Space · Moons
Luna
A moon of Earth — Earth's only natural satellite — the only body beyond Earth that humans have visited.
Quick facts
Parent planet
Diameter (mean)
3475 km
Mass
7.35 × 10²² kg
1 Moon masses
Mean orbital radius
384,400 km
Orbital period
27.32 Earth days
Discovery year
prehistoric
Discoverer
Known to all of humanity
Naming origin
Latin for ‘moon’
Surface conditions
The Moon is a heavily-cratered ball of rock with no atmosphere, no liquid water, and surface temperatures swinging from 127°C in direct sunlight to −173°C in shadow. The bright highlands are ancient anorthosite crust; the dark maria are basaltic plains flooded by lava 3.0–3.9 billion years ago after large impacts thinned the crust on the Earth-facing side. Water ice exists in permanently shadowed craters near the poles, confirmed by lunar prospector and LCROSS data in 2009.
Six Apollo missions (11, 12, 14, 15, 16, 17) landed twelve astronauts on the Moon between 1969 and 1972, returning 382 kg of lunar samples and leaving behind retroreflector arrays still used today for laser ranging that measures the Moon's recession from Earth (currently 3.8 cm per year). The Moon is tidally locked, always presenting the same face to Earth; the 'far side' was first photographed by the Soviet Luna 3 probe in 1959.
Naming etymology
'Luna' is the Roman personification of the moon, equivalent to the Greek Selene — both represent the female deification of the lunar body. In English the planet is most commonly just called 'the Moon' (with capital M when referring to Earth's specific moon), but 'Luna' is the formal IAU designation and the adjective 'lunar' carries through to all moon-related vocabulary. Every other moon in the solar system uses 'moon' as a common noun; only Earth's gets the proper name.
Methodology & sources
Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.