Why the Moon has phases
The Moon is always half lit by the Sun. What changes is how much of that half we see, and that explains every phase and moonrise.
The Moon makes no light of its own. The Sun always lights exactly one half of it; the phase is how much of that lit half faces us as the Moon orbits Earth every 29.5 days.
The cycle
- New Moon: the Moon is between us and the Sun; we see its dark side, and it is up during the day.
- Waxing crescent and first quarter: a growing sliver in the western evening sky. At first quarter we see half the disc lit, and the Moon is highest at sunset.
- Full Moon: Earth is between the Sun and Moon; the whole face is lit. It rises at sunset and sets at sunrise.
- Last quarter and waning crescent: the Moon rises after midnight and shows in the morning sky.
Rules of thumb
- The Moon rises about 50 minutes later each day.
- In the Northern Hemisphere a Moon lit on the right is growing; lit on the left is shrinking. South of the equator it is the other way round.
- The best time to look at the Moon in binoculars is not full Moon but the days around the quarters, when long shadows along the terminator show craters and mountains.
Why it matters for stargazing
A bright Moon hides faint stars, the Milky Way and weak aurora. Plan deep-sky nights and meteor showers around new Moon; see the Moon calendar.