Moon Phase on a Date

Pick any date, past or future, and see the moon's phase that night, how far through the cycle it was, and the nearest full and new moons.

How it works

The moon's phases repeat every 29.530588 days on average, the synodic month. If you know the exact moment of one new moon, you can count forward or back any number of days, divide by the cycle length, and the remainder tells you where in the cycle any other date falls. This calculator uses the new moon of 6 January 2000 at 18:14 UTC as its anchor, a conventional reference used by many almanacs and astronomy libraries.

The eight named phases are divided evenly: 0 to 1.85 days is new, then waxing crescent to about 7.4 days, first quarter around 7.4, waxing gibbous to about 14.8, full moon around 14.8, waning gibbous, last quarter around 22.1, waning crescent to 29.5. The calculator also gives illumination, the percentage of the visible disc that is lit, which follows a cosine curve from 0% at new to 100% at full.

Accuracy and limits

Because the moon's orbit is elliptical and perturbed by the sun, individual months run anywhere from about 29.2 to 29.8 days. Using the average means this calculator can be off by up to roughly half a day, which matters only if your date sits right at a boundary (say, the day a full moon technically peaks at 11pm). For an exact instant, the US Naval Observatory's phase tables are the standard reference. For "what phase was the moon on my birthday," this is more than enough.

Dates are treated as local midnight in your browser's time zone. For a date in the early morning the previous evening's phase may be what you remember seeing; the result shows days-since-new-moon so you can judge.

Northern and southern hemispheres

The phase is identical everywhere on Earth at a given moment. What flips is the orientation: in the northern hemisphere a waxing moon is lit on the right, in the southern on the left. Near the equator it looks like a bowl or an umbrella. The hemisphere selector changes the little icon accordingly; it does not change the phase.

Why people look this up

Birthdays are the big one: knowing you were born under a full moon or a sliver of waning crescent is a small, free piece of personal lore. Gardeners planting by the moon, anglers, photographers planning a shoot, people checking whether a half-remembered night really was a full moon, and anyone curious what the sky looked like on a historical date all use the same arithmetic. The calculator does not attach meanings to phases; if you want those, the moon-phase traditions in folklore and astrology are extensive and easy to find.

Reference new moon: 2000-01-06 18:14 UTC. Synodic month: 29.530588 days (Meeus, Astronomical Algorithms, 2nd ed., 1998).