Pick any date to see the moon’s phase, how far through the 29.5-day cycle it is, approximate illumination, and the next full moon.
How the phase is worked out
The moon repeats its phases every 29.53 days — the synodic month. Count the days from any known new moon, take the remainder of that cycle, and the phase falls out: 0 is new, halfway is full, with crescents, quarters and gibbous phases between. This calculator counts from the well-recorded new moon of 6 January 2000 and names the phase for any date you pick — past or future — with the approximate illumination and the next full moon.
How to use it
- Pick a date (it defaults to today).
- Read the phase name and how many days into the cycle it is.
- Check the illumination and the next full moon date.
The method
A worked example
For 13 August 2026 the calculator reads New Moon — just 1.2 days into the cycle, about 2% illuminated — with the next full moon around 27 August 2026. New-moon nights are the ones stargazers plan around: no moonlight means the darkest skies of the month for meteor showers and the Milky Way.