How the Moon’s phases work
The Moon does not make its own light. Half of it is always lit by the Sun, and what changes over a month is how much of that lit half faces Earth. When the Moon sits between Earth and the Sun, we see its dark side: that is the new moon. Two weeks later, it is on the far side of Earth and we see the whole lit face: the full moon.
How the calculator works
The time from one new moon to the next, the synodic month, averages 29.530589 days. The calculator counts the days since a known new moon, divides by that period and keeps the remainder. That remainder is the age of the Moon in days, and it gives:
- the phase name, from new moon (age near 0) through first quarter (about 7.4 days), full moon (about 14.8 days) and last quarter (about 22.1 days);
- the illuminated share, (1 − cos(360° × age ÷ 29.53)) ÷ 2;
- the approximate dates of the next full moon and next new moon.
Worked example
On November 3, 2026, the Moon is about 23.1 days old:
| Result | Value |
|---|---|
| Phase | Last quarter, waning |
| Illuminated | 40% |
| Progress through the cycle | 78% |
| Next new moon | November 9, 2026 |
| Next full moon | November 24, 2026 |
How accurate is it?
Because the Moon’s orbit is elliptical and tugged by the Sun, real lunar months range from about 29.3 to 29.8 days. A method based on the average period can therefore be off by up to about a day. That is fine for planning a night of stargazing or photographing a full moon rise, but for the exact minute of a phase, use an almanac such as the tables of the U.S. Naval Observatory.
Phase names at a glance
| Age (days) | Phase |
|---|---|
| 0 to 1.8 | New moon |
| 1.8 to 5.5 | Waxing crescent |
| 5.5 to 9.2 | First quarter |
| 9.2 to 12.9 | Waxing gibbous |
| 12.9 to 16.6 | Full moon |
| 16.6 to 20.3 | Waning gibbous |
| 20.3 to 24.0 | Last quarter |
| 24.0 to 27.7 | Waning crescent |
Tips
- In the Northern Hemisphere, a waxing Moon is lit on the right and a waning Moon on the left. South of the equator, it is the other way around.
- The best nights for faint stars and the Milky Way are a few days either side of the new moon.
- A full moon rises around sunset and sets around sunrise, so it is up all night.