Moon Phase Calculator

The moon cycles through eight commonly named phases about every 29.5 days — one synodic month from new moon back to new moon. Phase names describe how much of the near side is sunlit as seen from Earth: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. They describe lighting geometry, not where the moon sits in the sky at night.

Illumination is the percentage of the lunar disk that appears lit, from 0% at new moon to roughly 100% at full moon. Waxing phases grow brighter after new moon; waning phases dim after full moon. A first or last quarter moon shows near 50% illumination even though half the sphere is always lit — we see only the portion facing Earth.

Lunar age counts days since the last new moon in this model, from 0 days at new moon to about 29.5 days just before the next new moon. Age 7–8 days often aligns with first quarter; age 14–15 days aligns with full moon; age 22–23 days aligns with last quarter. Pair age with the named phase for gardening folklore or photography planning.

This tool uses astronomy-engine calculations aligned with almanac-style tables. Real skywatching can differ slightly from a calendar date because phase instants occur at specific times in Universal Time. A phase name can flip near local midnight while almanacs list the exact UT moment — compare with USNO data linked below for official times.

The moon is not visible at every phase from every location on every night. New moon sits near the sun in the sky; full moon rises around sunset. High latitudes see extreme seasonal shifts in moon path. Use phase and illumination as planning hints, then confirm rise times with a location-aware tool for critical outings.

Moon phase calendars appear in gardening guides, fishing folklore, and cultural holidays, but scientific outcomes tied to phase alone are mixed. Treat illumination and age as skywatching context — not weather forecasts or medical guidance. For navigation or eclipse planning, use professional ephemeris data. Waxing moons set later each night; waning moons rise later — visibility windows shift through the month even when clouds stay clear.

Worked example with default date August 7, 2026: the moon is in last quarter at about 36.6% illumination with a lunar age near 23.4 days — waning toward the next new moon. That means less than half the disk appears lit and brightness drops each night until the cycle resets.