Orbit of the Moon
The Moon orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days (a tropical month and sidereal month) and one revolution relative to the Sun in about 29.53 days (a synodic month). Earth and the Moon orbit about their barycentre (common centre of mass), which lies about 4,670 km (2,900 mi) from Earth's centre (about 73% of its radius), forming a satellite system called the Earth–Moon system. On average, the distance to the Moon is about 385,000 km (239,000 mi) from Earth's centre, which corresponds to about 60 Earth radii or 1.282 light-seconds.
Diagram of the Moon's orbit with respect to the Earth. While angles and relative sizes are to scale, distances are not. | |
| Semi-major axis | 384,748 km (239,071 mi) |
|---|---|
| Mean distance | 385,000 km (239,000 mi) |
| Inverse sine parallax | 384,400 km (238,900 mi) |
| Perigee | 363,228.9 km (225,700.0 mi), avg. (356400–370400 km) |
| Apogee | 405,400 km (251,900 mi), avg. (404000–406700 km) |
| Mean eccentricity | 0.0549006 (0.026–0.077) |
| Mean obliquity | 6.687° |
| Mean inclination | |
| of orbit to ecliptic | 5.15° (4.99–5.30) |
| of lunar equator to ecliptic | 1.543° |
| Period of | |
| orbit around Earth (sidereal) | 27.322 days |
| orbit around Earth (synodic) | 29.530 days |
| precession of nodes | 18.5996 years |
| precession of line of apsides | 8.8504 years |
With a mean orbital velocity around the barycentre between the Earth and the Moon, of 1.022 km/s (0.635 miles/s, 2,286 miles/h), the Moon covers a distance approximately its diameter, or about half a degree on the celestial sphere, each hour. The Moon differs from most regular satellites of other planets in that its orbit is close to the ecliptic plane instead of to its primary's (in this case, Earth's) equatorial plane. The Moon's orbital plane is inclined by about 5.1° with respect to the ecliptic plane, whereas the Moon's equatorial plane is tilted by only 1.5°.