Moon Phase Calculator
Find the moon phase and approximate illumination percentage for any date — runs 100% in your browser with no upload.
🌖Waning Gibbous
🔒 Calculated in your browser — nothing is uploaded. Phase is approximate.
What this tool does
Pick any date and the calculator shows which of the eight moon phases the moon is in — new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, or waning crescent — along with the approximate percentage of the disc that is lit. It is handy for planning stargazing and astrophotography, checking whether a camping trip lands near a full moon, or simply finding out what the moon looked like on your birthday. The phase name, matching moon emoji, and illumination update live as you change the date.
How the phase is calculated
The moon completes one cycle of phases in an average synodic month of about 29.53 days. The tool counts the days since a known reference new moon on 6 January 2000, takes that count modulo the synodic month to get the moon's age, and maps the age onto the eight named phases. Illumination is estimated from the age with a cosine curve that reads 0% at new moon and 100% at full moon. Because the real lunar orbit is slightly irregular, these figures are close approximations rather than exact ephemeris values, but they are accurate enough to tell the phase and roughly how bright the moon will be.
Frequently asked questions
How accurate is the illumination percentage?
It is an approximation based on the mean length of the lunar month, so it can be off by a small amount and by up to about a day near the exact phase boundaries. It is reliable for telling the phase and gauging how bright the moon is, but it is not a substitute for a precise astronomical ephemeris.
Which time of day does the result reflect?
The calculation uses local noon of the date you pick, which is representative of that whole day. The moon's phase changes gradually, so the named phase is the same for the entire day and the illumination shifts only slightly from morning to night.
Is my date sent to a server?
No. The date you choose never leaves your device. The phase and illumination are computed entirely in your browser with JavaScript — nothing is uploaded, logged, or stored.