To find the moon phase for a date, measure how long it has been since a known new moon and divide that by the average length of a lunar month, about 29.53 days. Where the date lands in that cycle gives the phase and how much of the disc is lit. The moon phase calculator does this in your browser and names the phase for any date you enter.
The cycle behind it
The moon takes about 29.53 days to go from one new moon to the next. This is the synodic month. Starting from a new moon that astronomers have recorded precisely, you can count forward in whole synodic months and find where any later date sits in the current cycle. A position near zero is a new moon, halfway through is a full moon, and the points between are the crescent, quarter and gibbous phases.
The eight named phases
The cycle is split into eight named stages:
- New moon, the disc is dark.
- Waxing crescent, a thin lit sliver that is growing.
- First quarter, half lit and growing.
- Waxing gibbous, more than half and growing.
- Full moon, the disc is fully lit.
- Waning gibbous, more than half but shrinking.
- Last quarter, half lit and shrinking.
- Waning crescent, a thin sliver that is fading.
Waxing always means growing toward full, and waning means fading toward new.
How accurate it is
This method uses the average lunar month rather than the slightly varying real one, so the result can be off by up to about a day. For knowing whether a night will be dark for stargazing or bright for a walk, that is close enough, and the named phase and lit percentage match what you would see. For precise eclipse work you would need a fuller astronomical model, but for everyday use the moon phase calculator gives a reliable answer.
Where it helps
Knowing the phase ahead of time is useful for a few things:
- Stargazing, where a new-moon night gives the darkest sky.
- Photography, where a full moon lights a landscape.
- Curiosity, such as checking the moon on a memorable past date.
Pick any date in the moon phase calculator to see the phase, the age of the moon and the percentage lit.