How to use it
- Choose a date, or press Today.
- Read the phase name, how much of the Moon is lit and its age in days.
- Below it are the previous new moon and the next full and new moon, in UTC.
The eight phases
The cycle is divided by the Moon’s angle from the Sun (its elongation). The Moon moves about 12° from the Sun per day, so a day either side of the exact moment counts as that phase, and new moon, first quarter, full moon and last quarter each last about two days. Between them are the waxing crescent, waxing gibbous, waning gibbous and waning crescent. Waxing means the lit part is growing; waning means it is shrinking.
How it works
The times of new and full moons are calculated with the algorithm from Jean Meeus’s book Astronomical Algorithms (chapter 49): a mean time for each lunation plus the largest periodic corrections for the Sun and Moon’s motion. The small planetary terms are left out, which costs at most a minute or so. The illuminated share uses the Moon’s elongation from the Sun (chapter 48) and rounds to a whole percent. The date is judged at noon UTC.
Worked example
The new moon of 29 January 2025 happens at about 12:36 UTC; the full moon before it was on 13 January at about 22:27 UTC. On 19 January the Moon is 5 days past full, so it is a waning gibbous, with the next new moon on 29 January and the next full moon on 12 February.
Good to know
- The phase is the same everywhere on Earth at the same moment; only the time of day on the clock differs.
- Eclipses and the exact tide times need more detail than a phase calculator gives.