Moon Phase Calculator
Select a date to find the approximate moon phase, age, and illumination.
This gives an approximate phase based on the average lunar cycle length, not precise astronomical ephemeris data.
How moon phase is calculated
This calculator counts the days since a known reference new moon, then takes the remainder after dividing by the synodic month (about 29.53 days — the average time between one new moon and the next) to find how far into the current lunar cycle a given date falls.
The illumination percentage is approximated using the moon's age within its cycle, following a smooth curve that's 0% at new moon, rises to 100% at full moon (halfway through the cycle), and falls back to 0% by the next new moon.
Frequently asked questions
Why is this only approximate, not exact?
Real lunar motion varies slightly due to the moon's elliptical orbit and other gravitational influences, so the actual synodic month length fluctuates by a few hours around its average. This calculator uses that fixed average, which can be off by up to roughly a day from precise astronomical data.
What are the 8 standard moon phases?
New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent — these divide the roughly 29.5-day lunar cycle into 8 named stages based on how much of the moon is illuminated and whether illumination is increasing or decreasing.
What's the difference between "waxing" and "waning"?
Waxing means the illuminated portion of the moon is increasing (growing toward Full Moon), while waning means it's decreasing (shrinking toward New Moon) — the two halves of the lunar cycle.