How the moon phase is worked out
The moon runs on a remarkably steady clock. One complete cycle of phases, called a synodic month, averages 29.53059 days from one new moon to the next. Because the cycle is so regular, you can find the phase for any date by counting the days since a known new moon and seeing how far through the current cycle that lands you. This calculator counts from the new moon of 6 January 2000 at 18:14 UTC, a standard reference point.
The "age" of the moon is that remainder: 0 days at new moon, about 7.4 at first quarter, about 14.8 at full moon, and about 22.1 at the last quarter. Illumination follows the age too. The lit fraction grows from 0% at new moon to 100% at full and back again, following a smooth cosine curve rather than a straight line, which is why the moon seems to fatten quickly around the quarters and linger near full.
The eight phases in order
Astronomers name four principal phases (new, first quarter, full, last quarter) and four transitional ones between them. Waxing means growing towards full; waning means shrinking towards new. A crescent is less than half lit and a gibbous moon more than half. So the full sequence runs: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, and back to new. In the northern hemisphere the right-hand side lights up first as the moon waxes; from the southern hemisphere the whole show appears mirrored.
Worked examples
How accurate is this, and what is it good for?
Checked against published astronomical tables for 2026, this method lands within a day of the true moment of each phase, and usually within half a day. The wobble exists because the moon's orbit is an ellipse, so it runs a little ahead of or behind the average clock; a real cycle can be anywhere from about 29.2 to 29.9 days. A full ephemeris models all of that to the second, but for everyday questions the average is plenty.
Those everyday questions are more common than you might think. Photographers plan shoots around a low full moon or the thin evening crescent. Stargazers do the opposite, hunting the dark skies near new moon when faint objects show best. Anglers and gardeners follow lunar calendars by tradition, and anyone can settle "what was the moon doing the night I was born?" in a couple of clicks by entering their date of birth. Pair it with the star sign calculator for the full birthday picture, or the days between dates tool to count the days separating two full moons. Everything is calculated in your browser and nothing you enter is sent anywhere.
Frequently asked questions
How accurate is the phase?
To within about a day. The calculator uses the average length of the lunar cycle, while the real moon runs slightly fast or slow because its orbit is elliptical. For naming the phase, planning a moonlit walk or checking a photo date that is plenty; for eclipse timing you would want a full astronomical ephemeris.
Is the moon phase the same everywhere in the world?
Essentially yes. The whole planet sees the same moon, so everyone shares the same phase at the same moment. The only difference is the calendar date: a full moon late in the evening in London can fall on the next calendar day in Auckland.
What is a blue moon?
In popular use, a second full moon within one calendar month. Because the lunar cycle is about 29.5 days and most months are 30 or 31 days, it happens roughly every two to three years. The moon does not actually look blue.
Why does the moon have phases at all?
Half of the moon is always lit by the sun. As the moon orbits the Earth roughly once a month, we see that lit half from changing angles, from none of it at new moon to all of it at full moon. The Earth's shadow is not involved, except during a lunar eclipse.
Does the full moon affect sleep or behaviour?
Large studies have found no reliable effect on behaviour, and only small, disputed effects on sleep. The extra light of a bright full moon through thin curtains is the most plausible culprit for a restless night.