Moon Phase Calculator

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Next new moon
Next full moon

Estimated using low-precision astronomical formulas (Meeus, Astronomical Algorithms), accurate to within about an hour. For precise work, use an official ephemeris.

About moon phase calculation

The Moon's phase — how much of it appears lit from Earth — changes over a roughly 29.5-day cycle (the synodic month) as it orbits the Earth and the angle between the Sun, Earth, and Moon shifts. This tool shows the moon phase, illumination percentage, and moon age for any date, plus the dates of the next new moon and full moon.

How the phase is calculated

The tool computes the angular separation (elongation) between the Sun and Moon using standard low-precision solar and lunar position formulas, then converts that angle into an illumination percentage and one of the 8 named phases (new moon, waxing crescent, first quarter, and so on). Moon age is the number of days since the most recent new moon, derived from the same elongation angle.

Frequently asked questions

How accurate is this calculator?
It uses standard low-precision astronomical formulas that are accurate to within a fraction of a day for the phase and illumination — more than sufficient for everyday use, though not a precision ephemeris used for eclipse prediction or scientific work.
What does "moon age" mean?
Moon age is simply the number of days that have passed since the most recent new moon, ranging from 0 (new moon) to about 29.5 (just before the next new moon).
Why is the synodic month about 29.5 days instead of the Moon's ~27.3-day orbit?
The Moon's orbital period around Earth (the sidereal month) is about 27.3 days, but because Earth is also moving around the Sun, the Moon needs a few extra days to return to the same position relative to the Sun — that's the 29.5-day synodic month this tool uses for phases.