Moon Phase Calculator
Phase + illuminated fraction + age of moon for any date. Synodic-cycle approximation from a known new moon reference (Jan 6, 2000).
Result
- Date__TODAY__
- Phase🌓 First Quarter
- Illumination41.31%
- Age (days since new moon)6.561
- Phase fraction (0 = new, 0.5 = full)0.2222
- Synodic period29.530589 days
- — Upcoming phases —
- Next New Moon2026-06-15
- Next First Quarter2026-05-24
- Next Full Moon2026-05-31
- Next Last Quarter2026-06-07
Step-by-step
- Reference new moon: 2000-01-06 18:14 UTC (Meeus 1998).
- Synodic month = 29.530589 days.
- Days since reference = 9,633.53; cycles = 326.2222.
- Phase fraction = cycles − floor(cycles) = 0.2222.
- Illumination = (1 − cos(2π · 0.2222)) / 2 = 0.4131 = 41.31%.
How to use this calculator
- Pick any date — past or future.
- Read the phase name + emoji + illumination %.
- Use the "upcoming phases" section to plan around full / new moons for tides, astronomy, or photography.
About this calculator
A synodic-cycle approximation good to ±6 hours over decades — fine for everyday "what phase is the moon tonight?" planning. The synodic month (29.530589 days, Meeus 1998) is the time between successive new moons as observed from Earth. It differs from the sidereal month (27.322 days, time to return to the same star) because the Earth-Moon system is also orbiting the Sun. The illumination model uses the simplified cosine fraction: (1 − cos(2π · phase)) / 2, which is accurate to within ~3% of the precise terminator integration (sufficient for naked-eye observation). For precise full-moon timing to the minute (e.g. for harvest festivals or astrophotography), use a dedicated ephemeris like JPL HORIZONS or the U.S. Naval Observatory Astronomical Almanac.