Why the Moon changes shape
It doesn’t. We see a different slice of its sunlit half as it goes round us.
- say why half the Moon is always lit
- draw where the Moon is at full and at new Moon
- explain why phases take 29.5 days, not 27.3
1. The Sun lights one half
Always half. The Moon makes no light of its own; it is a rock in sunshine.
2. The Moon goes round us
Once every 27.3 days, the same face turned towards us.
3. We see part of the lit half
Between us and the Sun: the lit half faces away, new Moon. Behind us: it faces us, full Moon.
Why 29.5 days and not 27.3?
While the Moon goes once round, Earth moves about a twelfth of its way round the Sun. The Moon needs two more days to catch up with the same sunlight angle.
Try itWalk round a chair while a friend walks round the room. You need extra steps to face the window again.The dark part is Earth’s shadow.
That only happens during a lunar eclipse, a few times a year, and only at full Moon.
The dark part is the Moon’s own night side, turned partly towards us.
Check yourself
At full Moon, where are the Sun, Earth and Moon?
Show the answer
Roughly in a line, with Earth in the middle. The Moon’s lit half faces us.
Half lit, always.
Phases are our view of that half. A full cycle takes 29.5 days because Earth moves too.