Definition · Plain-language
The Moon’s phases
The Moon’s phases are the different shapes of the sunlit Moon we see from Earth as the Moon orbits our planet.
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Why the Moon changes shape
The Moon does not make its own light; it shines by reflecting sunlight. The Sun always illuminates one half of the Moon, but as the Moon travels around the Earth, we see that lit half from different angles. When the Moon is between us and the Sun, its lit side faces away and we see darkness — a new Moon. When the Moon is on the opposite side of Earth from the Sun, we see its fully lit face — a full Moon. In between, we see varying slices, which is what produces the familiar changing shapes.
The eight phases in order
The cycle is usually divided into eight phases. It begins with the new Moon (unseen), then the thin waxing crescent, the first quarter (half lit on the right, from the Northern Hemisphere), and the waxing gibbous as more becomes visible. At the full Moon the whole face is lit. Then the Moon wanes: the waning gibbous, the third (or last) quarter half-Moon, and the slim waning crescent, before returning to new. “Waxing” means the lit portion is growing; “waning” means it is shrinking. The quarter Moons appear as half-discs, named for being a quarter of the way through the cycle.
The length of the cycle
One full cycle of phases, from one new Moon to the next, takes about 29.5 days — a little over four weeks, which is the origin of the word “month”. This is slightly longer than the time the Moon takes to circle the Earth once against the stars, because the Earth is also moving around the Sun, so the Moon must travel a bit further to return to the same phase. The phases are predictable and have been used as a calendar for thousands of years. Importantly, phases are caused by geometry, not by Earth’s shadow falling on the Moon.
Key facts
At a glance
- Definition: the changing shapes of the sunlit Moon seen from Earth
- Cause: our changing view of the Moon’s lit half as it orbits Earth
- Eight phases: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent
- Waxing/waning: waxing means growing; waning means shrinking
- Cycle length: about 29.5 days — the origin of the “month”
- Not caused by: Earth’s shadow (that is a lunar eclipse)
Common misconceptions
What people often get wrong
Often heard: The Moon’s phases are caused by the Earth’s shadow falling on the Moon.
Actually: Phases are caused by our changing view of the Moon’s sunlit half as it orbits Earth. Earth’s shadow on the Moon is a separate, rarer event — a lunar eclipse.
Often heard: The Moon produces its own light.
Actually: The Moon shines only by reflecting sunlight. The Sun always lights one half of it; the phases are simply how much of that lit half we can see from Earth.
Often heard: During a new Moon, the Sun is not lighting the Moon at all.
Actually: The Sun still lights half the Moon at new Moon — but that lit half faces away from Earth, so we see only the unlit side and the Moon appears dark.
