Moon Phase Calculator — Today's Moon, Illumination, Full Moon Dates
Date & TimeA moon phase calculator computes the lunar phase (new, crescent, quarter, gibbous, full) and illumination percent for a selected date using the mean synodic month approximation.
What is it
The moon phase is the sunlit shape of the Moon as seen from Earth, repeating on average every 29.530589 days (the synodic month). This tool derives the moon age from the mean synodic month and a reference new moon (2000-01-06 18:14 UTC), as used in Jean Meeus' astronomical algorithms, then maps the age to a phase name and illumination percent and lists the new moon, first quarter, full moon, and last quarter dates of the selected month. Because it is a mean-value approximation, results can differ from actual phase times by up to about one day; for official times consult the U.S. Naval Observatory or your national almanac. All calculations run in your browser.
How to use
- 1Pick a date (defaults to today).
- 2Press calculate to see the phase name and illumination percent.
- 3Check the monthly table for new moon, quarter, and full moon dates plus the next full moon.
Reference
| Moon age (days) | Phase | Illumination |
|---|---|---|
| 0 | New moon | 0% |
| about 7.4 | First quarter | 50% |
| about 14.8 | Full moon | 100% |
| about 22.1 | Last quarter | 50% |
| 29.53 | Next new moon | 0% |
Sources & standards
- Phases of the Moon - U.S. Naval Observatory
- Six Millennium Catalog of Phases of the Moon (computed with Meeus algorithms) - NASA Goddard Space Flight Center
FAQ
How accurate is this moon phase calculator?
It uses the mean synodic month (29.530589 days), so results can differ from actual phase times by up to about one day. The Moon's elliptical orbit makes real lunations vary between roughly 29.27 and 29.83 days; use USNO data for official phase times.
How do I find the next full moon date?
Select a date and calculate: the tool lists the full moon of that month and the next full moon after the selected date. A full moon occurs when the moon age passes 14.765 days.
Is moon phase input sent to a server?
No. This tool runs in your browser and does not send the selected date to a server.