Orbit of the Moon

The orbit of the Moon is, while stable and known, highly complex, and as such still studied by lunar theory. Most models describe the Moon's orbit geocentrically since the Moon is mainly bound to Earth, but it also orbits together with Earth, as the Earth-Moon system, around their shared barycenter. Furthermore from a heliocentric view its geocentric orbit is the result of Earth perturbing the Moon's orbit around the Sun. It orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the fixed stars in about 27.3 days (a tropical month and a sidereal month), and one revolution relative to the Sun in about 29.5 days (a synodic month).

On average, the distance to the Moon is about 384,400 km (238,900 mi) from Earth's centre, which corresponds to about 60 Earth radii or 1.28 light-seconds. The barycentre lies about 4,670 km (2,900 miles) from Earth's centre (about 73% of its radius). With a mean orbital speed around the barycentre of 1.022 km/s (2,290 mph), the Moon covers a distance of 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 orbital plane is closer to that of its primary – the ecliptic, the plane of Earth's orbit – than to the primary's equatorial plane. The Moon's orbital plane is inclined about 5.1° from the ecliptic (while Earth is tilted about 23.4°).