8th grade science · 25 minutes
The moon never changes shape. Our view of it does.
Today you will explain the phases using a lamp, a ball and your head.
Five steps take us from the puzzle to proving it ourselves
- 1 · The orbit
- How long the moon takes to go around Earth.
- 2 · The lit half
- What the sun lights, and what really changes.
- 3 · Eight phases
- Every phase name, in order.
- 4 · A wrong idea
- Is the dark part Earth's shadow?
- 5 · Try it
- Be the Earth in a dark room, then check yourself.
The moon goes around Earth about every 29.5 days
In one lap we see the whole set of phases, from new moon back to new moon.
Drawing not to scale.
The sun always lights half of the moon, all the time
Wherever the moon is, the half facing the sun is bright. The other half is dark.
Seen from above. Not to scale.
What changes is how much of that lit half we can see
Moon between Earth and sun
The lit half faces away from us. New moon.
Moon off to the side
We see half of the lit half. First quarter.
Moon behind Earth
The whole lit half faces us. Full moon.
One lap around Earth walks the moon through eight phases
From Earth, the lit part grows to full, then shrinks back
As seen from Earth. The whole set repeats about every 29.5 days.
Phases are not Earth's shadow falling on the moon
The dark part of the moon is Earth's shadow.
Earth's shadow on the moon is a lunar eclipse.
It happens only sometimes, and only at a full moon. Phases happen on every lap.
At a quarter moon, Earth's shadow points the other way
It always falls away from the sun, and a first-quarter moon sits off to one side.
Seen from above. Not to scale.
Make the phases yourself with a lamp, a ball and your head
- Darken the room. Switch on the lamp: it plays the sun.
- Hold the ball at arm's length. Your head plays Earth.
- Turn slowly in a circle and keep your eyes on the ball.
While you turn, hunt for these three moments
| What you see | Where is the ball? | Which phase? |
|---|---|---|
| The ball looks all dark | ||
| The ball looks fully lit | ||
| Exactly half is lit |
Copy this table and fill it in as you turn.
Check yourself with one question and a bonus
Tonight there is a full moon. Where is the moon?
Between Earth and the sun
On the far side of Earth from the sun
Off to one side of Earth
Bonus: does every full moon have a lunar eclipse?
Answers: where the ball sat, and where the full moon is
- All dark
- Ball between you and the lamp: new moon.
- Fully lit
- Lamp behind you: full moon. If your head's shadow hits the ball, you made a mini lunar eclipse.
- Half lit
- Ball off to your side: a quarter moon.
- Check
- B, the far side. Bonus: no, eclipses happen only sometimes.
The moon is always half lit; we just see it from new angles
- Half
- The sun always lights half of the moon.
- 29.5 days
- One lap around Earth, one full set of phases.
- Our view
- We see more or less of the lit half as the moon moves.
- Not a shadow
- Earth's shadow is a lunar eclipse: sometimes, at full moon.