Science: Earth and space

Why does the Moon have phases?

What do you think?

Why does the Moon seem to change shape during the month?

0

Hands up for A, B or C. We find out who is right later in the lesson.

By the end of this lesson you can…

say where the Moon's light comes from
name the eight phases of the Moon in order
use a model to explain why the phases happen
explain why Earth's shadow does not cause the phases

Three facts to start with

The Moon makes no light of its own.

Moonlight is sunlight that bounces off the Moon.

The Sun always lights half of the Moon.

The half that faces the Sun has day. The other half has night.

The Moon goes around Earth about once a month.

From one new moon to the next takes 29.5 days.

Move the Moon around Earth

Drag the Moon or the slider. The white curve marks the half of the Moon that faces Earth.

Seen from above the North Pole. Not to scale.

Waxing crescent

Day 4 of 29.5, 15% lit

Lit on the right, and growing.

Seen from Earth's Northern Hemisphere

About 8 minutes. This is the heart of the lesson. The left side is a view from far above the North Pole, with the Sun far off to the left. The right side is what we see from Earth, from the Northern Hemisphere. In the Southern Hemisphere the picture is flipped left to right. Start by clicking the new moon button under the slider: the Moon is between Earth and the Sun, and we see only its dark night side. Ask: "Which half of the Moon is lit?" Always the half facing the Sun. "Which half can we see?" The half facing Earth, marked by the white curve. The phase is the part of the lit half that we can see. Drag the Moon slowly around, or press Play a month. Stop at first quarter (lit on the right), full moon (Earth is between the Sun and the Moon, and we see the whole lit half) and third quarter (lit on the left). Point out the day counter: one full cycle takes 29.5 days. The model keeps the Sun still and is not to scale: the real Moon is about 30 Earth widths away. Return to the vote: which explanation fits what the model shows?

The eight phases, in order

As seen from the Northern Hemisphere. Days are counted from new moon.

New moonDay 0
Waxing crescentDay 4
First quarterDay 7
Waxing gibbousDay 11
Full moonDay 15
Waning gibbousDay 18
Third quarterDay 22
Waning crescentDay 26

Waxing: growing
Waning: shrinking
Crescent: less than half lit
Gibbous: more than half lit

What will the Moon look like in 4 days?

Tonight the Moon is a waxing crescent. New moon was 3 days ago.

A

B

C

D

BFirst quarter, lit on the right. 3 + 4 = 7 days after new moon, so the Moon has gone about a quarter of the way around Earth.

Is it Earth's shadow?

Phases

We see them all month long.
The dark part is the Moon's own night side.

Lunar eclipse

Earth's shadow falls on the Moon. It turns a dim red.
Only at full moon, and not at every full moon.

Our voteC is right: we see different amounts of the Moon's sunlit side.

Think, pair, share

Your friend says: "The Moon has phases because Earth's shadow covers part of it." What would you tell them?

Think on your own.
Pair: explain your answer to a partner.
Share with the class.

2:00

Be the Earth: make the phases yourself

You need

A lamp without a shade: the Sun
A pale ball on a pencil: the Moon
Your head: Earth
A dark room

Safety: do not stare into the lamp. It gets hot.

Face the lamp. Hold the ball at arm's length, a little higher than your head.
You see the dark side of the ball. This is new moon.
Turn slowly to your left. Keep the ball in front of your face. Watch the lit part grow on the right.
Stop with your back to the lamp. Keep the ball out of your head's shadow. This is full moon.
Keep turning left until you face the lamp again. Name each phase as you go.

About 7 minutes. Pupils work alone or in pairs around one lamp; up to about eight pupils can stand in a circle around the same lamp. Put the lamp at head height in the middle of the room and make the room as dark as you can. Click to show one step at a time and do each step together. In step 3, turning left is the same direction the Moon goes around Earth, seen from above the North Pole. At first quarter the lamp is on your right and the ball is lit on the right, just like the model. In step 4, if your head's shadow falls on the ball, you have made a lunar eclipse: hold the ball a little higher, the way the Moon's tilted path usually passes above or below Earth's shadow. Ask pupils to stop at first quarter and third quarter and say which side is lit. Polystyrene balls on pencils work well. A phone torch can stand in for the lamp in a small group.

Exit ticket

Write your answers on a slip of paper. We look from the Northern Hemisphere.

Where does the Moon's light come from?

From the Sun: moonlight is sunlight bouncing off the Moon.
Earth is between the Sun and the Moon. Which phase do we see?

Full moon: we see the whole sunlit half.
The left half of the Moon is lit. Is it waxing or waning?

Waning: it is a third quarter, and the lit part is shrinking.

What we found out

The Sun always lights half of the Moon.
As the Moon goes around Earth, we see more or less of that lit half.
A full cycle of phases takes 29.5 days.

Homework: keep a Moon diary

Each evening for a week, look for the Moon. Draw what you see, and write the date and time.

No Moon? Write "not seen" or "cloudy". That is an observation too.

Night 1DateTime

Night 2DateTime

Night 3DateTime

Night 4DateTime

Night 5DateTime

Night 6DateTime

Night 7DateTime