This tool finds the moon phase for any date — New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter or Waning Crescent — along with the Moon’s age in days and the percentage of the disk that is illuminated. It is for stargazers, photographers and event planners.
How it works
The calculator converts your date to a Julian Date, then measures the days elapsed since a reference new moon (6 January 2000, 18:14 UTC). Dividing by the mean synodic month of 29.530589 days and taking the remainder gives the Moon’s age within the current cycle. The fraction through the cycle maps to one of the eight named phases, and the illuminated fraction is computed as (1 − cos(2π × fraction)) ÷ 2 — 0% at new moon, 100% at full.
Example
For a date about 22 days into the cycle, the fraction is 22 ÷ 29.53 ≈ 0.745, which falls in the Waning Gibbous range, with illumination roughly:
(1 − cos(2π × 0.745)) ÷ 2 ≈ 38%.
| Age (days) | Phase | Illumination |
|---|---|---|
| 0 | New Moon | 0% |
| 7.4 | First Quarter | 50% |
| 14.8 | Full Moon | 100% |
| 22.1 | Last Quarter | 50% |
Results are accurate to within about a day. The calculation runs entirely in your browser with no network requests.