Earth's Moon is the natural satellite of Earth and the fifth largest natural satellite in the Solar System by diameter. It is an approximately spherical rocky body with a mean radius of about 1737.4 kilometres and a mass of about 7.35 times 10 to the 22 kilograms, roughly 1.23 percent of Earth's mass. Its average distance from Earth is about 384400 kilometres, although its orbit is elliptical and the distance varies over each revolution.
The Moon has no substantial global atmosphere comparable with Earth's atmosphere, although it possesses an extremely tenuous exosphere containing atoms and molecules supplied by processes including solar wind interaction, micrometeoroid impacts and surface release. Its surface gravity is approximately one sixth of Earth's surface gravity, so an object weighs much less on the Moon while retaining the same mass.
The Moon is tidally locked to Earth, meaning that its rotational period and orbital period are approximately equal, so nearly the same hemisphere remains directed towards Earth. The Moon nevertheless rotates on its axis, and the familiar statement that it does not rotate is incorrect.
Earth's Moon is thought to have formed approximately 4.5 billion years ago, relatively soon after the formation of Earth. The leading scientific explanation is the giant impact hypothesis, according to which a large planetary body collided with the young Earth and generated debris that eventually accumulated into the Moon. Modern versions of the hypothesis incorporate substantial mixing and complex dynamical evolution rather than treating the Moon as simply a collection of material from one impactor.
Evidence relevant to lunar origin includes the Moon's relatively small iron core, its similarities to Earth's mantle in isotopic composition, its depleted volatile inventory compared with Earth and the angular momentum of the Earth Moon system. The exact sequence of impact, debris disc formation, accretion, melting and subsequent differentiation remains an active area of planetary science. The Moon initially experienced extensive heating and melting and subsequently differentiated into a metallic core, mantle and crust. Its geological history therefore records processes occurring during the earliest stages of terrestrial planet formation.
The Moon's surface is dominated by ancient highlands and darker basaltic plains known as maria. The highlands are generally older and more heavily cratered, while the maria formed later when large impact basins were partly filled by basaltic lava generated by internal melting. Impact cratering has been the dominant geological process shaping the visible surface over much of lunar history.
Because the Moon lacks substantial atmosphere, liquid water at the surface and active plate tectonics comparable with Earth's, ancient surface features can persist for extremely long periods. Large impact basins, crater rims, ejecta blankets, rays, rilles, scarps and regolith provide evidence of its geological evolution. The lunar regolith is a fragmented layer of rock and dust produced primarily by repeated impacts and modified by space weathering. It can contain extremely fine particles, glassy material and chemically altered grains. The absence of terrestrial style erosion means that lunar landscapes preserve an unusually detailed record of impacts and surface processes.
The Moon's interior has been investigated through observations and measurements from spacecraft, remote sensing and instruments deployed during crewed and robotic missions. It contains a small metallic core, a silicate mantle and a crust, although the exact physical and chemical properties of its deep interior remain subjects of research. Seismic instruments deployed during Apollo missions recorded moonquakes and impacts and provided important evidence about lunar internal structure. The Moon also exhibits strong differences between its near side and far side.
The near side has extensive maria, whereas the far side contains much more heavily cratered highland terrain and a generally thicker crust. This hemispheric asymmetry is one of the major unresolved questions in lunar geology. Possible explanations involve differences in crustal thickness, thermal evolution, radioactive element distribution, tidal heating and early impact history. Lunar gravity measurements have also revealed mass concentrations called mascons, particularly beneath major impact basins, providing additional evidence about the Moon's internal and geological evolution.
The Earth Moon system has major physical consequences for Earth. The Moon's gravity produces most of the astronomical component of ocean tides, interacting with the gravitational influence of the Sun and the geometry of Earth's oceans and coastlines. Tidal effects also dissipate rotational energy and have gradually altered Earth's rotation and the Moon's orbital distance over geological time.
The Moon's presence contributes to the long term dynamical behaviour of Earth's rotational axis, although the precise climatic consequences involve complex interactions within the Earth system. The Moon also reflects sunlight and produces the familiar changing lunar phases as its position relative to Earth and the Sun changes. Its orbital motion determines the timing and geometry of solar and lunar eclipses. Because the Moon is approximately spherical and has no substantial atmosphere, its surface environment differs radically from Earth's, with extreme temperature variation between illuminated and shadowed regions and persistent darkness in some polar depressions.
Human exploration has made the Moon one of the most extensively investigated extraterrestrial bodies. Robotic spacecraft first established detailed observations of the lunar surface, while the Apollo programme between 1969 and 1972 carried astronauts to the lunar surface and returned approximately 382 kilograms of lunar rocks, soil and other material to Earth. These samples have provided information about lunar ages, mineralogy, volcanic activity, impact history and geochemical evolution.
Later missions from several countries have used orbiters, landers and rovers to investigate topography, composition, gravity, thermal properties, water related signatures and the polar environment. Lunar exploration has demonstrated that the Moon contains chemically bound and potentially accessible water ice in some permanently shadowed polar regions, although the quantity, distribution, physical state and practical accessibility remain subjects of continuing investigation. The Moon therefore functions both as a scientific archive and as an important target for developing planetary exploration technologies.
The Moon has no globally active plate tectonic system comparable with Earth's, but it is not geologically featureless or completely inactive. Thermal contraction has produced lobate scarps and other tectonic structures, while moonquakes indicate continuing internal activity. Meteorite impacts remain an ongoing process, and solar radiation and charged particles continuously modify exposed surfaces. Space weathering changes the optical and chemical properties of lunar soil through micrometeoroid bombardment and solar wind irradiation.
Volcanism was particularly important in the past, with extensive basaltic eruptions filling large impact basins, but large scale lunar volcanism is not occurring today in the manner seen on active terrestrial volcanic worlds. The Moon's present geological activity is therefore dominated by impact processes, thermal contraction, seismic activity and surface modification by the space environment rather than widespread tectonic or volcanic renewal.
Earth's Moon is scientifically significant because it provides a relatively accessible natural laboratory for studying planetary formation, differentiation, impact processes, volcanism, surface evolution and the history of the inner Solar System. Its synchronous rotation, unusual near side and far side differences, ancient crater record, relationship with Earth's tides and origin from the early Earth make it particularly important in planetary science.
The Moon is also useful for testing instruments and technologies intended for more distant exploration because communication delays are small and mission logistics are comparatively manageable. Common misconceptions include the idea that the Moon produces its own visible light, that the far side is permanently dark, or that lunar phases are caused by Earth's shadow. In reality, sunlight illuminates the Moon, the far side receives sunlight just as the near side does, and normal phases result from changing viewing geometry. Earth's Moon is therefore simultaneously a geological body, astronomical companion, historical archive and dynamical component of the Earth Moon system.
