Moon Phase Calculator

Explore a rough mean-cycle estimate of the Moon's phase for dates from 1900 through 2100. Enter a date and fixed UTC offset to estimate the phase name, illuminated fraction, Moon age, and the next primary phases. Everything runs locally in your browser.

Date and time

Fixed hours offset from UTC

Initially uses your browser's current offset. Adjust it for daylight-saving or historical dates.

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Enter a date.

The lunar cycle

This is a mean-cycle estimate, not an astronomical ephemeris. It uses a fixed 29.530588-day synodic month from the reference new moon at 2000-01-06 18:14 UTC; primary-phase times can differ from authoritative ephemerides by many hours. The disk is schematic and does not show its local-sky orientation. Do not use these estimates for navigation, tides, eclipse viewing, or precise observation planning. Compare precise events with the U.S. Naval Observatory Moon phase service.