Why the Moon changes shape

The Moon doesn't change shape. The Sun always lights half of it, like a lamp lighting half a ball. As the Moon goes around Earth once a month, we see that lit half from different angles: all of it (full Moon), none of it (new Moon), or a slice in between.

Make it in a room

You need a lamp with the shade off, a ball (an orange works) and a dark room. The lamp is the Sun. Your head is Earth. The ball, held at arm's length, is the Moon.

Stand facing the lamp with the ball in front of your face, a bit higher than the lamp so your head doesn't block it. The side of the ball you see is dark: new Moon. Now turn slowly to your left, keeping the ball in front of you. A thin lit edge appears on the right: a crescent. A quarter turn round and you see half lit. With your back to the lamp (ball held high), the whole face is lit: full Moon. Keep turning and the light shrinks from the right until it's gone.

Lamp = Sun, head = Earth, ball = Moon.
Turn left slowly, ball always in front.
Watch the lit part grow, then shrink.

Why a month

One full turn of you in the room is one trip of the Moon around Earth, about 29 and a half days from new Moon to new Moon. The word month comes from the same root as Moon.

Growing and shrinking

In the Northern Hemisphere, a Moon lit on the right is growing (waxing) and is seen in the evening; one lit on the left is shrinking (waning) and is up late at night and in the morning. In Australia they see it the other way round.

What it isn't

The dark part is not Earth's shadow. Earth's shadow only falls on the Moon during an eclipse of the Moon, a few times a year, and it looks completely different: a reddish dimming of a full Moon.

Check it in the sky

The model only counts if the real Moon agrees. Look at the Moon calendar for this month and pick the first quarter date. On that evening, the real Moon should be half lit on the right and high in the south after sunset, with the Sun just gone down in the west: the lit side faces the Sun, exactly like the ball and the lamp.

Draw the Moon every clear night for a week on the Moon journal sheet. The shapes your child draws will match the ones the model made in the room, and they will have found the pattern for themselves.

More guides

“Read it, then go and look. The sky is the best page in the book.”