The Moon is Earth’s constant companion, most likely born when a Mars-sized body struck the young Earth and the debris coalesced in orbit. It stabilises Earth’s tilt and raises the ocean tides.
It is drifting away from Earth by ~3.8 cm per year.
Its gravity is 1/6 of Earth’s - you could jump six times higher.
12 people have walked on it, all between 1969 and 1972.
Why we only see one side
The Moon rotates exactly once per orbit - 27.3 days for both. Earth’s gravity slowed its spin until the two locked together. That is why the same face always points at us: it IS rotating, just in perfect sync.
Physical characteristics
Diameter
3,475 km
Mass
7.350e+22 kg
Surface gravity
1.62 m/s²
Density
3.34 g/cm³
Rotation period
655.7 h
Tidally locked
Yes
Axial tilt
6.7°
Mean temperature
-23 °C
Temperature range
-173 °C to 127 °C
Geometric albedo
0.12
Atmosphere and composition
Atmosphere
Essentially none - a vanishingly thin exosphere of argon, helium and sodium.
Composition
Rocky, with a small iron core; the crust is anorthosite highlands and basalt maria.
Interior structure
Solid inner coreobserved
A small iron heart, ~480 km across.
Fluid outer coreobserved
A thin molten shell - too small to power a magnetic field today.
Mantleobserved
Solid rock, source of the ancient mare basalts.
Crustobserved
~40 km thick on the near side, thicker on the far side.
How we know: Apollo seismometers recorded moonquakes for eight years, and NASA’s GRAIL mission mapped the gravity field - together they resolved the core, mantle and crust.
Missions
Apollo 11NASA · Crewed lander · 1969–1969 · completed
The first crewed landing on another world. On 20 July 1969 Neil Armstrong and Buzz Aldrin walked on the Moon’s Sea of Tranquility while Michael Collins orbited above.
Lunar Reconnaissance OrbiterNASA · Orbiter · 2009 · active
LRO has been mapping the Moon in extraordinary detail since 2009, measuring some of the coldest temperatures known in the Solar System inside permanently shadowed polar craters. Its maps guide the Artemis programme’s choice of landing sites.
Artemis INASA · Uncrewed test flight · 2022–2022 · completed
The first flight of NASA’s giant SLS rocket sent an uncrewed Orion capsule around the Moon in late 2022, reaching about 432,000 km from Earth - farther than any spacecraft built for humans has ever gone. It cleared the way for crewed Artemis flights.