Moon Phase Calculator
Find the Moon's phase, illumination, and days until the next new or full moon for any calendar date.
- Moon phase
- Last Quarter
- Illumination
- 39.1%
- Moon age
- 23.2 days since new moon
- Next new moon
- in 6.4 days
- Next full moon
- in 21.1 days
How it works
The Moon cycles through its phases roughly every 29.530588 days on average — the mean synodic month, measured from one new moon to the next. Starting from a known reference new moon (January 6, 2000, 18:14 UTC), this calculator counts how many days have elapsed to your chosen date, takes that count modulo the synodic month length, and uses the remainder as the Moon's 'age' within its current cycle.
Illumination — the percentage of the visible disk that's lit — follows from that age as (1 − cos(2π × age ÷ 29.530588)) ÷ 2, which is 0% at a new moon, rises to 100% at a full moon halfway through the cycle, and falls back to 0% by the next new moon. The named phase (New Moon, Waxing Crescent, First Quarter, and so on) comes from splitting the cycle into 8 equal segments of about 3.69 days each.
Because the Moon's real orbit is elliptical rather than perfectly circular, the true time between new moons varies by several hours around this mean, so results can be off by roughly a day compared to a precise astronomical ephemeris — accurate enough to know what phase the Moon is in, not for time-critical astronomical work.
FAQ
Why might this differ slightly from another moon phase calculator or app?
This calculator uses a fixed average synodic month length rather than a full orbital ephemeris, so small differences of up to about a day compared to precise astronomical sources are expected — the real lunar cycle length varies slightly depending on where the Moon and Earth are in their own elliptical orbits at the time.
What time of day does the calculation use?
Since only a calendar date is entered, the calculation is evaluated at noon UTC on that date so the whole day maps to a single moon age — near a phase boundary, the actual phase at your local time of day could read one segment earlier or later than shown here.