Lunar Phase Calculator
Calculate the Moon's current phase and illumination percentage for any calendar date. Useful for photographers, gardeners, and astronomers planning around the lunar cycle.
Last updated: September 2026
Formula below · 2 sources (nasa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The Moon completes one synodic cycle — new moon to new moon — in 29.530589 days on average. The calculator counts the days from a known new moon (6 January 2000, 18:14 UTC) to noon UTC on your date, takes the remainder of that count divided by 29.530589 to get the Moon's age in days, and converts the age to the illuminated fraction of the disk: illumination = (1 − cos(2π × age / 29.53)) / 2. New moon is 0%, the first and last quarters are 50%, and full moon is 100%. Because the real Moon's orbit is elliptical, actual phase times can differ from this mean-cycle model by up to about 14 hours, which changes the illumination by at most a few percent.
How to use
Let's calculate the illumination for March 15, 2024 (year = 2024, month = March, day = 15). Step 1 — days from the reference new moon (6 Jan 2000 18:14 UTC) to 15 Mar 2024 12:00 UTC: 8,834.7 days. Step 2 — 8,834.7 mod 29.5306 ≈ 5.1 days, so the Moon is about 5 days old (a waxing crescent; the actual new moon was on 10 March). Step 3 — illumination = (1 − cos(2π × 5.1 / 29.53)) / 2 ≈ 0.27, so the calculator shows 27%.
Frequently asked questions
How accurate is this lunar phase calculator for planning observations?
This calculator uses the mean synodic month rather than a full ephemeris, so phase timing can be off by up to about half a day because the Moon's orbit is elliptical. For casual planning — choosing a dark-sky weekend for stargazing or scheduling a photography session — it is perfectly adequate. For precise lunar rise/set times or exact illumination percentages needed for scientific work, a dedicated ephemeris tool such as JPL Horizons is recommended.
What is the difference between waxing and waning moon phases?
The lunar cycle begins at new moon (0% lit), when the Moon is between Earth and Sun and its lit face points away from us. During the waxing half (about 14.8 days), the illuminated portion grows through crescent, first quarter (50%) and gibbous to full moon (100%). During the waning half, illumination decreases through gibbous, last quarter (50%) and crescent back to new moon. The calculator's percentage is the same for a waxing and a waning Moon of the same illumination. Waxing moons are visible in the evening sky, while waning moons rise progressively later and are better seen in the pre-dawn hours.
Why does the Moon's phase cycle last 29.53 days instead of exactly 28?
The 29.53-day synodic month is the time for the Moon to return to the same phase as seen from Earth — say, full moon to full moon. This differs from the Moon's sidereal orbital period of 27.32 days because Earth is also orbiting the Sun. By the time the Moon completes one orbit relative to the stars, Earth has moved roughly 1/13 of the way around the Sun, so the Moon must travel an extra ~2.2 days to 'catch up' and realign with the Sun as seen from Earth. This extra time accounts for the difference between the sidereal and synodic months.