Orbit of the Moon
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| Diagram of the Earth–Moon system | ||||||||||||||||||||||||||||
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The Moon orbits Earth in the prograde direction and completes one revolution relative to the stars in approximately 27.323 days (a sidereal month). Earth and the Moon orbit about their barycentre (common center of mass), which lies about 4,600 km (2,900 mi) from Earth's center (about 3/4 of the radius of Earth). On average, the Moon is at a distance of about 385,000 km (239,000 mi) from Earth's centre, which corresponds to about 60 Earth radii. With a mean orbital velocity of 1.022 km/s (2,290 mph), the Moon appears to move relative to the stars each hour by an amount roughly equal to its angular diameter, or by about half a degree. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to Earth's equatorial plane. The plane of the lunar orbit is inclined to the ecliptic by about 5°, whereas the Moon's equatorial plane is inclined by only 1.5° to the ecliptic.
The properties of the orbit described in this section are approximations. The Moon's orbit around Earth has many irregularities (perturbations), whose study (lunar theory) has a long history.
The orbit of the Moon is distinctly elliptical, with an average eccentricity of 0.0549. The non-circular form of the lunar orbit causes variations in the Moon's angular speed and apparent size as it moves towards and away from an observer on Earth. The mean angular movement relative to an imaginary observer at the barycentre is 13.176° per day to the east (Julian day 2000.0 rate).
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