Moon Phase Calculator
Pick any date and see which moon phase it falls on, how bright the moon is that night, and an emoji that matches — from a slim crescent to a full moon.
How it works
The moon repeats its cycle of phases roughly every 29.53 days — the synodic month. Starting from a known new moon in early 2000, the calculator counts how many days have passed and finds where your date lands within that cycle.
That position, the moon's age in days, decides the phase: near zero is a new moon, around fourteen and a half days is full, and the points in between are the crescents, quarters, and gibbous phases.
The illumination percentage comes from a cosine of the phase angle, which climbs smoothly from 0% at new moon to 100% at full and back down. It's an approximation, not an observatory reading, but it's close enough to plan a stargazing night or a moonlit walk.
Frequently asked questions
How accurate is this?
It uses a steady 29.53-day cycle from a fixed reference new moon, which tracks the real phases within a day or so. The moon's orbit wobbles slightly, so treat it as a solid estimate rather than a precise astronomical almanac.
What does the illumination percentage mean?
It's the share of the moon's disc that's lit as seen from Earth. Zero percent is a new moon you can barely see, and 100% is a full moon. A first or last quarter sits near 50%.
Can I check a date in the past or future?
Yes. The cycle math works in both directions, so you can look up the phase for a birthday years ago or an eclipse-watching trip years ahead.