The Moon runs through the same cycle of shapes every month, from an invisible new moon to a brilliant full moon and back. Understanding why explains a surprising number of things: why the full moon always rises at sunset, why "blue moons" are rare, and why eclipses cluster the way they do.

Sunlight and geometry

The Moon produces no light of its own — it simply reflects sunlight. Half of it is always lit, but from Earth we see that lit half at different angles as the Moon orbits us. The changing slice of the lit side that faces us is what we call a phase. One full cycle, new moon to new moon, takes about 29.5 days (the "synodic month"), which is where the rough length of a calendar month comes from.

The eight phases

  • New moon — the Moon is between Earth and Sun; its lit side faces away from us, so it is invisible (and this is when solar eclipses can happen).
  • Waxing crescent — a thin sliver appears in the evening sky after sunset.
  • First quarter — half the disk is lit; it rises around midday and sets around midnight.
  • Waxing gibbous — more than half lit and growing.
  • Full moon — the Earth sits between Sun and Moon, so we see the whole lit face; it rises at sunset and sets at sunrise.
  • Waning gibbous — more than half lit and shrinking.
  • Last (third) quarter — half lit again, rising around midnight.
  • Waning crescent — a thin sliver in the pre-dawn sky before it returns to new.

"Waxing" simply means growing, "waning" means shrinking. A handy trick in the Northern Hemisphere: if the lit edge is on the right, the Moon is waxing; on the left, waning.

Why the full moon rises at sunset

Because a full moon is opposite the Sun in the sky, it does the opposite of the Sun: it rises in the east as the Sun sets in the west, sits highest around midnight, and sets as the Sun rises. That is why a full moon is up all night, and why it is such an effective spoiler of meteor showers — it is in the sky during exactly the dark hours you want for faint meteors.

Blue moons, harvest moons and supermoons

These popular names describe when a full moon falls, not a different kind of Moon:

  • A blue moon is the second full moon in a single calendar month (the modern usage) — a consequence of the 29.5-day cycle being slightly shorter than most months, so occasionally two full moons squeeze into one. It has nothing to do with colour, and happens roughly every two to three years ("once in a blue moon").
  • A harvest moon is the full moon nearest the September equinox, traditionally giving farmers extra evening light.
  • A supermoon is a full moon near the Moon's closest approach to Earth, so it looks slightly larger and brighter — see what is a supermoon.

Phases and eclipses

Eclipses are tied directly to phase. A solar eclipse can only happen at new moon, and a lunar eclipse only at full moon — but not every month, because the Moon's tilted orbit usually carries it above or below the perfect line-up. When a new or full moon does land on that line, we get an eclipse; the different types of eclipse follow from the exact geometry. You can track every 2026 full moon, with local moonrise times, on our full moon pages.