Moon Phase Calculator
The Moon's 29.5-Day Cycle
The Moon doesn't produce its own light — every phase we see is sunlight reflecting off whatever half of the Moon happens to be facing the Sun at that moment, combined with how much of that lit half faces Earth. A full cycle from new moon to new moon takes 29.53059 days on average, the synodic month, which is slightly longer than the Moon's actual 27.3-day orbit around Earth because Earth itself keeps moving around the Sun in the meantime. This calculator finds where in that cycle any given date falls.
The Formula
Phase = Moon Age divided into 8 equal bins of the synodic month
The calculator anchors its count to a known new moon (January 6, 2000) and counts forward or backward in whole synodic months to find the Moon's age — days elapsed since the most recent new moon — for any date.
Why Track Moon Phase
- Astrophotography and stargazing — a bright Moon washes out faint deep-sky objects, so observers deliberately plan sessions around the new moon window.
- Tides — the largest spring tides occur near new and full moon, when the Sun and Moon's gravitational pull align; tracking phase is a quick way to anticipate them.
- Cultural and religious calendars — numerous calendar systems and observances are timed to specific lunar phases.
- Wildlife and outdoor activity planning — hunting, fishing, and some research fieldwork schedules are influenced by moonlight levels tied to phase.
The Eight Phases
| Phase | Moon age (days) | Approx. illumination |
|---|---|---|
| New Moon | 0.00 – 3.69 | ~4% |
| Waxing Crescent | 3.69 – 7.38 | ~31% |
| First Quarter | 7.38 – 11.07 | ~69% |
| Waxing Gibbous | 11.07 – 14.77 | ~96% |
| Full Moon | 14.77 – 18.46 | ~96% |
| Waning Gibbous | 18.46 – 22.15 | ~69% |
| Last Quarter | 22.15 – 25.84 | ~31% |
| Waning Crescent | 25.84 – 29.53 | ~4% |
Illumination is modeled with a simple cosine curve over the synodic month, computed directly from the calculator's own formula. This uses the average synodic month, so results can be off by up to roughly a day compared to a precise lunar ephemeris.
How to Use This Calculator
- Enter the date you want the Moon phase for.
- Select Calculate to see the phase name and approximate percentage illumination.
Related Calculations
Planning a viewing session around the Moon's position too? Check the Sunrise Sunset Calculator for how much dark sky you'll have that night.