Science
How do moon phases work?
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.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how moon phases works.
Step by step
- 1The Moon reflects sunlight; it makes no light of its own.
- 2As it orbits Earth, we see different parts of its lit side.
- 3This cycle of phases takes about 29.5 days.
- 4Phases run new → crescent → quarter → full → back again.
Frequently asked questions
- 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.