The moon doesn't produce its own light — everything we see is sunlight reflecting off its surface. As the moon orbits Earth, the portion of its sunlit half that's visible to us changes, which is what creates the cycle of phases. A full cycle, from one new moon to the next, takes about 29.53 days — the synodic month.
The eight phases in order
- New moon — the moon sits between Earth and the sun, so its sunlit side faces away from us. It's effectively invisible in the night sky.
- Waxing crescent — a thin sliver of light appears on the right side (Northern Hemisphere view) and grows night by night.
- First quarter — exactly half the visible face is lit. Despite the name, this is the midpoint of the waxing half, not a quarter of the way through.
- Waxing gibbous — more than half lit, continuing to grow toward full.
- Full moon — Earth sits roughly between the sun and moon, and the entire visible face is lit.
- Waning gibbous — illumination starts shrinking back from full.
- Last quarter (also called third quarter) — half lit again, this time on the opposite side from the first quarter.
- Waning crescent — a thinning sliver, shrinking back toward new moon.
Waxing vs. waning
"Waxing" means growing (more illumination each night); "waning" means shrinking. It's a useful shorthand independent of the exact phase name.
Synodic vs. sidereal month
The 29.53-day cycle of phases (synodic month) is slightly longer than the 27.3 days it takes the moon to complete one orbit relative to the stars (sidereal month), because Earth is also moving around the sun, so the moon has to travel a bit further each cycle to catch up to the same sun-Earth-moon alignment.
Want to see which phase we're in right now, and how many days until the next new or full moon? Check the homepage for a live view.