Moon Phase Calculator

Find the moon's phase for any date and time. Enter a date (defaults to now) to get the phase name, illuminated fraction, age of the Moon, and the dates of the next new, first-quarter, full, and last-quarter moons. A simple synodic-month model (lunation 29.530588 days from the J2000 reference new moon). Everything runs locally in your browser.

Date and time

Hours offset from UTC (auto from browser)

—

Enter a date.

The lunar cycle

Phase angle is computed from the lunation number relative to the J2000 reference new moon (2000-01-06 18:14 UTC). The synodic month is 29.530588 days; the model is accurate to within a few hours of the true phase. Runs locally.