Moon phase calculator
Estimate the moon phase (new moon, first quarter, full moon…) for any date, using a simplified astronomical calculation based on the lunar synodic cycle. The calculation runs locally in your browser.
How it works
The moon takes about 29.53 days to complete a full cycle of phases (the synodic cycle) between two successive new moons. This calculator starts from a precisely known reference new moon (January 6, 2000), counts the days elapsed up to the chosen date, then works out the position in the current cycle and the corresponding phase.
Is this result astronomically exact?
No, it is an approximation based on an average length of the synodic cycle: the actual length of a lunar cycle varies slightly (by a few hours) from one month to the next because of the moon's elliptical orbit. For general use (planning a stargazing evening, anticipating a rough full moon date), this accuracy is more than enough, but for scientific or navigation purposes you should use official ephemerides.
Does the result depend on the time zone?
The calculation is done at noon UTC on the chosen date, which limits time zone differences to a few hours at most — well below the margin of error already introduced by approximating the mean synodic cycle.
No data is sent to a server: the astronomical calculation runs entirely in your browser.