Moon Phase Calculator

We measure the time from a known reference new moon (January 6, 2000, 18:14 UTC) to your selected date, then take the remainder when divided by the synodic month (29.530588853 days). That remainder is the moon's age in days — 0 at new moon, ~14.77 at full.

Illumination is derived from the phase angle: lit fraction = (1 − cos(2π × phase)) ÷ 2. At new moon this is 0%, at first or last quarter it is 50%, and at full moon it is 100%.

The eight named phases are buckets along the cycle: roughly 1 day on either side of a primary phase (new, first quarter, full, last quarter) and ~7 days for each crescent or gibbous in between.

New moon — 0% lit. The moon is between Earth and the sun, invisible from Earth's night side.

Waxing crescent — 1% to ~49% lit, growing. Visible in the western sky after sunset.

First quarter — exactly half lit (right side, Northern Hemisphere). Rises around noon, sets around midnight.

Waxing gibbous — more than half lit, less than full. Best for evening viewing.

Full moon — 100% lit. Rises at sunset, sets at sunrise.

Waning gibbous — more than half lit and shrinking. Visible late evening into morning.

Last quarter — half lit (left side, Northern Hemisphere). Rises around midnight, sets around noon.

Waning crescent — small sliver, shrinking. Visible in the eastern sky before sunrise.

Blue moon — the second full moon in a single calendar month. Happens about every 2.5 to 3 years.

Supermoon — a full moon at or near perigee (closest approach). Appears about 7% larger and 15% brighter.

Micromoon — a full moon at apogee (farthest point). Looks about 14% smaller than a supermoon.

Harvest moon — the full moon nearest the September equinox. Rises unusually close to sunset for several nights in a row.

Hunter's moon — the first full moon after the harvest moon, usually in October.

Wolf, snow, worm, pink, flower, strawberry, buck, sturgeon, corn, beaver, cold — the traditional month-by-month names for the year's full moons.