What this calculator does
This online tool estimates the moon's phase — new, crescent, quarter, gibbous, or full — and its approximate illumination percentage for any date, using a known reference new moon and the average length of the lunar cycle.
Worked example
Enter today's date or any date of interest above to see its estimated phase and illumination — the underlying cycle repeats roughly every 29.5 days, so any date within that window relative to a known new moon can be estimated this way.
Common mistakes
- Expecting exact-to-the-minute precision. This is an approximation based on average cycle length — official astronomical almanacs account for small orbital variations this calculator doesn't model.
- Confusing "new moon" with an invisible moon forever. A new moon is temporary — the moon becomes visible again as a crescent within a day or two.
- Assuming the same phase is visible everywhere at once. While the phase itself is roughly global, local visibility (whether the moon is above the horizon) still depends on your location and the time of day.