The Moon Changes Nothing

Go outside tonight and look at the Moon. Do it again in a week, and again the week after. The shape will be different every time — a thin curved sliver, then half a disc, then a full bright circle, then shrinking back again.

Now here is the strange part. The Moon is not changing. It is the same ball of grey rock it was last night, the same size, lit by the same Sun. Nothing about it is growing or being eaten away.

What changes is where you are looking at it from.

At every single moment, the Sun lights exactly one half of the Moon — the half pointing at the Sun. That never varies. But as the Moon travels round the Earth, the amount of that lit half you can see from here changes, and it changes on a rhythm so reliable that people built calendars out of it long before anyone knew why it worked.

This lesson is about the sky doing simple things that look complicated. By the end you'll be able to:

• explain why we have day and night, and why we have seasons
• explain what causes the phases of the Moon — and what does not
• say how long the cycle takes, and why you'll see two different numbers for it
• explain what actually happens during a solar and a lunar eclipse