Moon Phase Calculator

This tool uses a lightweight approximation (synodic month = 29.530588853 days) and a standard epoch. It’s generally within a few hours for phase timing — plenty for planning casual observation, photography, or outreach.

Lunar phases are continuous. The value depends on the exact time of day. If you change your system time or test different times, the number shifts smoothly.

Waxing means the illuminated portion is increasing after New Moon. Waning means it’s decreasing after Full Moon.

A common name for a total lunar eclipse. During totality, sunlight passing through Earth’s atmosphere is reddened and refracted into the shadow, giving the Moon a coppery hue (Rayleigh scattering).

The full Moon immediately after the Harvest Moon (the full Moon nearest the September equinox). It often occurs in October and historically provided bright evenings helpful for hunting.

A full Moon that occurs near perigee (its closest point to Earth). It appears slightly larger and brighter to the eye (up to ~14% larger and ~30% brighter compared with a distant apogee full Moon).

It’s an idiom for something that happens very rarely. “Blue Moon” has two accepted meanings: (1) the second full Moon in a single calendar month (modern usage); or (2) the third full Moon in a season containing four full Moons (older almanac usage). Very occasionally the Moon can look bluish after large wildfires or volcanic eruptions due to airborne particles.

Gibbous comes from the Latin gibbosus, meaning “hump-backed.” In astronomy it describes a Moon that is more than half but not completely illuminated. A waxing gibbous Moon occurs between the first quarter and full Moon, while a waning gibbous appears after the full Moon as the illuminated portion decreases.

The Moon’s gravity pulls on Earth’s oceans, causing the water to bulge slightly toward it, creating a high tide on the side facing the Moon and another on the opposite side due to inertia. As Earth rotates, most coastal areas experience two high and two low tides roughly every 24 hours and 50 minutes. When the Sun, Moon, and Earth align (during new and full Moons), their combined gravity produces spring tides with greater range; when at right angles (first and last quarters), they create neap tides with smaller range.