Orbit of the Moon

The Moon’s path around Earth, shaped mainly by Earth’s gravity and perturbed by the Sun. Its changing distance, orientation, and speed produce cycles such as the lunar phases and eclipses.

Newtonian gravity: A theory describing gravity as a force between masses, proportional to their product and inversely proportional to distance squared. Earth’s attraction supplies the dominant force bending the Moon’s path.
Sidereal month: The time the Moon takes to return to the same direction relative to the distant stars, about 27.3 days. It measures one revolution of the Moon around Earth against a nearly fixed sky.
Lunar phases: The changing illuminated shapes of the Moon visible from Earth during its orbit. The Moon’s changing position relative to Earth and Sun creates the phase cycle.
Ptolemaic astronomy: An ancient geocentric system that represented celestial motions using combinations of circles. Its lunar model predicted changing distance imperfectly and implied implausible apparent size changes.
Geostationary orbit: A circular equatorial orbit in which a satellite remains above the same longitude on Earth. Unlike a geostationary satellite, the Moon follows an inclined, eccentric orbit with a much longer period.
Two-body problem: The idealized problem of predicting two objects moving under their mutual gravitational attraction. It approximates Earth and Moon motion before solar perturbations are added.
Synodic month: The average interval between successive identical lunar phases, about 29.5 days. It exceeds the sidereal month because Earth and Moon move together around the Sun.
Solar eclipse: An event in which the Moon passes between Earth and the Sun and obscures the Sun partly or fully. A solar eclipse occurs when new Moon coincides with a node crossing.
Tycho Brahe: A Danish astronomer whose precise naked-eye observations improved measurements of planetary and lunar positions. His observations supplied data for refining models of lunar motion.
Earth’s orbit: The path Earth follows around the Sun, approximately elliptical with a period of one year. The Moon’s motion must be understood within the moving Earth–Sun system.

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