 382 kilograms of lunar material. (science.nasa.gov)
Later missions extended orbital mapping, surface measurements, and sample collection. China’s Chang’e 4 achieved the first far-side soft landing in 2019, and Chang’e 6 returned the first far-side samples in 2024. NASA’s Lunar Reconnaissance Orbiter has supplied detailed observations of lunar terrain and composition, supporting geological research and the assessment of exploration sites. (science.nasa.gov)