Moon Phase Calculator
Approximate moon age, fraction and phase for any date.
Overview
The Moon Phase Calculator reports the approximate age of the moon (days since the last new moon), the illuminated fraction (0% at new, 100% at full), and the named phase — new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent — for any date you pick.
Useful for astrophotographers planning deep-sky shoots when the moon is dim, gardeners following lunar planting guides, anglers and hunters consulting solunar tables, ocean kayakers checking spring versus neap tide timing, and writers researching the night sky for a particular scene's date.
How it works
The synodic month — the time between successive new moons as seen from Earth — averages 29.530589 days. The calculator counts days since a known reference new moon (2000-01-06 18:14 UTC, JD 2451550.260) and takes the remainder modulo that synodic period to find the current age. Illuminated fraction is then 0.5 × (1 − cos(2π × age / 29.530589)).
Because the moon's orbit is elliptical and inclined, real new moon timing wobbles by up to about half a day either side of the average. The result here is therefore approximate — accurate to within a few hours, plenty for amateur observation but not for predicting the exact instant of new moon for an eclipse calculation.
Examples
2026-05-18 → age 1.5 days, 7% illuminated, waxing crescent
2026-06-01 → age 14.3 days, 99% illuminated, full moon
2026-12-25 → age 7.4 days, 50% illuminated, first quarter
2000-01-06 → age 0 days, 0% illuminated, new moon (reference epoch)
FAQ
What is the difference between sidereal and synodic month?
Sidereal (~27.32 days) is the time for the moon to return to the same position against the stars. Synodic (~29.53 days) is the time between successive new moons as seen from Earth. The synodic period is what matters for phase, because phase depends on the Sun-Earth-Moon angle.
Why are the times approximate?
The moon's orbit is elliptical and the calculation here uses a constant-rate model. Real new moon instants can sit half a day either side of the average. For exact phase timing, use a full ephemeris like JPL's HORIZONS.
What is a blue moon?
Two definitions exist: the third full moon in a season of four full moons (the older astronomical meaning), or the second full moon in a calendar month (the modern folk meaning). Either is rare.
Does the phase depend on hemisphere?
The phase is the same everywhere on Earth, but the orientation flips — a waxing crescent appears as a "C" from the southern hemisphere and a backwards "C" from the northern hemisphere.
What is the maximum age?
A full synodic cycle is 29.53 days, so ages run from 0.00 to about 29.53 before wrapping back to 0.