How to Explain the Phases of the Moon to Kids

“Why does the Moon change shape?” is a question children ask once they start noticing the night sky. The answer is neat: the Moon does not change at all. What changes is how much of its sunlit side we can see.

The key idea

The Moon does not make its own light. The Sun always lights half of it, and as the Moon travels around Earth about once a month, we see different amounts of that lit half.

That is all of it. Every phase, from thin crescent to full Moon, comes from one fact: half the Moon is always lit, and our viewing angle changes.

Make the phases with a lamp and a ball

You need a lamp with no shade (or a strong torch on a stack of books), a ball on a stick or held in your hand, and a dark room. Your child’s head is Earth.

Put the lamp at one side of the room, bulb at about head height.
Stand a few steps away, holding the ball at arm’s length, slightly above your head.
Turn slowly in a circle, keeping the ball at arm’s length. Stop at four spots: facing the lamp, with the lamp to your left, with your back to the lamp and with the lamp to your right.
At each stop, look at the ball and describe the lit part.

Facing the lamp, the lit side faces away from you, so you see a dark ball: new Moon. With the lamp to one side, you see half lit: a quarter Moon. With your back to the lamp, you see the whole face lit: full Moon. Moving on you see a half again, then a thin sliver.

Ask your child to draw each stop. Our lesson Why does the Moon change shape? uses this same demonstration and the matching blog post is why does the Moon change shape: a lesson idea.

The vocabulary

New Moon: we see almost none of the lit side.
Crescent: a thin curve.
Quarter: half of the disc is lit (a quarter of the way around its orbit).
Gibbous: more than half lit.
Full Moon: the whole disc lit.
Waxing: growing from new to full. Waning: shrinking from full to new.

A handy tip: in the northern hemisphere, a waxing Moon is lit on the right. In the southern hemisphere, a waxing Moon is lit on the left, and it often looks tipped on its side near the equator. Check which way round it looks from where you live.

The Earth’s shadow mix-up

Many people believe the phases are caused by Earth’s shadow falling on the Moon. They are not. Earth’s shadow causes a lunar eclipse, which is rare and happens only at full Moon. Ask your child, “If the shadow made the phases, why would we see a crescent when the Moon is nowhere near the other side of Earth?” Then show it with the ball: the shadow of your head only falls on the ball when you line it up on purpose.

Start a Moon diary

Over two to four weeks, step outside at the same time each evening, or when you can, and draw the Moon’s shape. Note the date, time, which direction you faced and roughly where it sat in the sky. Cloudy nights are fine: write “cloud” and move on. After a while you will see:

The shape changes gradually through the month.
It rises later each day, by about 50 minutes.
Sometimes it is up in daylight. This surprises many children, and it is true.

Do not look at the Sun to compare. Never point binoculars or a telescope near the Sun. Binoculars on the Moon are fine and make craters appear on a quiet night.

Answering follow-up questions

“Why do we always see the same face?” The Moon turns once on its axis in the same time it takes to go around Earth, so the same side faces us.
“Is there a dark side?” Every part of the Moon gets sunlight in turn. The far side is not always dark, just always hidden from us.
“Does the Moon cause the tides?” Partly: its gravity pulls on the ocean. The Sun contributes too.

Keep it light

You do not need to memorise all the names. Try asking, “What shape is the Moon tonight? Is it growing or shrinking?” That is a lovely habit. For a bedtime routine, a minute at the window is enough.

Where to go next

The Sun, Earth and Moon unit has the full sequence. If your child is also asking about light and dark, try why do we have day and night: ball and flashlight. Our home education page shows how families fit lessons like these into a week.