🌙 Moon Phase Calculator

The Moon orbits the Earth roughly every 29.53 days (a synodic month), and as it does, the portion illuminated by the Sun as seen from Earth changes shape. When the Moon is fully invisible it is called new moon, and when it appears fully round it is called full moon. In between, the phases are named waxing crescent, first quarter, waxing gibbous, waning gibbous, last quarter, and waning crescent.

Using a known reference new moon at 2000-01-06 18:14 UTC, the calculator finds the Julian Day for your selected date (at 12:00 UTC) and computes the remainder of the elapsed days divided by the synodic month length (29.530588853 days) — this is the "moon age." An age near 0 days means new moon, and near 14.77 days means full moon; the full cycle is split into 8 buckets to name the phase. Illumination is approximated with the formula (1 - cos(2π × age / synodic month)) / 2.

This calculator is a simple periodic approximation that does not account for actual orbital perturbations (small variations caused by gravitational interactions between the Sun, Earth, and Moon), so it can differ from precise observational ephemerides by up to a day (±1 day). Please use it as a general reference rather than for precision work.