Sun
Earth and Space

Why is it hotter in the summer than the winter?

Teaching Guidance for 11-14 Supporting Physics Teaching

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.

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