About Moon Phase Calculator
The Moon's phase is entirely a matter of geometry — how much of the half facing Earth also happens to be lit by the Sun, which changes predictably as the Moon orbits. A full cycle, new moon to new moon, is called a synodic month and averages about 29.53 days; this calculates a date's position in that cycle from a known reference new moon, the same basic approach any lunar calendar uses.
Illumination percentage is the more precise figure sitting behind the eight familiar phase names — new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent. Only new and full are true instants; everything else is a continuous, gradually changing illumination that the named phases carve into conventional ranges.
'Age' means days since the most recent new moon, which is also why the two 'days until' figures below it work the way they do: full moon sits at almost exactly half a synodic month after new moon, so the calculator can tell how far away it is without needing a separate lookup.
How to check a moon phase
Pick a date
Defaults to today.
Read the phase and illumination
The eight named phases, plus the precise percentage.
Check the next full or new moon
Both shown as days from the selected date.