Last updated: 2026-06-16

The Moon phase calculator reports which of the eight phases the Moon is in on a given date, how many days have passed since the last new moon, and what fraction of the visible disc is lit by the Sun. It is useful for planning stargazing and astrophotography, anticipating bright or dark nights, gardening by the lunar calendar, and simply understanding why the Moon looks the way it does on a particular evening.

How the phase is calculated

The Moon takes an average of 29.53 days to return to the same phase — one new moon to the next. This interval is the synodic month. Starting from a known new moon, the age of the Moon on any later date is the number of elapsed days reduced to that cycle:

The illuminated fraction then follows a cosine curve across the cycle, where the cycle position $p$ runs from 0 at new moon to 1 at the next new moon:

This gives 0% at new moon, 50% at the first and last quarters, and 100% at full moon.

Worked example

For a date 15 days after a new moon, the cycle position is $p = 15 / 29.53 \approx 0.508$, just past the midpoint. The illuminated fraction is:

So the disc is about 99.9% lit — effectively a full moon. An age near 7.4 days would instead give a first-quarter Moon with half the disc lit.

The eight phases and accuracy

The cycle is conventionally divided into eight named phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. A waxing Moon is growing brighter, a waning Moon fading; a crescent is less than half lit and a gibbous more than half. Because this calculator uses the mean synodic month and whole-day differences, the named phase and illumination are accurate to within about a day. The real lunar orbit speeds up and slows down slightly, so a published almanac may place the exact moment of a new or full moon a few hours either side. To measure the number of days between two dates directly, see the Date Difference Calculator calculator.