Explain · analogy
Why the Moon changes shape in terms of a pizza being eaten through the month
The Moon is like a pizza that gets eaten a slice at a time over the month and then turns up again as a fresh one.
The lunar cycle takes about twenty-nine and a half days. At the Full Moon the pizza arrives whole and glowing, and everyone stands outside admiring it. Over the next two weeks the lit part appears to shrink, first a fat gibbous that has lost a corner, then a neat half at the Last Quarter, then a thin crescent crust that looks suspiciously like what is left after a Friday night in Croydon. At the New Moon there is nothing but the empty box, and then a fresh crescent starts to grow back towards a full pizza.
Nothing has actually been eaten, of course. The Moon is always a complete ball, and the Sun always lights one half of it. As the Moon travels around the Earth we see more or less of that lit half, so the pizza only looks smaller because we are looking at the sunny side from a different angle. It is less a feast than a very slow game of peekaboo, played over the whole month with the Sun holding the torch.
Where the analogy breaks downThe Moon remains a solid sphere throughout its phases, whereas a pizza typically becomes a pile of crumbs and profound regret.