Moon Phase & Illumination

Moon illumination (or "illuminated fraction") measures how much of the Moon's visible disk is lit by the Sun as seen from Earth. At New Moon it's near 0%, at Full Moon it's ~99.9%. The percentage changes daily as the Moon orbits Earth, cycling fully every ~29.53 days (a synodic month).

Our calculator uses the Meeus astronomical algorithm with a synodic month of 29.53058867 days, referenced to the precise New Moon of January 6, 2000 (JD 2451550.1). The illumination percentage is typically accurate within ±0.3%. Moonrise/set times are approximate and may vary by 10–30 minutes depending on your exact location and atmospheric conditions.

The 8 primary phases are: New Moon (0%), Waxing Crescent (~0–25%), First Quarter (~25%), Waxing Gibbous (~25–50%), Full Moon (~50% illumination / 100% visible disk), Waning Gibbous (~50–25% illuminated portion), Last Quarter (~25%), and Waning Crescent (~25–0%). Note: "Waxing" means growing brighter, "Waning" means growing dimmer.

As the Moon orbits Earth eastward (~13° per day), Earth must rotate an extra ~50 minutes each day for the Moon to reappear at the same position in the sky. This is why moonrise shifts later daily. The exact delay varies between ~30 and ~70 minutes depending on the season and your latitude.

Moon age is the number of days elapsed since the last New Moon. It ranges from 0 (at New Moon) to about 29.5 days (just before the next New Moon). A moon age of ~7.4 days corresponds to First Quarter, ~14.8 days to Full Moon, and ~22.1 days to Last Quarter.

Yes! The Moon is visible during daylight for about 2 weeks each month — specifically during the waxing and waning phases when it's not too close to the Sun. The best times are late afternoon for a Waxing Moon and mid-morning for a Waning Moon. Only near New Moon is it invisible due to the Sun's glare.

Spring tides (highest high tides, lowest low tides) occur during Full Moon and New Moon when the Sun, Earth, and Moon align. Neap tides (smaller tidal range) occur during First and Last Quarter when the Sun and Moon are at right angles relative to Earth. The Moon's gravitational pull is the primary driver of ocean tides.