Moon Phase Calculator

The tool counts the days from a known new moon on 6 January 2000 at 18:14 UTC and divides the remainder by the mean synodic month of 29.530589 days. That fraction is the moon age, which is then mapped to one of eight named phases.

It is how far the moon has travelled through the current lunation, counting from the most recent new moon. Zero is a fresh new moon, about 14.8 days is full, and the count resets near 29.5 days when the next new moon begins.

It is the share of the moon's visible disc lit by the sun, computed from the moon age with a cosine curve. It reads 0 percent at new moon, climbs to 100 percent at full moon, and sits near 50 percent at the two quarter phases.

The 29.5-day cycle is split into eight equal bands of about 3.69 days each, centred on the principal phases. A date sitting exactly on a new, quarter, or full moon lands in the middle of its band, so the name stays stable for the day or two around each principal phase.

In cycle order they are New, Waxing Crescent, First Quarter, Waxing Gibbous, Full, Waning Gibbous, Last Quarter, and Waning Crescent. Waxing means the lit part is growing, and waning means it is shrinking back toward new.

This uses the mean synodic month, a long-run average. The real interval between new moons varies by several hours because the moon's orbit is elliptical, so a precise almanac can differ by up to a day. The phase name and illumination here are reliable for planning.

The phase is evaluated at 00:00 UTC on the date you enter. Since the moon changes slowly, the phase name is effectively the same worldwide on a given calendar day, so you do not need to convert your local time.

Yes. The date is turned into a Julian Day with the standard Fliegel and Van Flandern formula, which works across the Gregorian calendar, so historical and future dates both return a phase. Accuracy stays within about a day of the true moon.

No. The whole calculation runs in your browser from a fixed reference new moon and the synodic month constant, so once the page has loaded it works offline and sends nothing to a server.