Free interactive tool
Moon Phase Viewer
Scrub through the 29.5-day lunar cycle and read tonight's illumination.
Scrub through the Moon's month
Drag the day of the 29.5-day cycle; the lit shape, the phase name, and the illumination follow.
Phases are geometry, not weather: the Moon is always half lit — the cycle is about how much of that lit half faces Earth.
- Find the two days when exactly half the disc is lit.
- Read off how many days remain to the next full moon tonight.
The Moon is always half lit — phases are about how much of that lit half faces Earth. The cycle runs 29.53 days from new moon to new moon, and the illuminated fraction follows a clean cosine curve.
Scrub the day slider to identify a phase, plan a stargazing session around moonlight, or find the two nights when exactly half the disc shows.
Why 29.5 days and not 27?
The Moon orbits in 27.3 days, but Earth moves too — catching back up to the Sun's alignment takes about two extra days.
When is the Moon brightest?
Full moon, around day 14.8 of the cycle — bad for faint-object stargazing, wonderful for walks.