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The Moon’s changing shape — Earth science, 4–6 years

Why the Moon seems to change shape while travelling around Earth, even though it is always round.

The Moon’s phases

The Moon is a round, rocky world, but we do not always see all of its sunlit side. As it travels around Earth, we see different amounts of the bright half: a thin curve, a half Moon, or a full Moon. These repeating appearances are called phases.

Why the Moon changes

Long ago, people watched the Moon to count time and to plan journeys, planting and celebrations. The Moon does not make its own light; sunlight illuminates half of it. Its position around Earth changes which part of that bright half faces us, so the Moon appears to change while its round shape stays the same.

Watching one month

Imagine drawing the Moon every few nights. Night 1: you see a thin bright curve. About one week later: you see a half Moon. About another week later: the whole face facing Earth looks bright, so it is a full Moon. After about two more weeks, the thin curve returns, and the pattern starts again.

The Moon is not sliced

It is easy to think that a half Moon means half of the Moon has disappeared. We see only the part lit by the Sun from our viewpoint, like seeing part of a torch-lit ball in a dark room. The Moon is still round; the dark-looking part is simply not facing the Sun in the right way for us to see it lit.

The Moon outside school

You can use the phases to describe what you see in the sky and to keep a simple Moon diary. The Moon’s gravity also helps make ocean tides rise and fall, although the Sun helps too. A calendar may show a full Moon, but clouds can hide it on the actual night.

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