Moon Phase Calculator

See the Moon's phase for any date, drawn to scale — from new to full and back, with the percentage lit. Computed with the standard synodic-month method in your browser.

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Waning Gibbous
83% litMoon age · 18.8 days

The Moon cycles from new to full and back roughly every 29.5 days (the synodic month). We count the days from a known new moon — 6 January 2000 — and see how far through the current cycle your date lands, which gives both the named phase and the rough percentage lit. A waxing moon is filling toward full; a waning moon is emptying toward new. This is the standard almanac approximation, accurate to about a day; it doesn't fine-tune for the Moon's slightly uneven orbit. Everything is computed in your browser.

How to check a moon phase

Choose a date. We work out how far through the lunar cycle it falls, name the phase, draw the Moon with the correct portion lit, and show the approximate percentage illuminated and the Moon's age in days.

Questions

How is the moon phase calculated?
We count the days from a known new moon (6 January 2000) and see how far through the 29.5-day lunar cycle your date falls. That fraction gives the named phase and the approximate percentage of the Moon that's lit.
How accurate is it?
It's the standard almanac-grade approximation, good to roughly a day. It uses the mean length of the lunar cycle and doesn't fine-tune for the Moon's slightly uneven orbit, so the exact moment of a full or new moon can be off by up to about a day.
What's the difference between waxing and waning?
A waxing moon is growing brighter on its way from new to full; a waning moon is fading from full back to new. In the northern hemisphere a waxing moon is lit on the right side and a waning moon on the left.