Dates and Days

Moon phase

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

Computed from the Moon's actual position rather than a 29.5-day remainder, so the day it names is the day it happens. Every figure is worked out here in your browser.

Phase— Illuminated— Moon age— Next full moon— Distance—
Moon phase —

The month at a glance

Phase times — exact instants this month, in UTC
PhaseDateTime (UTC)

Why most moon phase pages can be a day out

The common way to compute a phase is to count the days since a known new moon and divide by 29.530588. That figure is the mean synodic month, and the real interval between new moons is never exactly it: the Moon's orbit is elliptical, so it runs fast near perigee and slow near apogee, and consecutive lunations vary from about 29.27 to 29.83 days. Errors from the mean do not cancel — they accumulate 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 page instead computes where the Moon and the Sun actually are, using the standard series from Meeus's Astronomical Algorithms, and finds the instants at which they reach the relevant separation. Two calendars can disagree about a date for three quite different reasons, only one of which is an error. Checked against 250 phase instants published by the US Naval Observatory across 1900, 2006, 2026, 2048 and 2100, it agrees to within a minute everywhere Earth's rotation has been measured, and it never lands on the wrong day. The check against the Naval Observatory table is described in full.

What "waxing" and "waning" mean here

Waxing is the half of the cycle where the lit fraction is growing, from new through first quarter to full; waning is the half where it shrinks, from full through last quarter back to new. The disc above is drawn from the computed fraction rather than chosen from a set of stock images, so the picture and the percentage beside it always agree. Phase is not the only lunar cycle people keep calendars for: the sign the Moon is standing in runs on a different clock entirely, completing in 27.3 days rather than 29.5.

A named phase is an instant, not a day

New, first quarter, full and last quarter are moments — the Moon reaches each one at a specific second. A day is called by one of those names on this page only when that instant falls inside it. Every other day gets a crescent or gibbous name, because that is what it actually is. Pages that assign the four names by an illumination band will call two or three days in a row "full moon", which is friendly and not true.

Questions

Why does this disagree with a moon phase app by a day sometimes?
Because most of them divide by 29.530588 and read the remainder. That is the MEAN length of a lunar month, and the real one is not constant — 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 truth, which is more than enough to print a full moon on the wrong day, and it gives no sign that it has. The phases here are computed from the Moon's actual position, and they agree with the US Naval Observatory's published table to within a minute across every year tested from 1900 to 2101.
What does the percentage actually measure?
The fraction of the Moon's visible disc that is lit, which is not the same as how far through the cycle it is. The relationship between the two is a cosine rather than a straight line, so a Moon halfway from new to full is not 50 percent lit — it is a little under. It also means the number moves fastest around the quarters and barely at all for the two or three days either side of full, which is why the full moon looks full for most of a week.
Why is a full moon not exactly 100 percent lit?
Because 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 joining the Earth and Sun rather than exactly on it. A sliver of the disc stays unlit. The only full moons that reach a true 100 percent are the ones where the alignment is exact — and those are lunar eclipses, where the Earth's shadow then takes the light away again.
Which timezone are the phase times in?
UTC, and they are labelled that way rather than silently converted. A phase is a single instant that happens everywhere at once, so it has one true time; what changes with your location is only which local date that instant falls on. Converting silently is how a page ends up showing a full moon on the 12th to one reader and the 13th to another with no explanation of why.
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 is turning — a quantity that is measured, not predicted. Reasonable sources already disagree by a minute or two about what time a full moon in 2100 will happen, and nobody can settle it in advance. This page uses the standard long-term projection and says so rather than implying a precision it does not have.