The moon's phase for any date
The moon runs through its phases in a cycle of about 29.53 days — the synodic month — from new moon (fully dark) through waxing crescent, first quarter and waxing gibbous to full moon, then back down through waning gibbous, last quarter and waning crescent. This calculator finds where any date sits in that cycle and reports the phase name, the moon's age in days, the percentage of its face that is lit, a picture of the phase, and the dates of the next new moon, quarters and full moon.
How the phase is calculated
Everything hangs on the age: count days from a precisely known new moon (this calculator uses January 6, 2000, 18:14 UTC) and take the remainder modulo the average cycle length. The illumination formula follows from the geometry — the lit fraction of the visible disc grows with the sun–moon angle, reaching 50% at the quarters (age ≈ 7.4 and 22.1 days) and 100% at full moon (age ≈ 14.8). Because the moon's real orbit is elliptical, its true speed varies, so computed phase dates can be off by several hours, occasionally up to about a day — the honest limit of any average-cycle calculator.
Phases and what they mean
"Waxing" means the lit part is growing (the lit edge is on the right in the Northern Hemisphere — the illustration here uses that view), "waning" means it is shrinking. Gibbous means more than half lit; crescent means less than half. Gardeners, photographers, anglers and anyone planning around night light use the phase to predict how bright the night will be: around full moon the sky is lit all night, while the nights near new moon are darkest — pair the phase with the Sunrise and Sunset Calculator to plan a full day and night outdoors.
Worked example
On October 12, 2026 the moon is about 1.3 days into its cycle — only just past new, with under 2% of its face lit; this calculator puts the following full moon on October 25, with the true moment (per almanacs) falling within about a day of that.
How to use this calculator
- Pick any date — tonight, a birthday, a planned trip.
- Read the phase name in the green panel and see it drawn next to the illumination percentage.
- Check the stats for the moon's age and the next new and full moons.
- See the table below for all four upcoming quarter phases in order.
Why “once in a blue moon” is rare
A calendar month of 30 or 31 days is longer than the 29.53-day cycle, so most months contain exactly one full moon — but a month can occasionally hold two (when a full moon falls in the first day or two). That second full moon is the popular "blue moon": it happens about once every two to three years. This calculator will show it directly — pick the first of such a month and both full moons appear in the upcoming-phase table across two lookups.