Moon of Earth
Moon
Earth's tidally locked companion, drifting away about an inch a year.
The Moon spins exactly once per orbit, so the same face always points toward Earth. It circles Earth every 27.3 days, but because Earth is moving around the Sun too, the cycle of phases takes 29.5 days. Its orbit is slowly widening, by about an inch each year.
How it moves
- The Moon orbits Earth every 27.3217 days relative to the stars.
- It rotates in the same time it takes to orbit (synchronous rotation), so one hemisphere always faces Earth.
- New moon to new moon takes 29.53 days (synodic period), longer than the orbit because Earth moves along its own path around the Sun.
- Its orbit is tilted 5.145 degrees to the ecliptic, and that tilted plane swings around once every 18.6 years.
- The Moon's distance ranges from about 357,000 to 407,000 km, and it recedes from Earth by about an inch per year.
Facts
- Mean radius
- 1,737.4 km
- Mass
- 7.346e+22 kg
- Surface gravity
- 1.62 m/s²
- Rotation period
- 655.72 hours
- Orbital period
- 27.322 days
- Average orbital distance
- 384,400 km
- Eccentricity
- 0.0549
- Axial tilt
- 6.68°
- Mean orbital speed
- 1.022 km/s
- Discovered
- Known since antiquity
The Moon is slowly moving away from Earth, about an inch farther each year.
Sources: nssdc.gsfc.nasa.gov, science.nasa.gov, ssd.jpl.nasa.gov