The Moon phase cycle, explained
- Phases come from the changing angle between Sun, Earth and Moon — not from Earth's shadow.
- One full cycle (new → full → new) is the synodic month: about 29.53 days.
- The four primary phases are instants; the four in-between phases are the weeks spanning them.
The eight phases in order
| Phase | Type | Sun–Moon angle | Lit side visible | When visible |
|---|---|---|---|---|
| New Moon | Primary | 0° | ~0% | Up with the Sun (not visible) |
| Waxing Crescent | Intermediate | 0–90° | rising | Evening, west |
| First Quarter | Primary | 90° | 50% | Afternoon to midnight |
| Waxing Gibbous | Intermediate | 90–180° | rising | Late afternoon to late night |
| Full Moon | Primary | 180° | 100% | All night |
| Waning Gibbous | Intermediate | 180–270° | falling | Late night to morning |
| Last Quarter | Primary | 270° | 50% | Midnight to noon |
| Waning Crescent | Intermediate | 270–360° | falling | Morning, east |
Synodic vs sidereal: two different months
The Moon takes about 27.3 days to orbit Earth once relative to the stars — the sidereal month. But while it orbits, Earth is also moving around the Sun, so the Moon needs roughly two extra days to 'catch up' to the same Sun–Earth–Moon alignment.
That alignment cycle — the time between one new moon and the next — is the synodic month, about 29.53 days. Phases follow the synodic month, which is why a calendar month usually holds one full moon, and occasionally two (a 'blue moon').
Waxing, waning, gibbous, crescent
'Waxing' means the lit portion is growing (new toward full); 'waning' means it is shrinking (full toward new). 'Crescent' is less than half lit; 'gibbous' is more than half but not full.
A lunar eclipse is a separate event: it only happens at a full moon when the Moon passes through Earth's shadow, which is why eclipses are rare rather than monthly.