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Moon Phase Calculator

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The Moon's phase for any date between 1900 and 2100 — which eighth of the cycle it is in, what percentage of the disc is lit, how many days old it is, and when the next new moon, full moon and quarters fall. Computed from Meeus's series in your browser, with no location needed: the phase is the same everywhere on Earth.

Local clock time

Include daylight saving if it applies

How to use the moon phase calculator

  1. 1Choose the date and time you want the phase for.
  2. 2Set your UTC offset so the phase dates come back in your own clock time.
  3. 3Press Calculate.
  4. 4Read the phase and illuminated percentage, and use the table for the next four phase instants.

Examples

A date in the first half of a lunation

Input
A moment four days after a new moon
Result
Waxing Crescent, around 15% lit, with the first quarter three days away

Checking a full moon date

Input
Any date in the month, with the next phases table
Result
The exact instant of the full moon in both your local time and UTC

Full moon is an instant, not a night — it can fall at 03:00 and belong to the date before the one people call it.

About the moon phase calculator

Phase, age and illumination are three different numbers

The phase name is which eighth of the cycle the Moon is in. The age is how many days have passed since the new moon that started the lunation. The illuminated fraction is how much of the disc is lit. They move together but they are not interchangeable, and the difference shows up most at the quarters.

At first quarter the Moon is about 7.4 days old and exactly half lit — but it reaches half illumination slightly before it reaches the quarter point of the cycle, because the cycle's halves are not equal in length. Reporting all three separately is more honest than deriving one from another and presenting the result as if it were measured.

Why the four principal phases are instants

New, first quarter, full and last quarter are defined by exact geometric conditions — the Moon's ecliptic longitude relative to the Sun's — and each is satisfied at a single moment. The Moon is exactly full for no measurable length of time at all.

So a date labelled Full Moon means the day containing that instant in whatever time zone the label was printed for, which is why the same full moon appears on different dates in different places. When the phase name is applied to a date rather than an instant, as it is in the headline here, it means the eighth of the cycle that moment falls in.

Dynamical time and the clock

Astronomical series are written in Terrestrial Time, which runs at a uniform rate. Civil time is tied to the Earth's rotation, which does not: the planet is a slightly unreliable clock, and the gap between the two — ΔT — is around 69 seconds in the 2020s and can only be measured, never predicted.

Ignoring it would put every phase time here about a minute late. The correction applied is Espenak and Meeus's polynomial for 2005 to 2050; outside that range the value is held at the nearest endpoint and the result says so, because extrapolating a polynomial for ΔT produces a confident number with nothing behind it.

Frequently asked questions

Why does the full moon date sometimes differ from my calendar?
Because a full moon is an instant, not a day, and calendars print it in one time zone. A full moon at 00:40 UTC on the 5th is at 19:40 on the 4th in New York and at 06:10 on the 5th in Delhi — the same instant, three different dates in two cases. This shows the instant in both your offset and UTC so the discrepancy is visible rather than mysterious.
Do I need to enter my location?
No, and nothing here asks for one. The phase is geocentric: the fraction of the Moon lit is the same for everyone on Earth at a given moment. What does change with location is the orientation — the crescent that leans right in London leans left in Sydney and lies on its back near the equator — and so do moonrise and moonset, which are a different calculation not attempted here.
How accurate is it?
The instants of the phases come from Meeus's series in Astronomical Algorithms, which is stated as accurate to a few seconds, plus the ΔT correction that converts dynamical time to the clock. The illuminated fraction uses his approximate phase angle, good to about 0.0014 — a tenth of a percentage point, well inside the precision anyone reads off the number.
Why is the length of the lunation not always 29.53 days?
29.530588 days is the mean. The real interval between new moons runs from about 29.27 to 29.83 days, because the Moon's orbit is elliptical and the Sun's pull on it varies through the year. The tool computes the actual new moons either side of your date and reports the length of that particular lunation, which is why the number moves.
What is the lunation number?
Brown's count, which numbers lunations from the new moon of 17 January 1923. It is the standard reference in almanacs and observing logs, and it is unambiguous in a way that “the March full moon” is not.

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