Moon Phase Calculator
Phase of the Moon, its age and how much of the disc is lit on any date, with the days to the next full and new moon.
Results
Age of the Moon (days since new moon)
28.65
d
Share of the disc lit
1
%
Phase
New moon
Days to the next full moon
15.7
d
Days to the next new moon
0.9
d
The dashed line marks the value you entered.
What this tool does
The Moon goes through its phases every 29.53 days on average — the synodic month — so counting days from a known new moon gives the phase on any date. Because the Moon's orbit is elliptical, real new and full moons can come up to about half a day earlier or later than this average, so treat times within a day of a phase change as approximate. The lit share is what you see from Earth: 50 % at the quarters, which are named after the quarter of the orbit, not the quarter of the disc.
Formula
age = (JD − 2451550.26) mod 29.530589 days · lit = (1 − cos(2π × age ÷ 29.53)) ÷ 2
Variables
| Symbol | Meaning | Unit |
|---|---|---|
yr | Year | — |
mo | Month (1–12) | — |
dy | Day of the month | — |
hu | Hour (UTC) | h |
AG | Age of the Moon (days since new moon) | d |
IL | Share of the disc lit | % |
PH | Phase | — |
NF | Days to the next full moon | d |
NN | Days to the next new moon | d |
Worked example
- Year2026
- Month (1–12)10
- Day of the month10
- Hour (UTC)12 h
- Age of the Moon (days since new moon)28.65 d
- Share of the disc lit1 %
- PhaseNew moon
- Days to the next full moon15.7 d
- Days to the next new moon0.9 d
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Dates are handled in the proleptic Gregorian calendar, which is the one computers use. For dates before the Gregorian reform of 1582 this differs from what was written at the time.