Phases of the Moon
Earth and Space

Why does the Moon change?

Teaching Guidance for 11-14 Supporting Physics Teaching

Three unhelpful tracks

Wrong Track: The Moon changes shape because the clouds shade it and you only see the part which hasn't been shaded. The clouds might cover a bit of the Moon and the clouds move so it might get bigger or smaller, depending on how big the clouds are.

Wrong Track: I think the Moon changes its shape because the planet next to it keeps moving forwards and backwards covering and uncovering the Moon. And when there is no Moon at all it's because the planet is covering it.

Wrong Track: The Moon changes shape because the Earth gets in the way of the Sun's rays.

A better line

Right Lines: The phases of the Moon are what you see. You see a fraction of the illuminated half of the Moon. This fraction changes as the Moon orbits the Earth.

Learning about phases

Thinking about the learning

Clouds cover the Moon: These explanations involve clouds passing in front of the Moon.

Shadow of planet on Moon: These explanations are based on the idea that the shadow of a planet is projected onto the Moon.

Shadow of the Earth on Moon: These explanations are based on the shadow of the Earth being projected onto the Moon. Here is a caricature of the suggestion.

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A cautionary tale from five classes of 11-year-old children, from comprehensive schools, who responded to the challenge: Use words and diagrams to explain your ideas about why the Moon changes shape.

You might expect to find a similar mix in your classes.

Planning for teaching

Thinking about the teaching

You might start by probing your pupils' understandings of phases of the Moon. You might simply ask pupils to respond to the question:

Teacher: Why does the shape of the Moon change?

Given the relative difficulty of the explanation for the phases of the Moon, don't be surprised when many pupils struggle to offer any ideas at all about what is happening. Thus pupils who suggest that it's something to do with clouds (see wrong tracks) may well be offering this in the absence of any other ideas.

The response certainly does not mean that they are firmly committed to the explanation.

The shadow of a passing planet idea (second wrong track) should alert you to deficiencies in the pupil's understanding of the layout of the solar system.

The third wrong track response is, in effect, a lunar eclipse model where the Earth stops the light from the Sun reaching the Moon. Expect a lot of these as this is common among children and adults.

The correct explanation is demanding since it requires the ability to stand outside yourself and to imagine what the Moon will look like, in different positions, when viewed from the Earth.

Here is the essence of the argument: The phases of the Moon are explained in terms of the part of the illuminated side of the Moon which is visible from Earth. There is much more in the Physics Narrative.

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Phases of the Moon
can be exhibited by Moon
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