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.
The eight phases in detail
The lunar cycle divides into eight named phases, each covering roughly one-eighth of the 29.53-day synodic month. Here is how they progress:
| Phase | Age range (approx) | Description |
|---|---|---|
| New Moon | 0–1.5 days | Moon is between Earth and Sun; invisible at night |
| Waxing Crescent | 1.5–7.4 days | A thin crescent grows in the western evening sky |
| First Quarter | 7–8 days | Right half illuminated; rises at noon, sets at midnight |
| Waxing Gibbous | 8–14.8 days | More than half lit, growing toward full |
| Full Moon | 14–15 days | Fully illuminated; rises at sunset, sets at sunrise |
| Waning Gibbous | 15–22 days | Still more than half lit, shrinking from full |
| Last Quarter | 22–23 days | Left half illuminated; rises at midnight, sets at noon |
| Waning Crescent | 23–29.5 days | Thin crescent visible in the pre-dawn eastern sky |
Why the Moon appears to change shape
The Moon does not change shape — the same sphere orbits Earth throughout. What changes is the angle from which we see its sunlit half. As the Moon moves around Earth, the geometry shifts: at new moon, the lit hemisphere faces away from us entirely; at full moon, it faces toward us completely. The crescent and gibbous phases are the in-between views.
Practical uses for moon phase information
Knowing the current or upcoming moon phase is useful in more situations than you might expect:
- Night photography and astronomy: A full or gibbous moon illuminates the sky enough to wash out faint stars and deep-sky objects. Astrophotographers and star-gazers often plan sessions around new moon when the sky is darkest.
- Fishing and wildlife: Many anglers and hunters track moon phases, as feeding and movement patterns in some species are correlated with lunar cycles — particularly solunar tables around new and full moons.
- Agriculture and gardening: Some traditional and biodynamic farming systems use moon phases to time planting, pruning, and harvesting. Planting root crops during waning phases or leafy crops during waxing phases are examples of these traditions.
- Tidal planning: Tides are strongest (spring tides) at new and full moon, when the Moon and Sun align gravitationally. The moon phase is a quick indicator of tidal range for coastal planning.
- Event planning: Outdoor events, ceremonies, or activities that benefit from moonlight can be scheduled around the full moon.
Accuracy and limitations
This calculator uses the mean synodic month (29.530589 days) anchored to a well-known reference new moon. Because the Moon’s orbit is elliptical, actual new and full moons can fall up to about a day either side of the mean — this is why you may occasionally notice a one-day discrepancy with a precise astronomical almanac. For planning purposes the calculation is more than adequate; for exact phase times in seconds, consult a dedicated ephemeris.