Moon phase calculation methods

Moon phase is not stored as a label on a calendar. It is calculated for an instant from the changing geometry of the Sun, Earth, and Moon. The phase calculator first resolves the submitted civil date, clock time, and IANA time zone to UTC, then evaluates the lunar state at that single instant.

Phase angle and illumination answer one question about geometry and visible fraction at an instant. Lunar age answers another: the elapsed time since the preceding new Moon. A rounded phase name or a mean month length cannot stand in for either quantity.

Physical lunar phase wheel with a photographed Moon and engraved angle scale

Calculation sequence

How this method is divided

01
A continuous phase angle tracks the Moon through a 360-degree cycle, while illuminated fraction measures the visible sunlit disc. Names such as waxing gibbous are display bands; the four principal phases occur at defined angular targets.
02
Lunar age is the interval between the selected instant and the strictly preceding new Moon. The previous and next principal-phase timestamps fall on opposite sides of the selected instant rather than at fixed calendar intervals.

Working sheets

Two focused guides

01Moon Phase Angle and IlluminationSee how Moon phase angle and illuminated fraction describe different parts of lunar geometry, with an exact-instant example and observer conventions.

02Lunar Age Calculation from New MoonLearn how lunar age is measured as elapsed UTC time from the preceding calculated new Moon instead of scaling phase angle by a mean month length.

Method selection

Where this method fits

Use this method for exact-instant phase state, illumination, lunar age, and principal-event timing.
Use the Desktop Lunar Calendar when a day-by-day monthly overview is the better starting point.
Use the position method when moonrise, horizon direction, or observer coordinates matter.

Put the method to work

Use the working calculation

Evaluate another exact instant with the calculator described here, or return to the overview for the shared time conventions and supported era.

Open Moon Phase Calculator

Method overview · Calculation sources