Everyday & Conversions
Moon Phase Calculator
Estimate the Moon's age in days and its illuminated fraction for any date using the mean synodic month.
Your inputs
Change any value to update the result instantly.
Illuminated fraction
99.7 %
Phase
Full moon
Moon age
15.27 days
Days to the next new moon
14.26 days
Mean synodic month
29.5306 days
Mean-phase approximation: the Moon's orbit is elliptical, so the true phase can lead or lag this estimate by up to roughly half a day.
What this calculator does
Estimates the Moon's age and illuminated fraction using the mean synodic month.
What your result means
How many days have passed since the last new moon and roughly what fraction of the disc is lit.
Formula and working
Moon age = (Julian Date − reference new moon) mod 29.530588853 days; illumination = (1 − cos(2π × age ÷ synodic month)) ÷ 2
- 1
Collect the inputs
Date (UT) = 2026-06-01; Universal Time = 0 h UT
- 2
Apply the formula
Moon age = (Julian Date − reference new moon) mod 29.530588853 days; illumination = (1 − cos(2π × age ÷ synodic month)) ÷ 2
- 3
Result
Illuminated fraction = 99.7 %
Frequently asked questions
Symbols and assumptions
- a
- Moon age in days since the new moon
- θ
- Phase angle
- • Mean synodic month of 29.530588853 days measured from a reference new moon.
- • Illumination is modelled as (1 − cos θ) / 2 for phase angle θ.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Mean-phase approximation: the elliptical lunar orbit can shift the true phase by up to roughly half a day.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
Report a mistake or request an improvement
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