Why the Moon has phases
The short answer
Half the Moon is always lit by the Sun. Its phases come from the angle we see that half from, not from Earth’s shadow.
0:28ScienceBeginnerMoonlunar phasesastronomy
Chapters and transcript
- 0:00
Half the Moon is always lit
The Moon makes no light of its own. Sunlight lights the half that faces the Sun; the other half is in night.
- 0:07
It isn’t Earth’s shadow
As the Moon circles Earth, we see that lit half from a changing angle. Earth’s shadow reaches the Moon only during a lunar eclipse.
- 0:14
New to full in about two weeks
Night by night we see more of the lit half: new Moon, crescent, first quarter, gibbous, then full.
- 0:21
A full cycle takes about 29.5 days
About 29.5 days after a new Moon, the Moon is new again, and the cycle repeats.