Earth's natural satelliteOur companion in space
The Moon
The Moon steadies Earth's seasons, drives the tides and preserves a four-billion-year record of impacts.
- Host
- Earth
- Type
- Rocky natural satellite
- Mean diameter
- 3,475 km
- Mean distance
- 384,400 km
- Orbital period
- 27.32 days
- Rotation
- Synchronous · 27.32 days
- Surface gravity
- 1.62 m/s²
- Temperature
- About −173 to 127 °C
The Moon up close
NASA imagery
Born from a giant impact
The leading explanation for the Moon's origin is a collision between the young Earth and a Mars-sized body about 4.5 billion years ago. Debris thrown into orbit gathered into a partly molten Moon, then cooled into the grey world we see today.
The Moon is unusually large for a planetary satellite. Its gravity raises ocean tides, and its steadying influence on Earth's axial tilt helps keep the climate within a relatively stable range.
Two faces, one world
Tidal locking keeps the same lunar hemisphere facing Earth. The familiar near side has broad dark maria: ancient basalt plains formed by volcanic eruptions. The far side is more heavily cratered and has a much thinner basaltic crust.
The lunar surface is covered in regolith, a layer of broken rock and glass produced by impacts. Without a substantial atmosphere or weather cycle, footprints and tracks can survive for millions of years.
Worth knowing
The Moon is moving away from Earth by about 3.8 centimetres each year.
Sunlight takes about 1.3 seconds to cross the Earth-Moon distance.
The far side is not permanently dark; it experiences sunrise, sunset and a two-week long lunar night.
Watch and explore
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Moon 101 National Geographic
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Research & further reading
Mission imagery and measurements are interpreted alongside orbital data. Follow the source links for the current NASA overview and precise satellite parameters.