Anomalistic month
The time the Moon takes to return to perigee, about 27.55 days; longer than the sidereal month because the lunar orbit's perigee slowly advances.
Sidereal month: The Moon's orbital period relative to the fixed stars, about 27.32 days. Roughly 0.23 days shorter: the perigee advances while the Moon orbits, so returning to perigee takes extra time.
Perturbation (astronomy): Deviations of a body's motion from a simple Keplerian orbit caused by other gravitating bodies. Solar and terrestrial gravitational perturbations drive both the Moon's perigee advance and orbital variations.
Supermoon: A full moon occurring near perigee, appearing up to 14 percent larger than an apogee full moon. Defined by coincidence of the synodic and anomalistic cycles, roughly every 14 lunar months.
Babylonian astronomy: The Mesopotamian tradition of systematic observation and calculation of lunar and planetary motion. Babylonian lunar ephemerides used perigee-based periods, effectively tracking the anomalistic month.
Synodic month: The Moon's phase cycle from new moon to new moon, about 29.53 days. The longest of the lunar months; phase depends on both Moon and Sun, not on perigee position.
Kepler's laws of planetary motion: Laws describing elliptical orbits with the central body at a focus and varying orbital speed. Explains why perigee and apogee matter: the Moon moves fastest near perigee, slowest at apogee.
Perigean spring tide: An unusually large tide occurring when perigee coincides with new or full moon. Predicting these extreme tides requires tracking the anomalistic month's perigee timing.
Hipparchus: Greek astronomer of the second century BC who determined the lunar orbit's eccentricity and apogee motion. First measured the Moon's eccentric orbit and the perigee's advance from eclipse observations.
Draconic month: The Moon's period relative to its ascending node, about 27.21 days; governs eclipse timing. Shorter than the anomalistic month because the nodes regress while the perigee advances, in opposite directions.
Evection: The largest periodic lunar inequality, a roughly 31.8-day variation in eccentricity caused by solar perturbation. Modulates the lunar orbit's eccentricity and perigee motion over a period near the anomalistic month.