Why Does the Moon Change Shape? Teaching Moon Phases with a Ball and a Lamp

The Moon looks different almost every night, and children notice. Yet “why does the Moon change shape?” is one of the most commonly misunderstood questions in elementary science. A lot of people, adults included, believe Earth’s shadow is what makes the phases.

It isn’t. Here’s a way to show the real reason in ten minutes, using a ball and a lamp.

The real explanation

The Moon doesn’t make its own light. It reflects sunlight. At any moment, half the Moon is lit by the Sun, just as half of Earth is in daylight. The phases depend on how much of the Moon’s lit half we can see from Earth, and that depends on where the Moon is in its orbit.

New Moon: the Moon is between Earth and the Sun. The lit half faces away from us.
Full Moon: Earth is between the Moon and the Sun. We see the whole lit half.
Quarter moons: we see half of the lit half, so it looks like a half circle.
Crescents and gibbous: in between.

The cycle takes about 29.5 days from one new Moon to the next.

Why it isn’t Earth’s shadow

Earth’s shadow on the Moon is real, but it’s a lunar eclipse, which happens only occasionally. A crescent Moon and a half Moon happen every month. If Earth’s shadow caused phases, we’d see an eclipse every full Moon. We don’t, because the Moon’s orbit is slightly tilted, so it usually passes above or below Earth’s shadow.

The ball-and-lamp model

You need a lamp with the shade removed (a bright LED bulb is ideal), a small ball on a pencil or skewer, such as a polystyrene ball, tennis ball or ping-pong ball, and a dim room.

Stand a couple of metres from the lamp, holding the ball at arm’s length. Your head is Earth, the ball is the Moon and the lamp is the Sun.
Turn slowly in a full circle, keeping the ball out in front of you.
Watch the ball. At different points it shows a crescent, half, gibbous and full lit side, just like the Moon.
At the point where you’re facing away from the lamp, the ball looks fully lit. When you face the lamp, the ball is dark on the side you see. Your own head may block some light, so hold the ball slightly above your head to see the full phase.

Children usually say “Ohh!” at the moment the crescent appears in their hand.

Our lesson Why does the Moon change shape? uses this model, tests the shadow idea with evidence, and ends with a two-week Moon diary so students can check their model against the real sky.

Keep a Moon diary

For two weeks, go outside at about the same time each evening and sketch the Moon’s shape and where it is in the sky. Notice:

Is it getting bigger or smaller (waxing or waning)?
Where is it relative to the horizon?
Can you see it in the daytime? (You often can.)

Patterns from a diary are evidence, and evidence is what we want students to use.

A note on hemispheres

People in the Southern Hemisphere see the Moon “upside down” compared with the north. A crescent that looks like a “D” or a “C” shape will flip depending on where you stand. The phases are the same, and our model explains the difference too.

Common mix-ups

“Earth’s shadow causes the phases.” Shadow causes eclipses.
“The Moon only comes out at night.” It’s often up in daylight.
“The Moon makes its own light.” It reflects the Sun’s.

More space science

This activity belongs to the Sun, Earth and Moon unit. The night sky has its own unit, Stars and the Night Sky. Sign up for three free lessons or browse the library.