Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• organise what pupils have observed into a coherent whole
• build three-dimension models where illumination is important
• draw out what children believe about the Earth-Moon-Sun system and how this is related to everyday phenomena

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• build a sequence that uses increasingly complex models
• keep each step of the simplification in building the models connected with the whole
• distinguish between spinning on an axis and orbiting another object
• keep what is noticed and recorded somewhat separate from what is modelled

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Starting points

What would you expect children, on entering secondary school, to know about the force of gravity?

We asked a group of 11-year-old pupils the following question:

Teacher: I throw an object up into the air. Are there any forces acting on it after it has left my hand?

Here are a couple of sample answers.

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Anya (clip 1) states that gravity is pulling the object down. She then goes on to suggest that there is also an upthrust acting on the object and that if the gravity overpowers the upthrust the object will fall. In reality the object is accelerated by the upwards force of the hand (during the act of throwing), but after that the only force acting on the object is the downward pull of gravity (ignoring air resistance).

Sarah (clip2) knows that gravity is acting on the object at the top and as it comes down, but is not sure about what happens on the way up. Pupils often struggle with the idea that a force can act in the opposite direction from the direction of travel of an object (gravity acts down as the object moves up).

Common unhelpful approaches

Wrong Track: We get day and night so that we can sleep at night and do things in the day.

Wrong Track: It is when the Moon gets in front of the Sun and stops us getting sunlight and we call that night and then it moves and gives us the sunlight and we call that day.

Wrong Track: The Sun lights up the Earth so the Earth is shining brightly. It comes over the hills and mountains and when it starts to get dark the Sun goes back behind the hills and mountains.

Wrong Track: Because at night it isn't so warm and maybe the Sun goes behind a cloud.

Wrong Track: When the Sun comes to our side of the Earth, then it's day time.

Wrong Track: The Earth goes around the Sun every day. When it faces the Sun it's day.

Getting off along the right lines – a spinning Earth

Right Lines: The Earth spins on its axis, completing one turn a day. Day time for you is when your side faces the Sun.

Correct approaches and unhelpful starting points

Thinking about the learning

A reference to everyday life: We get day and night so that we can sleep at night and do things in the day.

Simple blocking explanations: These explanations all involve something getting in the way of the Sun, blocking its light and thereby creating the darkness of night. Common wrong tracks include arguing that the Moon, clouds or hills and mountains cause sunlight to be blocked.

Sun orbiting the Earth: Here the Earth is stationary and the Sun is in an orbit around it. According to this view, day-time comes when the Sun is on your side of the Earth and night-time when it is on the opposite side. For this model to work the Sun must orbit the Earth once a day.

Earth orbiting the Sun: Here the Sun is stationary and the Earth is in orbit around it. According to this view, day-time comes when your side of the Earth faces towards the Sun and night-time when it faces away from the Sun. It's the orbiting of the Earth around the Sun that is used here to explain day and night. For this model to work, the Earth must orbit the Sun once per day and also maintain the same orientation (facing the same way) in space, with no spinning. Here is a caricature of this suggestion.

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Thinking about the teaching

Here the Earth is spinning on its axis in front of the Sun, once every 24 hours. According to this view, day-time comes when your side of the Earth is facing towards the Sun and night-time when it faces away from the Sun. Here is a caricature of the model.

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Pupils' responses

Thinking about the teaching

There is an important progression in understanding here. For pupils to appreciate that the Earth is spinning, and that this is the cause of day and night, is a major step and suggests a more sophisticated understanding which you can build on.

Here are some responses from a mixed ability science class of 11-year-olds, before the year's work on the solar system, in a well established, rural comprehensive school to the challenge:

Teacher: Use words and diagrams to explain your ideas about why we get day and night.

These results suggest that it is worth probing your pupils' understanding of day and night right at the start of the teaching.

You might simply ask pupils to respond to the question:

Teacher: Why do we get day and night?

Alternatively you may wish to use a diagnostic question which actually confronts the pupils with possible alternative views and probes the link between ideas and evidence.

Rotating and orbiting

When describing the motion of the Earth, be very careful to distinguish between:

• The spin or rotation of the Earth, on its own axis.
• The orbiting of the Earth around the Sun.

Pupils tend to confuse these terms and may talk about the Earth spinning around the Sun once in a year. You will need to emphasise the difference in meaning of spin and orbit in your teaching.

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Teacher Tip: Are we looking at spinning or orbiting here?

Three common, but unhelpful tracks

Wrong Track: Clouds stop heat from the Sun – there are more clouds in the winter, blocking out the Sun's heat.

Wrong Track: Distance of Earth from the Sun – we get the four seasons by the Sun moving around and if the Earth moves away from the Sun it will get colder and if the Earth moves closer to the Sun it will be warmer. They change because in the summer the Sun is closer to the Earth and in the autumn the Sun is a lot farther away and in the winter it is cold because the Sun is a long way away.

Wrong Track: Sun on other side of the Earth – in the summer the Sun is next to us but in the winter it goes around to the other side of the world.

Getting off on the right lines – it's all about angles

Right Lines: The seasonal changes are due to the tilt of the Earth's axis and the effect that has on the angle at which the Sun's rays meet the surface of the Earth as the Earth orbits the Sun.

Considering what might go wrong

Thinking about the learning

Clouds stop heat from Sun: These explanations involve clouds stopping the Sun from warming us in the winter months.

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Distance of the Earth from the Sun: These explanations all involve the Earth being closer to the Sun in the summer and farther away from the Sun in the winter.

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Sun on the other side of the Earth: The idea here is that when the Sun is on your side of the Earth it's summer and when it's on the other side of the Earth it's winter.

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Here are the responses of two mixed ability science classes of 11-year-olds (one had completed their year's work on the solar system, while the other had not yet started – total of 50 pupils) in well established comprehensive schools to the challenge: Use words and diagrams to explain your ideas about why we get the seasons.

A large-scale study focusing on simple astronomical ideas was recently carried out with the general adult public and 16-year-old pupils. The correct scientific explanation for seasonal changes was offered by less than 50 % of the pupils. As for the public, 60 % of them knew that the Earth orbits the Sun rather than vice versa but only 33 % knew that it took one year.

These are not easy ideas to teach.

Planning for teaching

Thinking about the teaching

By far the most common response to explaining the seasons is in terms of distance from the Sun. This is based on the common-sense reasoning that if you go closer to a glowing source, then you become warmer.

Voice of the street: The Earth is closer to the Sun in the summer, so it must be warmer.

This seems to be a reasonable line of argument, further developed as

Voice of the lane: The closer you are, the warmer it gets.

Unfortunately it's on the wrong track!

The slightly elliptical orbit of the Earth around the Sun means that we (in the Northern Hemisphere) are 152 million km from the Sun in summer but 147 million km from the Sun in winter. The Earth is actually nearer to the Sun in winter!

(And it is only a 3 % difference, probably not enough to explain the difference in average temperatures between summer and winter in any case.)

This idea is, unfortunately, supported by those textbooks which show the orbit of the Earth around the Sun as a very obvious and flattened ellipse.

This not an accurate representation. The orbit of the Earth is only slightly elliptical.

One thing you should look out for are those children who accept the idea that seasonal changes are due to the tilt of the Earth but then suggest that the tilt makes one hemisphere closer to the Sun, thereby bringing summer.

This is not correct; it is a projected area effect not a distance effect.

Here is the essence of the argument. There is much more in the Physics Narrative.

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There is another line of thinking, that experience suggests you are more likely to come across in adults, but it is worth watching out for.

Wrong Track: The Sun is lower on the horizon in the winter, so there the rays have to travel through a greater thickness of atmosphere, so they are weaker.

As ever, there is some reason for believing this, but the numbers just don't stack up. The Sky does appear more red in the morning and evening because some of the blue fraction of the light is affected by the atmosphere, and is scattered (which is why the sky overhead appears blue). Notice that this is an effect that depends on direction, rather than on thickness.

There is, of course, some absorption of the insolation by each kilometre of atmosphere, but a model of the differences between a summer and winter day do not suggest that there is enough of a difference for it to be a significant factor in accounting for the seasons.

The atmosphere is a variable and complex filter – the quantity of water vapour locally is a big factor – but a justifiable range for the absorption is 5–16 watt metre^-2 kilometre^-1.

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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There's a good chance you could improve your teaching if you were to:

Try these

• teaching a sequence determined by the complexity of model explaining the illumination
• relating your models to observations that the children make, both inside and outside the laboratory
• paying attention to the issue of relative scale in your depictions
• ensuring that the phenomena to be modelled are well characterised before the modelling starts

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• assuming that the models are obvious
• building models without the physical experiences that make them seem real
• not running the models once they are built, so that you can show how they work
• not relating the models back to the observations

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.