Moon Phase Date Calculator

About this calculator

The Moon cycles through its phases over a synodic month — about 29.53059 days, the time it takes to return to the same phase as seen from Earth (slightly longer than its 27.32-day orbital period because Earth itself moves around the Sun). This calculator finds a date's position within that cycle by measuring elapsed days since a known reference new moon, then reduces that count modulo the synodic month length to get the Moon's age in days and its phase name (New, Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Last Quarter, Waning Crescent).

From the phase angle, illumination percentage is derived from how much of the Moon's visible disk is lit — roughly (1 − cos(phase angle))/2 — which is why illumination peaks near 100% at Full Moon and bottoms out near 0% at New Moon. This kind of calculation underlies lunar calendars, tide prediction, astrophotography planning, and religious observances (like the Islamic and Hebrew calendars) that are tied directly to the Moon's visible phase rather than the solar year.