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How do moon phases work?
Quick answer
Moon phases happen because we see different amounts of the Moon's sunlit half as it orbits Earth. The Moon doesn't make its own light — it reflects the Sun's — so as its position changes, the lit portion we see grows and shrinks over about a month.
Step by step
- The Moon reflects sunlight; it makes no light of its own.
- As it orbits Earth, we see different parts of its lit side.
- This cycle of phases takes about 29.5 days.
- Phases run new → crescent → quarter → full → back again.
People also ask
Why does the Moon change shape?
We see varying amounts of its sunlit half as it orbits Earth — the Moon itself doesn't change.
How long is one full cycle of moon phases?
About 29.5 days, from one new moon to the next.
What's the difference between waxing and waning?
Waxing means the lit part is growing toward full; waning means it's shrinking toward new.
Part of the guide: Space & Astronomy →