Earth's ancient, ever-present companion
The Moon
Moon of Earth · Known since prehistory
The Moon is Earth's only natural satellite and the fifth-largest moon in the Solar System. It is the only world beyond our own that humans have walked on, and its steady gravitational pull shapes our tides, slows Earth's spin and helps keep our planet's axis stable over long spans of time.
Look closely and the familiar face in the night sky turns out to be a record book. Its dark plains, bright highlands and countless craters preserve more than four billion years of history that erosion and plate tectonics have long since erased from Earth.
Key facts
- Diameter
- 3,475 km27% of Earth
- Mass
- 7.35 × 10²² kg
- Surface gravity
- 1.62 m/s²0.165 g
- Density
- 3,344 kg/m³
- Distance from Earth
- 384,400 km≈ 60.3 Earth radii
- Orbital period
- 27.3 daysSame as its rotation (tidally locked)
- Discovered
- Known since prehistory
- Surface temperature
- About −173 °C at night to 127 °C at midday
- Atmosphere
- Near-vacuum exosphere of helium, argon and sodium
- Rotation
- Tidally locked: the same face always points at Earth
- Notable feature
- South Pole–Aitken basin, about 2,500 km across
- Named after
- Old English mōna; Luna and Selene in mythology
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
The Moon's face is split into two kinds of terrain. The bright highlands are ancient crust made largely of anorthosite, a pale rock rich in calcium and aluminium, and they are saturated with craters. The dark maria, Latin for 'seas', are vast plains of basaltic lava that welled up into giant impact basins between roughly 3 and 3.9 billion years ago, creating the patterns people describe as the 'man in the Moon'.
Impact craters range from microscopic pits to the South Pole–Aitken basin, one of the largest known impact structures in the Solar System. Younger craters such as Tycho, about 85 kilometres wide and around 108 million years old, are surrounded by bright rays of ejecta that stretch across much of the near side and can be seen with binoculars at full Moon.
Everything is blanketed in regolith, a layer of shattered rock and glassy dust produced by billions of years of meteorite bombardment. In permanently shadowed craters near the poles, where temperatures stay colder than −200 °C, spacecraft have found evidence of water ice, a resource future explorers hope to use for drinking water, oxygen and rocket fuel.
Interior and composition
Seismometers left by Apollo astronauts revealed that the Moon has a layered interior: a crust tens of kilometres thick (thicker on the far side), a rocky mantle, and a small iron-rich core with a radius of roughly 300–350 kilometres. Later analysis suggests the core has a solid inner part surrounded by fluid, possibly wrapped in a partially molten layer.
The Moon is poor in iron and volatile elements compared with Earth, yet the oxygen isotopes in lunar rocks are almost identical to those of Earth's mantle. That combination is one of the strongest clues to how it formed. The Moon also shakes: shallow moonquakes recorded by Apollo instruments reached magnitudes of about 5.
Orbit and rotation
The Moon takes about 27.3 days to circle Earth relative to the stars, but 29.5 days to cycle through its phases, because Earth is moving around the Sun at the same time. Its rotation is tidally locked, meaning it spins exactly once per orbit, so we always see the same hemisphere. A slight wobble called libration lets observers on Earth glimpse about 59 per cent of the surface over time.
Tidal interaction with Earth is slowly pushing the Moon outwards by about 3.8 centimetres per year, a rate measured by bouncing lasers off reflectors left by Apollo crews and Soviet rovers. The same process is gradually lengthening Earth's day.
Origin
The leading explanation is the giant-impact hypothesis. Around 4.5 billion years ago, scientists think a Mars-sized body, often called Theia, struck the young Earth a glancing blow. Debris from both bodies was flung into orbit and quickly gathered into the Moon.
The hypothesis neatly explains the Moon's small iron core and its isotopic kinship with Earth, but details are still debated. Some models favour a more energetic collision that thoroughly mixed the two worlds, and researchers continue to test the idea against new samples.
Exploration
The Moon was the first target of the Space Age. The Soviet Luna 2 struck it in 1959 and Luna 3 returned the first pictures of the far side the same year. Today a new wave of robotic landers from several nations, and NASA's Artemis programme, are reviving interest in its poles.
Apollo (1969–1972)
Six Apollo missions landed between July 1969 and December 1972, and 12 astronauts walked on the surface, beginning with Neil Armstrong and Buzz Aldrin on Apollo 11. Crews brought home 382 kilograms of rock and soil that still underpin lunar science.
Chang'e 4 and Chang'e 6 (2019–2024)
China's Chang'e 4 made the first soft landing on the far side in 2019. In 2024 Chang'e 6 returned the first samples from the far side, scooped from the South Pole–Aitken basin.
Chandrayaan-3 (2023)
India's Chandrayaan-3 touched down at a high southern latitude in August 2023, making India the fourth nation to achieve a soft lunar landing.
Artemis (2022–present)
Artemis I sent an uncrewed Orion capsule around the Moon in 2022, and Artemis II is the programme's first crewed mission: four astronauts on a flyby around the Moon, the first people to travel there since 1972. Later Artemis missions aim to return people to the surface near the south pole.
Surprising facts about the Moon
- Footprints left by Apollo astronauts could survive for millions of years, because there is no wind or rain to erase them, only the slow rain of micrometeorites.
- Despite looking brilliant at night, the Moon's surface is roughly as dark as worn asphalt, reflecting only about 12 per cent of the sunlight that hits it.
- The 'dark side' of the Moon is a myth: the far side receives just as much sunlight as the near side; it is simply never visible from Earth.
- The Moon is drifting away by about 3.8 centimetres a year, roughly the rate at which your fingernails grow.
- Total solar eclipses happen only because the Sun is about 400 times wider than the Moon and also about 400 times farther away, a coincidence that will not last forever as the Moon recedes.