Dates and Days

Moon phase

The Moon's phase on any date, and the exact times it changes

Moon phase —

The month at a glance

Phase times: the exact instants this month, in UTC
PhaseDateTime (UTC)
How this was counted
Phase— Illuminated— Moon age— Next full moon— Distance—

Checked on every build against 250 USNO phase instants and 105 JPL Horizons lunar positions. How it is checked

Pick a date to see the Moon's phase, how much of the disc is lit, the Moon's age and distance, and the next full moon. Each phase is computed from where the Moon and Sun are, so it is named on the day it happens. Every figure is worked out in your browser.

Why the 29.5-day shortcut can be a day out

The usual shortcut counts the days since a known new moon and divides by 29.530588. That figure is the mean synodic month. The real interval between new moons varies from about 29.27 to 29.83 days, because the Moon's orbit is elliptical: it runs fast near perigee and slow near apogee. The errors build up within a lunation and can reach around fourteen hours, which is enough to name the wrong calendar day for a full moon several times a year. This shortcut is one of three reasons two calendars can disagree about a date, and the only one of the three that is an error.

This page computes where the Moon and the Sun are, using the standard series from Meeus's Astronomical Algorithms, and finds the instants at which they reach the relevant separation. It has been checked against 250 phase instants published by the US Naval Observatory for 1900, 2006, 2026, 2048 and 2100: through 2048 every time matches to within a minute, and none of the 250 lands on the wrong day. The check against the Naval Observatory table is described in full on the method page.

What "waxing" and "waning" mean here

Waxing is the half of the cycle in which the lit fraction grows, from new through first quarter to full. Waning is the half in which it shrinks, from full through last quarter back to new. The disc above is drawn from the computed fraction, so the picture always matches the percentage beside it.

People also keep calendars by the sign the Moon is standing in, a cycle that completes in 27.3 days where the phase cycle takes 29.5. The Sun's four turning points, both equinoxes and both solstices, have their own page, timed to the exact instant each season turns, along with the harvest moon: whichever full moon falls nearest the September equinox.

Each named phase is an instant

New, first quarter, full and last quarter are moments: the Moon reaches each one at a specific second. This page gives a day one of those names only when that instant falls inside it, in UTC. Every other day gets a crescent or gibbous name. Calendars that assign the four names by an illumination band call two or three days in a row "full moon", although the full moon instant falls on only one of them.

Questions

Why does this disagree with a moon phase app by a day sometimes?
Because many phase apps divide by 29.530588 and read the remainder. That is the mean length of a lunar month, and the real length varies: the Moon's orbit is an ellipse, so successive new moons run anywhere from about 29.27 to 29.83 days apart. The remainder method drifts up to roughly fourteen hours from the true time, enough to print a full moon on the wrong day, and it gives no sign when it has. This page computes each phase from the Moon's position. Against the US Naval Observatory's published table, the times agree to within a minute in every year tested up to 2048, and every phase lands on the right day in every year tested from 1900 to 2101.
What does the percentage measure?
The fraction of the Moon's visible disc that is lit. That is a different number from how far through the cycle the Moon is: the two are linked by a cosine, so the lit fraction changes fastest around the quarters, when the Moon is about 50 percent lit, and barely at all for the two or three days either side of full. That is why the full moon looks full for most of a week.
Why is a full moon not exactly 100 percent lit?
The Moon's orbit is tilted about five degrees to the Earth's path around the Sun, so at most full moons the Moon sits slightly above or below the line through the Earth and Sun, and a sliver of the disc stays unlit. A full moon reaches a true 100 percent only when the alignment is exact, and then it is a lunar eclipse: the Earth's shadow takes the light away again.
Which timezone are the phase times in?
UTC. A phase is a single instant that happens everywhere at once, so it has one time, and your location changes only the local date it falls on. The times here are labelled UTC and are not converted; a page that converts to local time without saying so can show a full moon on the 12th to one reader and on the 13th to another.
How far into the future are the times reliable?
The phase dates stay right for centuries. The clock times are good to the minute through about 2050 and drift after that, because they depend on how fast the Earth will be turning, which can be measured but not predicted exactly. Reasonable sources already disagree by a minute or two about the time of a full moon in 2100. This page uses the standard long-term projection.