Moon Phase Age Calculator
The Moon's age, the number of days since the last new moon, tells you where it is in its monthly cycle and which phase to expect in the sky. This calculator counts the time from a known reference new moon and folds it into the 29.53 day synodic month to give the Moon's age, the matching phase name, and an approximate illuminated fraction for any date you choose. It uses a uniform mean-cycle model, accurate to about a day, which is plenty for knowing whether tonight is near full, new, or a quarter.
Moon age formula
days since reference = (date - reference new moon) in days
age = days since reference mod 29.53059
phase angle = 360 * age / 29.53059 degrees
illuminated fraction = (1 - cos(phase angle)) / 2
The reference new moon used is 2000-01-06 18:14 UTC, a standard astronomical epoch. The synodic month is 29.53059 days. Phase names are assigned by age band.
Worked example
For a date about 15 days after a new moon, the age is roughly 14.77 days, the phase angle is near 180 degrees, and the illuminated fraction approaches 100%: a full moon. Around 7.4 days gives a first quarter at about 50% illuminated.
Moon phase age: frequently asked questions
What is the Moon's age?
The Moon's age is the number of days that have passed since the most recent new moon. It runs from 0 days at new moon up to about 29.53 days, the length of the synodic month, at which point the cycle repeats. Age 0 is new moon, about 7.4 days is first quarter, about 14.8 days is full moon, and about 22.1 days is last quarter.
How does the calculator find the Moon's age?
It counts the days from a known reference new moon and divides by the synodic month of 29.53059 days. The remainder is the Moon's age. This mean-cycle method is accurate to about a day; the true new moon can vary by up to around half a day because the Moon's orbit is not perfectly uniform, so treat the result as a close approximation.
Why is illuminated fraction approximate here?
The illuminated fraction is estimated from the phase angle using a simple cosine model, (1 minus cos(phase angle)) divided by 2. The Moon's elliptical orbit makes the true value differ slightly. For navigation or precise observation planning, use a full ephemeris; for everyday phase awareness, this estimate is close.
Sources
- Synodic month length and Moon phase definitions: NASA Moon Fact Sheet (synodic period 29.53 days).
- U.S. Naval Observatory phases of the Moon background: USNO Moon Phases.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.