How accurate is the result?

It is computed from the mean synodic month (29.53 days) anchored to the new moon of 6 January 2000, accurate to within about a day. Exact phase times vary slightly because the Moon's orbit is elliptical.

What does illumination percentage mean?

It is the fraction of the Moon's visible disk lit by the Sun — 0% at new moon, 50% at the quarters, and 100% at full moon.

Is my date sent anywhere?

No — the calculation runs entirely in your browser with no network requests.

Moon Phase Calculator

Find the moon phase and illumination for any date.

Moon phase calculator

Find the moon phase for any date — New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter or Waning Crescent.

The result includes the Moon’s age in days and the percentage of the disk that is illuminated. Phases are derived from the mean synodic month (about 29.53 days) referenced to a known new moon, accurate to within roughly a day.