The prevailing scientific explanation for the origin of the Moon is the giant-impact hypothesis, which was first proposed in rudimentary form by Canadian geologist Reginald Daly in 1946 and gained broad scientific consensus in the 1980s.¹³¹⁴ According to this model, approximately 4.5 billion years ago—roughly 30 to 100 million years after the formation of the Solar System—the proto-Earth collided with a Mars-sized protoplanet designated Theia, named after the Greek Titan who was the mother of the moon goddess Selene.¹⁵¹⁶ The cataclysmic impact shattered Theia and Vaporized substantial portions of the mantles of both bodies, ejecting a dense cloud of silicate debris into orbit around the young Earth.¹⁷ Gravitational interactions caused this debris disk to coalesce into the Moon within a period that traditional models estimate at weeks to millions of years, though high-resolution supercomputer simulations published in 2022 suggest accretion may have occurred within a matter of hours.

The giant-impact hypothesis accounts for several fundamental characteristics of the Earth–Moon system, including its exceptionally high angular momentum and the small size of the lunar metallic core, as most of Theia's heavy iron core sank into the core of Earth during the collision. However, some aspects of the model remain subjects of active scientific debate. Rock samples returned from the lunar surface exhibit oxygen and titanium isotope ratios nearly identical to those of terrestrial mantle rocks, rather than showing a distinct chemical signature from a foreign impactor. To resolve this discrepancy, planetary scientists have proposed either that Theia formed in a similar orbital distance from the Sun and shared Earth's isotopic composition, or that the collision was a high-energy, direct impact that thoroughly homogenized the vaporized material of both bodies before the Moon condensed.

Following its accretion, the infant Moon was covered by a deep global magma ocean.¹⁸ As this molten layer cooled and crystallized over millions of years, dense iron- and magnesium-rich minerals such as olivine and pyroxene sank to form the mantle, while lighter plagioclase feldspar floated upward to solidify into a low-density anorthositic crust. Between 4.1 and 3.8 billion years ago, a period of intense asteroid and comet impacts excavated vast basins across the lunar surface. Over the subsequent billion years, heat generated by the decay of radioactive elements within the lunar interior melted portions of the upper mantle, driving flood basalts up through fissures in the crust to fill the low-lying impact basins and create the dark volcanic plains visible today.

The Moon is an oblate spheroid with an equatorial radius of 1,738.1 kilometers and a polar radius of 1,736.0 kilometers, yielding a total surface area of approximately 37.9 million square kilometers, or about 7.4 percent that of Earth. Its mean density is 3.34 grams per cubic centimeter, making it the second-densest natural satellite in the Solar System after Jupiter's moon Io, though substantially less dense than Earth. Because its mass is roughly one-eightieth that of Earth, gravitational acceleration on the lunar surface is only 16.6 percent, or approximately one-sixth, of terrestrial gravity.

Seismic data collected by instruments deployed during the Apollo missions, combined with gravitational mapping from orbital spacecraft such as the Gravity Recovery and Interior Laboratory, show that the lunar interior is differentiated into a core, mantle, and crust.¹⁹ At the center lies a metallic core comprising a solid, iron-rich inner core with a radius of about 240 kilometers, surrounded by a fluid outer core shell roughly 90 kilometers thick that contains iron alloyed with small quantities of nickel and sulfur. Above the outer core sits a partially molten boundary layer, which gives way to a rigid silicate mantle extending to the base of the crust. The lunar crust averages 40 to 45 kilometers in thickness globally, but exhibits a pronounced hemispherical asymmetry: the crust on the near side is roughly 30 to 38 kilometers thick, whereas the crust on the far side is approximately one-third thicker.²⁰

The Moon orbits Earth in a prograde, elliptical path with a mean semi-major axis of 384,399 kilometers—roughly 30 times the diameter of Earth. Because of its orbital eccentricity of 0.0549, the center-to-center distance between the two bodies varies between an average perigee of 362,600 kilometers and an apogee of 405,400 kilometers. Relative to the fixed stars, the Moon completes one revolution around Earth in 27.32 days, an interval known as the sidereal month. However, because the Earth–Moon system is simultaneously orbiting the Sun, it takes 29.53 days—a synodic month—for the Moon to return to the same phase as viewed from Earth.

Gravitational interactions early in the history of the Earth–Moon system slowed the Moon's initial rotation until it became tidally locked in synchronous rotation with its orbital period. Consequently, the same hemisphere always faces Earth, while the opposite hemisphere remains hidden from direct terrestrial view. Nevertheless, because the Moon's orbit is elliptical and inclined at 5.145 degrees to the ecliptic plane, slight variations in its orbital speed and viewing angle produce an apparent rocking motion called libration, which allows observers on Earth to see approximately 59 percent of the lunar surface over time.²⁶²⁷

The gravitational gradient exerted by the Moon across the diameter of Earth is the primary driver of terrestrial tides.²⁸ Water in the oceans bulges outward on both the side of Earth facing the Moon and the side directly opposite, generating two high tides and two low tides each day as the planet rotates beneath these bulges. Because Earth rotates faster than the Moon orbits it, friction between the oceans and the seafloor drags the tidal bulges slightly ahead of the Earth–Moon axis. This offset creates a gravitational torque that transfers rotational energy and angular momentum from Earth to the Moon.²⁹ Laser ranging experiments—which bounce laser beams off retroreflector arrays placed on the lunar surface by Apollo and Soviet Lunokhod missions—demonstrate that this tidal acceleration causes the Moon to recede into a higher orbit at a rate of 3.8 centimeters per year, while simultaneously slowing Earth's rotation and lengthening the terrestrial day by roughly 1.7 to 2.3 milliseconds per century.

As the Moon orbits Earth, the changing geometry of sunlight striking its surface produces the recurring sequence of lunar phases: new moon, crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent. When the Moon passes directly between the Sun and Earth at the new moon phase, it casts a shadow onto Earth, producing a solar eclipse; because the apparent angular diameter of the Moon in the sky is nearly identical to that of the Sun, the Moon can completely block the solar disk during a total eclipse.³⁰ Conversely, when Earth passes directly between the Sun and the full Moon, the terrestrial shadow falls across the lunar surface, causing a lunar eclipse.³¹

Before the advent of telescopic astronomy, observation of the Moon was limited to the naked eye, by which ancient astronomers tracked its phases and eclipses. The invention of the telescope in the early seventeenth century initiated the field of selenography—the mapping of lunar physical features—as early observers charted craters, mountain ranges, and the dark plains they mistook for seas. Direct physical exploration began during the Cold War space race of the late 1950s between the Soviet Union and the United States.³²

In January 1959, the Soviet spacecraft Luna 1 executed the first lunar flyby, followed later that year by Luna 2, which became the first human-made object to impact the lunar surface, and Luna 3, which returned the first photographs of the previously unseen far side. In 1966, the Soviet Union's Luna 9 achieved the first survivable soft landing on the Moon, demonstrating that the lunar regolith could support the weight of a spacecraft, and the United States followed with five robotic Surveyor landings between 1966 and 1968.³³

Crewed exploration commenced in December 1968 when NASA's Apollo 8 carried astronauts Frank Borman, James Lovell, and William Anders into lunar orbit. On July 20, 1969, the Apollo 11 Lunar Module landed in Mare Tranquillitatis, and American astronauts Neil Armstrong and Buzz Aldrin became the first humans to walk on the Moon. Between 1969 and December 1972, six Apollo missions (Apollo 11, 12, 14, 15, 16, and 17) landed a total of 12 astronauts on the surface. The astronauts deployed scientific instruments, traversed the terrain using battery-powered Lunar Roving Vehicles, and returned 382 kilograms of lunar rocks and soil to Earth. Concurrently, the Soviet Union conducted three uncrewed sample-return missions—concluding with Luna 24 in 1976—and deployed two robotic rovers, Lunokhod 1 and Lunokhod 2.

The international legal status of the Moon is governed primarily by the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies—commonly known as the Outer Space Treaty—which entered into force on October 10, 1967.⁴⁰⁴¹ The treaty designates the exploration and use of the Moon as the "province of all mankind" and guarantees free access to all areas of celestial bodies for all nations on a basis of equality. Article II of the treaty explicitly prohibits national appropriation, stipulating that no state may claim sovereignty over the Moon by means of use, occupation, or any other measure.⁴² Furthermore, the treaty mandates that the Moon be used exclusively for peaceful purposes, outlawing the placement of nuclear weapons or other weapons of mass destruction, the establishment of military bases and fortifications, the testing of weapons, and the conduct of military maneuvers on the lunar surface, while permitting the use of military personnel for scientific research.

As national space agencies and private corporations have developed plans to extract lunar resources—such as harvesting polar water ice to produce breathable oxygen and hydrogen rocket propellant—questions regarding the legal permissibility of space mining have generated international debate.⁴³⁴⁴ In October 2020, the United States Department of State and NASA introduced the Artemis Accords, a series of non-binding multilateral arrangements designed to establish operational principles for civil lunar exploration. By October 2026, 77 countries had signed the Accords.

The Artemis Accords affirm commitment to the 1967 Outer Space Treaty while introducing guidelines on system interoperability, the public release of scientific data, orbital debris mitigation, the protection of historic lunar heritage sites (such as the Apollo landing zones), and the creation of temporary "safety zones" to prevent harmful interference between nearby operations. Crucially, the Accords assert that the extraction and utilization of space resources does not constitute prohibited national appropriation under Article II of the Outer Space Treaty. Critics of the framework, including non-signatory spacefaring nations such as Russia and China, have contested this interpretation, arguing that unilateral rules on resource extraction and exclusive safety zones favor commercial mining interests and risk creating de facto territorial claims outside the framework of the United Nations.

Because of its prominence in the night sky and the regularity of its 29.5-day synodic cycle, the Moon has served as a foundational timekeeping instrument across human civilizations for millennia. Archaeological and historical records show that ancient societies, including the Babylonians and Egyptians by at least the fifth century BCE, structured their calendars around the lunar month. Purely lunar calendars, such as the Islamic Hijri calendar, consist of 12 lunar months totaling approximately 354 days and continue to govern religious observances including Ramadan and Eid al-Fitr. Because 12 lunar months fall roughly 11 days short of a solar tropical year, many civilizations developed lunisolar calendars—such as the Hebrew, Chinese, and ancient Babylonian calendars—which periodically insert an intercalary month to keep lunar phases aligned with the agricultural seasons.

Beyond timekeeping, the Moon has been personified as a deity in numerous mythologies, including Selene and Artemis in ancient Greece and Luna in ancient Rome, and has inspired diverse pareidolic interpretations of the dark patterns formed by the near-side maria, such as the Western "Man in the Moon". In Chinese culture, the full moon of the eighth lunar month is celebrated during the Mid-Autumn Festival as a symbol of peace, prosperity, and family reunion. Conversely, lunar and solar eclipses were historically viewed with awe or trepidation across many traditions: in Hindu mythology, eclipses (grahana) are attributed to the celestial entities Rahu and Ketu swallowing the Sun or Moon and are observed with fasting and prayer, while in Navajo tradition, an eclipse represents a sacred moment of death and renewal during which observers remain indoors in silent reverence.