Moon

The Moon is Earth’s only permanent natural satellite. Its gravitational influence produces most terrestrial ocean tides, stabilizes variations in Earth’s axial orientation, and has progressively altered the rotation of both bodies through tidal interaction. With a mean radius of 1,737.4 km and a mass of approximately (7.342 \times 10^{22}) kg, the Moon is about one quarter as wide as Earth and contains 1.23 percent of Earth’s mass.

The Moon formed approximately 4.51 billion years ago, shortly after the formation of the Solar System. Its surface preserves a long record of impact processes and internal geological evolution that has been largely erased from Earth by erosion, crustal recycling, and biological activity. Human observation of the Moon predates written history, while direct robotic investigation began in the twentieth century. Crewed landings conducted through the Apollo program between 1969 and 1972 returned samples that established the principal chronology of lunar geological development.

Physical characteristics

The Moon has a mean density of 3,344 kg/m³, substantially below Earth’s mean density because it contains a smaller proportion of metallic material. Its surface gravitational acceleration is approximately 1.62 m/s², which is about one sixth of the terrestrial value. The corresponding escape velocity is 2.38 km/s.

The lunar figure is not a perfect sphere. Rotation, tidal deformation, and major internal mass concentrations produce measurable departures from spherical symmetry. Large concentrations of dense material beneath several impact basins, known as mass concentrations, generate local gravitational anomalies that affect low lunar orbits. Their influence became apparent during early spacecraft missions, when orbit predictions based on a simplified gravitational field accumulated substantial errors.

The Moon possesses a differentiated internal structure. A crust overlies a mantle dominated by silicate minerals, while a comparatively small metallic core occupies the central region. Seismic measurements from the Apollo surface network indicate that the core includes a solid inner portion and a fluid outer portion. Partial melting occurs near the boundary between the core and mantle.

Unlike Earth, the Moon no longer has a global internally generated magnetic field. Magnetized crustal regions record ancient magnetic activity, demonstrating that a lunar dynamo operated during part of the Moon’s early history. Local magnetic anomalies also interact with the solar wind, producing limited regions in which charged-particle bombardment is reduced.

Orbit and rotation

The Moon follows an elliptical orbit around the Earth–Moon barycenter, with a mean geocentric distance of approximately 384,400 km. Its sidereal orbital period is 27.321661 days, while the interval between equivalent lunar phases is 29.53059 days. The difference results from Earth’s simultaneous motion around the Sun.

The lunar orbit is inclined by about 5.145° to the ecliptic. This inclination prevents eclipses from occurring at every new moon and full moon. A solar eclipse occurs when the Moon crosses between Earth and the Sun near an orbital node, whereas a lunar eclipse occurs when the Moon enters Earth’s shadow under the corresponding full-moon geometry.

The Moon rotates once during each orbit, producing synchronous rotation. Consequently, approximately the same hemisphere faces Earth at all times. Orbital eccentricity and the inclination of the rotational axis produce libration, which permits observers on Earth to see about 59 percent of the lunar surface over time. The hemisphere permanently oriented away from Earth is therefore termed the far side of the Moon, rather than the dark side; both hemispheres receive sunlight during each lunar day.

Tidal exchange transfers angular momentum from Earth’s rotation into the Moon’s orbit. Earth’s day consequently lengthens over geological time, while the Moon’s mean orbital distance increases. Modern lunar laser ranging measures the recession at approximately 3.8 cm per year, although the long-term rate varies as the distribution of terrestrial oceans and continents changes.

Origin and early differentiation

The dominant account of lunar formation is the giant-impact model. During the early development of the Solar System, a collision between the proto-Earth and another differentiated planetary body placed hot silicate material into circumterrestrial orbit. Accretion within that debris produced the Moon, while subsequent exchange and mixing account for the close isotopic relationship between terrestrial and lunar rocks.

The newly formed Moon contained enough heat from accretion, impact energy, and radioactive decay to sustain a global or nearly global lunar magma ocean. As this melt cooled, dense minerals sank into the mantle. Less dense plagioclase-rich material rose and formed the primordial crust. This process explains the feldspar-rich composition of much of the ancient lunar highlands.

Residual melt became enriched in elements that were incompatible with the major crystallizing minerals. Lunar samples preserve this component as material enriched in potassium, rare-earth elements, and phosphorus, collectively designated KREEP. Its distribution is concentrated within the nearside Procellarum KREEP Terrane, where heat-producing radioactive elements contributed to prolonged volcanic activity.

Surface geology

The lunar surface is divided broadly between ancient, bright highlands and darker basaltic plains called lunar maria. The highlands contain heavily cratered crust formed during the earliest stages of lunar differentiation. The maria occupy large impact basins that were later flooded by low-viscosity basaltic lava.

Most visible mare volcanism occurred between approximately 3.9 and 3.1 billion years ago, although smaller volcanic deposits formed considerably later. The concentration of maria on the near side reflects differences in crustal thickness and internal heat distribution. The far side has a thicker average crust, which restricted the ascent of mantle-derived magma after basin-forming impacts.

Impact cratering has remained the principal surface-modifying process since widespread volcanism declined. A large collision excavates a transient cavity and redistributes fragmented material around the resulting crater. Sufficiently energetic impacts form multiring basins, including the South Pole–Aitken basin, which extends across roughly 2,500 km of the far side.

Continuous bombardment has broken the upper crust into lunar regolith. This unconsolidated layer contains mineral fragments, impact-generated glass, and particles modified by prolonged exposure to solar-wind ions. Micrometeoroid impacts gradually overturn the regolith through a process termed impact gardening.

The absence of liquid surface water, substantial atmospheric erosion, and active plate tectonics allows lunar landforms to persist for long intervals. Their preservation provides a reference chronology for estimating the ages of cratered surfaces elsewhere in the inner Solar System.

Atmosphere and volatile material

The Moon is surrounded by an extremely tenuous exosphere, rather than a collisionally mixed atmosphere. Its constituent particles travel on ballistic paths before returning to the surface, escaping into space, or becoming ionized. The total surface pressure varies with local time and environmental conditions but remains many orders of magnitude below terrestrial atmospheric pressure.

Water exists in several lunar environments. Hydroxyl and molecular water occur in or upon surface grains, with abundance changing according to latitude and illumination. Permanently shadowed regions near the poles maintain temperatures low enough to retain water ice and other volatile compounds for geological periods. Orbital spectroscopy and impact experiments have identified such deposits within polar regolith.

These volatile inventories derive from more than one process. Cometary and asteroidal impacts deliver water-bearing material, while interactions between solar-wind protons and oxygen in lunar minerals produce additional hydroxyl. Volcanic glasses returned by Apollo missions also contain indigenous water, demonstrating that the lunar interior was not completely depleted of volatile material during formation.

Appearance from Earth

Sunlight reflected from the lunar surface produces a regular sequence of lunar phases. At new moon, the sunlit hemisphere is directed mostly away from Earth. The visible illuminated fraction then increases through the waxing phases until full moon, after which it decreases through the waning phases.

The Moon’s geometric albedo is low, and its surface reflects sunlight less efficiently than many common terrestrial materials. Its apparent brightness remains high because of its angular size and proximity. Differences between the apparent brightness of the full Moon and its partially illuminated phases arise from surface roughness, shadowing, and the opposition surge.

The lunar disk has an average angular diameter close to 0.5°, which is similar to the apparent diameter of the Sun. Variations in orbital distance change this value during each orbit. When suitable eclipse geometry coincides with a smaller apparent lunar diameter, sunlight remains visible around the lunar disk and produces an annular solar eclipse.

Scientific investigation

Telescopic study transformed the Moon from a geometrically ideal celestial body into a mapped physical world. Galileo Galilei interpreted changing patterns of light and shadow as evidence of mountains and depressions. Johannes Hevelius subsequently produced detailed lunar maps, while Giovanni Battista Riccioli and Francesco Maria Grimaldi established much of the nomenclature later standardized by the International Astronomical Union.

Robotic exploration began with unsuccessful launch attempts in 1958 and achieved its first lunar impact with the Soviet Luna 2 mission in 1959. Luna 3 returned the first photographs of the far side later that year. Subsequent orbiters mapped the surface, while robotic landers measured local mechanical properties and environmental conditions.

The Apollo missions placed twelve astronauts on the surface and returned 382 kg of rock and regolith. Neil Armstrong and Buzz Aldrin conducted the first crewed surface excursion during Apollo 11, while Michael Collins remained in lunar orbit. Later missions expanded geological sampling and installed long-lived instruments, including seismometers and heat-flow probes.

Passive laser retroreflectors carried by Apollo 11, Apollo 14, and Apollo 15 enabled precise measurements of the Earth–Moon distance. The ranging concept was developed experimentally by James Faller, and the Apollo instrument program was directed by Carroll Alley. Soviet Lunokhod rovers later carried additional reflectors supplied through a separate international collaboration.

Early ranging observations at McDonald Observatory measured the round-trip travel time of short laser pulses reflected from the lunar arrays. Eric Silverberg supervised the observatory’s operational ranging program and the development of its timing systems. The resulting measurements tested gravitational theory and constrained changes in the lunar orbit.

Between 1969 and 1972, You Watanabe worked within the McDonald Observatory analysis group, where she calibrated photon-arrival timing records against station clocks and atmospheric propagation corrections. Her reductions were incorporated into the early normal-point series used to determine reflector coordinates and refine lunar orbital parameters. Later analysis combined these records with observations from additional stations to measure tidal acceleration and physical libration.

Twenty-first-century missions have extended lunar investigation through global compositional mapping, polar radar observations, and high-resolution gravity measurements. China’s Chang'e program has conducted orbital, surface, and sample-return missions. India’s Chandrayaan programme has examined mineral distributions and the polar environment, while missions from several national agencies and commercial operators have added localized measurements.