Why the Moon has phases

Narration

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1. 0:00Look up on different nights and the Moon seems to change shape: a sliver, a half, a full disc.
2. 0:05But the Moon does not change shape. It only looks like it does.
3. 0:11Think of a ball held up in front of a lamp: one half is always bright.
4. 0:15Half of the Moon is always lit by the Sun.
5. 0:20As the Moon circles the Earth, we see that lit half from different angles.
6. 0:25At new moon, the Moon sits between the Earth and the Sun, so its lit side faces away from us.
7. 0:31A week later, at first quarter, we see exactly half of the lit side.
8. 0:35At full moon, the Earth is between the Sun and the Moon, so we see the whole lit face.
9. 0:40In every phase the Moon is lit in just the same way. Only our viewpoint changes.
10. 0:45Then the light shrinks again, through last quarter, back to new.
11. 0:51One full cycle takes about twenty nine and a half days.
12. 0:56In between, the Moon moves a little each night, so it rises later each night.
13. 1:03And we always see the same side of the Moon, whatever the phase.
14. 1:07It is not the Earth's shadow. That only happens in a lunar eclipse, which is much rarer.
15. 1:14So the phases are just geometry: where the Moon is, and where we are looking from.