How this calculator works
Enter any date and this calculator estimates the Moon's phase, its illuminated percentage, and how many days separate that date from the nearest new moon and full moon. It works by counting whole and partial synodic months forward from a known reference new moon, rather than by looking up live ephemeris data, so it works for any date without needing a network connection.
This is a mean-phase approximation, not a precise ephemeris calculation. The real interval between new moons varies from about 29.26 to 29.80 days because the Moon's orbit is elliptical, not circular, so the true phase can be off from this calculator's estimate by anywhere from a few hours to roughly half a day. For casual planning — like guessing whether a night will be dark enough for stargazing — that level of accuracy is normally sufficient; for precise astronomical or navigational work, use a dedicated ephemeris source.
The mean synodic month method
days since reference new moon = (target date − Jan 6, 2000, 18:14 UTC) ÷ 1 day, taken modulo 29.530588853illuminated fraction ≈ (1 − cos(2π × days since new moon ÷ 29.530588853)) ÷ 229.530588853 days is the mean synodic month (average new-moon-to-new-moon interval). January 6, 2000, 18:14 UTC is a documented new moon used here as the reference epoch. Dates are evaluated at 00:00 UTC, not the exact hour, which adds a small additional offset on top of the mean-phase approximation itself.
Worked example: a known full moon
- December 19, 2021 was a documented full moon at 04:36 UTC.
- Counting whole synodic months forward from the January 6, 2000 reference new moon lands this date about 14.45 days into its cycle — close to the halfway point of 14.77 days (half of 29.530588853).
- Because that's near the cycle's midpoint, the calculator reports 'Full Moon' with illumination just under 100%.
- The small gap between 14.45 and the exact 14.77-day midpoint (about 8 hours) illustrates the method's stated accuracy limit — still well within its 'several hours to about half a day' margin.
Frequently asked questions
How accurate is this calculator?
It uses a mean (average) synodic month rather than the Moon's true, slightly irregular orbit, so results can be off from the exact real-world moment by anywhere from a few hours up to about half a day. That's precise enough for everyday reference but not for telescope scheduling or navigation, which need true ephemeris data.
Why does the Moon's cycle length vary at all?
The Moon's orbit around Earth is an ellipse, not a perfect circle, and Earth's own orbit around the Sun adds a further small perturbation. Together these make any single synodic month run anywhere from about 29.26 to 29.80 days, even though the long-run average is 29.530588853 days.
What do the 8 named phases mean?
New Moon and Full Moon are the two 'exact' phases (0% and 100% illuminated on average); First Quarter and Last Quarter are the two half-illuminated phases; the remaining four (Waxing/Waning Crescent and Gibbous) describe the growing or shrinking sliver or bulge of light in between.
Does illumination percentage account for which side is lit?
No — it's a magnitude only, describing how much of the visible disk is lit, not whether the light is on the left or right side (which flips between waxing and waning phases, and also depends on the observer's hemisphere).
Why 00:00 UTC and not a specific time of day?
This calculator takes a single date as input, so it evaluates the phase at the start of that day in UTC for a consistent, reproducible answer. Since the Moon only moves through roughly 12-13 degrees of its cycle per day, using midnight instead of the exact hour of interest adds at most a small fraction of a day's worth of extra uncertainty on top of the mean-phase approximation itself.