lunations

Lunation is the period from one new Moon to the next, encompassing the complete cycle of lunar phases as observed from Earth. It is synonymous with the synodic month and is defined by the Moon's 29.53-day cycle relative to the Sun. Because the Earth-Moon system orbits the Sun, the Moon must travel a little farther to regain the same elongation from the Sun, making the synodic month longer than the Moon's orbital period about the Earth.

The sidereal month, about 27.3217 days, measures the Moon's orbital period with respect to the fixed stars, while the anomalistic month, about 27.5545 days, measures the cycle between perigees. Thus lunation length is variable and tied to the Sun-Earth-Moon geometry.

During a lunation, the Moon's phase progresses through new moon, first quarter, full moon, and last quarter, with waxing phases increasing in illuminated fraction and waning phases decreasing.

Common terms associated with specific lunation events include blue moon (two full moons in one calendar month), supermoon (full Moon near perigee appearing larger), and micromoon (full Moon near apogee appearing smaller).

In calendars and astronomy, lunations are used to track the Moon's cycle and to coordinate lunar-based timekeeping, including lunisolar calendars. Modern calculations rely on astronomical ephemerides, which predict the timing of each new Moon and phase with high precision. The concept underscores the Moon's synodic period and its observable, repeating sequence of phases.