Orbit of the Moon and Lunar theory

Main articles: Orbit of the Moon and Lunar theory

The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days[nb 6] (its sidereal period). However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days[nb 7] (its synodic period).[39] Unlike most satellites of other planets, the Moon orbits nearer the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbital motion gradually rotates, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.[77]





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