Moon · Phases
Moon phase calculator.
Pick a date and see the moon phase, how much of the Moon is lit and when the next new and full moons fall.
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How the moon phase is calculated
The Moon returns to the same phase every synodic month. NASA gives the mean length as 29.530588 days, although any single month can run from 29.26 to 29.80 days. The tool counts days since a reference new moon taken from NASA's table (18 January 2026, 19:52 UT), takes the remainder after whole cycles as the Moon's age, and works out illumination as (1 − cos(2π × age / 29.530588)) / 2.
Worked example. For 8 October 2026 at 12:00 UT the age is 26.4 days, giving 11 percent illumination: a Waning Crescent. The next new moon is estimated at 2026-10-11 (NASA's table has 10 October 2026, 15:50 UT) and the next full moon at 2026-10-26 (NASA: 26 October 2026, 04:12 UT).
Because the real Moon speeds up and slows down along its orbit, this mean-cycle method can be off by up to about a day from the true time of a phase. Compared with NASA's 2026 and 2027 tables, the estimate was within one day of NASA's new and full moons for 2026 and 2027; quarters can be just over a day off. Phase names follow a common convention: the new moon, quarters and full moon are each named for a window of about a day either side of the exact moment.
Frequently asked questions
How accurate is this moon phase calculator?
It uses the average month length, so the estimated moment of a phase can be about a day off the true one. The phase name and percentage for the day are usually right; for exact times, use a table such as NASA’s.
What does moon age mean?
The number of days since the last new moon, from 0 up to about 29.5.
How long is the cycle from new moon to new moon?
The mean synodic month is 29.530588 days, per NASA, with real months between 29.26 and 29.80 days.
Is the illumination the same everywhere on Earth?
Very nearly. The percentage lit is the same worldwide, but the Moon appears tilted or flipped depending on your latitude and hemisphere.
Sources
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Scope
Approximate method for planning and curiosity. For exact times of phases and eclipses, use the NASA tables.