🌙 Moon Phase Calculator
Enter any date to see the approximate moon phase, illumination percentage, and where that date falls in the 29.53-day lunar cycle.
How the phase is worked out
The tool counts days from one known new moon and wraps that count into a single average lunar month:
age = (your date at 12:00 UTC − 6 Jan 2000, 18:14 UTC) mod 29.53058867 days
illumination = (1 − cos(2π × age ÷ 29.53058867)) ÷ 2An age of 0 days is new and unlit, 7.38 days is half lit, 14.77 days is full at 100%, and 22.15 days is half again and shrinking. The eight names each own one eighth of the cycle — 3.69 days — centred on the exact moment, so a date 5.7 days in is called First Quarter while the readout still shows about 32% lit.
What it assumes about place and time
- No location, and none needed. The lit fraction is identical everywhere on Earth. Latitude changes only the tilt: a crescent hanging right in London looks mirrored from Sydney.
- Your date is read as 12:00 UTC. Timezone is ignored, so an evening in Los Angeles falls some 15 hours after the moment used, and near a boundary the name can sit one step either side of what you see.
- The month is a mean. Real lunations run from about 29.27 to 29.83 days, so a fixed 29.53 drifts.
Frequently asked questions
How dark will the night of a given date be?
Read the illumination figure. Under about 20% the moon is a thin crescent that sets early and barely interferes; over 70% it washes out faint galaxies and nebulae for the whole night.
Why does it say First Quarter when the moon is only a third lit?
Each of the eight names covers 3.69 days of the cycle. The exact first quarter, 50% lit, sits in the middle of that band at 7.38 days, so dates early in the band show less and dates late in it show more.
Is the moon phase the same in every country?
Yes. The same face is lit for everyone at any given instant, so the percentage does not change with your longitude — only the local date and the orientation of the crescent do.