Moon Detectives

Why does the Moon change shape?

Some nights it's a big round ball. Some nights it's a thin smile. Some nights it vanishes! Let's crack the case.

The evidence

Three clues solve the mystery

Every good detective needs clues. Read all three, then test them in the Moon Lab.

Clue 1

The Moon has no light of its own

It's a big grey rock. It only shines because sunlight bounces off it, like a torch shining on a football in a dark room.

Clue 2

Half the Moon is always lit

The side facing the Sun is bright. The other side is dark. Just like Earth has day and night, so does the Moon!

Clue 3

The Moon travels around Earth

One lap takes about 29½ days, roughly a month. As it moves, we see a different amount of its bright half.

Test the clues

The Moon Lab

Grab the Moon and drag it around the Earth, or press Play. Watch how its shape changes in the window on the right.

Looking down from space
What we see from Earth
First Quarter
Lit up: 50% Growing

When is the Moon in the sky?
noon6pmmidnight6amnoon
daytimenight-timeMoon is up

Spot them all

The eight Moon shapes

Scientists call these shapes phases. Tap any one to see where the Moon is in the Lab.

Detective words

Waxing, waning and the DOC trick

Waxing means growing: more of the Moon is lit each night. Waning means shrinking. Gibbous (say "GIB-us") means more than half but not full.

In the UK, remember DOC: the Moon grows like a D, is round like an O, then shrinks like a C.

Dgrowing
Ofull
Cshrinking
Myth buster
Myth!

"The Earth's shadow makes the Moon's shape."

Lots of grown-ups think this, but it's not true. The dark part of the Moon is just its own night side. Earth's shadow only lands on the Moon during a lunar eclipse, which happens just a few times a year and turns the Moon a coppery red.

And the Moon never really disappears. At New Moon it's still up there, in the daytime sky close to the Sun, with its dark side pointing at us.

Be the Earth

Make Moon phases in your living room

Ask a grown-up to help with this experiment. You'll be the Earth!

A lamp with no shadeA ball or an orangeA pencilA dark room
  1. Push the pencil into the ball so you have a handle. This is your Moon.
  2. Switch on the lamp and turn off every other light. The lamp is the Sun.
  3. Stand facing the lamp and hold the ball out at arm's length, just above your head.
  4. Slowly turn round on the spot to your left, keeping your eyes on the ball.
  5. Watch the bright part of the ball grow into a crescent, a half, then a full circle, then shrink again. You made every phase!
lamp = Sun you = Earth ball = Moon
Detective test

Earn your Moon Detective badge

Eight questions. Get them right to collect stars. Don't worry if you get one wrong: you'll find out why.