Why the Moon changes shape
The Moon doesn't change. Half of it is always lit by the Sun; we just see that lit half from different angles as it goes round us.
The idea
The Sun always lights half of the Moon. As the Moon goes round Earth, about once every 29.5 days, we see that lit half from a changing angle: all of it (full moon), none of it (new moon), or a slice in between. The shape we see is simply how much of the lit half is turned towards us.
Where kids get stuck
- Thinking Earth's shadow makes the phases. It doesn't; Earth's shadow on the Moon is a lunar eclipse, which happens a couple of times a year, not every month.
- Mixing up waxing (growing, after new moon) and waning (shrinking, after full moon).
- Not realising the Moon is up in the daytime too, for about half of every day.
Try it at home
A dark room, one lamp for the Sun, your child's head for Earth, and a ball on a stick for the Moon. Hold the ball out at arm's length, a little above eye level, and turn slowly on the spot. Watch the lit part of the ball grow and shrink. That is the whole month in one turn.