Moon Phase Calculator

From the moon's age — days elapsed since the most recent new moon — measured against the 29.53-day synodic month (new moon to new moon), anchored to a reference new moon and computed forward or backward to your date. This method lands within several hours of observatory tables, which is more than enough to know what tonight looks like; to-the-minute precision requires modeling the moon's elliptical orbit, which shifts exact phase moments by up to ±15 hours around the average.

Waxing is growing toward full, waning is shrinking toward new — and you can read direction from which side is lit: in the Northern Hemisphere, light on the right means waxing, light on the left means waning (flipped in the Southern Hemisphere, since you're viewing it upside-down relative to a northern observer). Crescent means less than half lit, gibbous more than half. The quarters confuse everyone: a 'first quarter' moon is half lit — it's a quarter of the way through the cycle.

Opposite answers. For deep-sky stargazing (Milky Way, meteor showers, faint objects), you want the new moon and the few nights around it — moonlight is light pollution from the sky itself. For watching the moon, the counterintuitive pro tip: full moon is the worst time, because head-on sunlight flattens every crater; the quarter and gibbous phases throw long shadows along the terminator line where detail pops, even in binoculars.

The evidence is thinner than the folklore. A few controlled studies have found small sleep effects — several minutes less sleep and slightly delayed bedtimes in the nights before full moon, plausibly from evening brightness — but the dramatic claims (ER admissions, crime, births) fail consistently when studied at scale; they persist through confirmation bias, because a wild night at full moon is memorable and one at waning gibbous isn't. The moon does verifiably drive one thing: tides.