Moon Phase Calculator
Moon phase on any date (or your birthday)
The phase is worked out in your browser from the mean synodic month (29.53 days) measured from a known new moon, so it is accurate to within about a day — plenty for a phase name and illumination, but not exact astronomical timing. Nothing is sent anywhere.
The eight moon phases
One lunar cycle runs through eight principal phases, from new moon back to new moon over about 29.5 days — the four quarter phases (new, first quarter, full, last quarter) with a crescent or gibbous phase between each.
- New MoonThe Moon sits between Earth and Sun; its lit side faces away, so it is invisible.
- Waxing CrescentA slim sliver grows on the right after the new moon, visible just after sunset.
- First QuarterHalf the disc is lit on the right; the Moon is a quarter of the way round its cycle.
- Waxing GibbousMore than half is lit and still brightening as it heads toward full.
- Full MoonEarth lies between Sun and Moon; the whole near side is lit and rises at sunset.
- Waning GibbousPast full and dimming, still more than half lit, now on the left.
- Last QuarterHalf the disc is lit on the left; three quarters of the way through the cycle.
- Waning CrescentA thin sliver on the left fades before the next new moon, visible before dawn.
What moon phase is it today?
This page shows the current moon phase the moment it loads: the phase name (from new moon through full moon and back), its emoji glyph, the percentage of the disc that is lit, whether the Moon is waxing (brightening toward full) or waning (dimming toward new), and the Moon's age in days since the last new moon. Pick a different month, day, and year to see the phase for any date — a birthday, an anniversary, or a night you want to plan around — and the next full moon and next new moon from that date.
How the moon phase is calculated
The Moon takes about 29.53 days to cycle from one new moon to the next — the mean synodic month. Starting from a known new moon (6 January 2000, 18:14 UTC), the calculator measures how far the chosen date falls through that cycle, turns it into an illuminated fraction, and names the phase. Because it uses the average cycle length rather than a full astronomical ephemeris, the result is accurate to within roughly a day. That is more than enough to tell you the phase name and rough illumination, but do not use it for precise eclipse or rise/set timing.
Waxing, waning, crescent and gibbous
"Waxing" means the lit portion is growing from new toward full; "waning" means it is shrinking from full back toward new. "Crescent" is less than half lit and "gibbous" is more than half lit. Combined, they name the four phases between the quarters: waxing crescent and waxing gibbous on the way up, waning gibbous and waning crescent on the way down, with first quarter and last quarter marking the half-lit points.