How the lunar calendar works

The lunar calendar shows the Moon phase and the illuminated percentage of the lunar disc for each day of a period of up to 60 days, starting on the date you choose (from the year 1900 onwards). The result is a chart with phase icons and a day-by-day table. Use it to plan photography or stargazing nights, to see when the next full moon is due, to follow traditions tied to the Moon, or simply to understand its monthly cycle.

The calculation is based on the synodic month, the average time between two new moons: about 29.53 days (29.530588853). For each date, taken at noon UTC, the page counts the days elapsed since a reference new moon (January 6, 2000, 18:14 UTC), divides by the length of the month and keeps the fractional part, which is the position in the cycle. The illuminated fraction is (1 − cos(2π × position)) / 2, and the cycle is split into eight phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent.

Example: near a new moon the illumination is close to 0%, it reaches about 50% at the first quarter, roughly a week later, and nearly 100% at full moon, after about 14.8 days, before decreasing again. Limitations: the real lunar month varies by several hours because the Moon’s orbit is elliptical, so this mean-cycle model can place a phase change up to about a day away from the exact astronomical moment, and it ignores your location and time zone. Use it as a guide, not for precise astronomical or navigational purposes.