How this calculator works
The moon repeats its phases about every 29.53 days, the mean synodic month. Counting from a known new moon at 2000-01-06 18:14 UTC gives the age of the current cycle in days. The illuminated fraction follows from the angle between the sun and the moon, which ranges from 0 at new moon to 1 at full moon. The eight common phase names divide the cycle into equal buckets around the principal phases.
Formula
Julian date at noon UTC = day number + 1721424.5 + 0.5. age = (JD - 2451550.26) mod 29.530588853. fraction = age / 29.530588853. Illuminated fraction = (1 - cos(2 pi x fraction)) / 2. Days to a target phase t = (t - age) mod 29.530588853.
Worked example
On 2026-11-01 the age is about 21.1 days, so the moon is in its last-quarter phase and about 60.9 percent lit. The next new moon is around 2026-11-09 and the next full moon around 2026-11-24.
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Frequently asked questions
How accurate are the dates?
They come from the mean synodic month, an average cycle. The real moon speeds up and slows down, so a phase can occur up to about a day earlier or later than the average predicts. Treat the dates as good approximations, not almanac times.
Why is the phase calculated at noon UTC?
The input is a date without a time. Noon UTC is the middle of the world day, which reduces the chance that the answer is on the wrong side of a phase boundary for most users. Exact phase times need a time and a more precise ephemeris.
What does the illuminated fraction mean?
It is the share of the moon's visible disc that receives sunlight as seen from Earth, from 0 at new moon to 1 at full moon. It is not the brightness of the moon in the sky, which also depends on its altitude and the atmosphere.
Why is the moon sometimes visible in daylight?
The phase cycle describes how much of the lit side faces Earth, not when the moon is above the horizon. A waxing moon can be up during the afternoon, when its lit face is still partially visible against a bright sky.