The Sun and the Seasons, Grade 7: Tilt, Solstice and Equinox

We are closest to the Sun in early January, in the middle of a South African summer. Seasons have nothing to do with distance.

The text usually gets this right. The diagram usually does not. You will see Earth's orbit drawn as a pronounced oval, with the planet obviously nearer the Sun at one end, and that one picture is the single commonest cause of the belief that summer happens when we are close to the Sun.

Earth's orbit is very nearly circular. If you drew it accurately on an A4 page you could not tell it from a circle. And the small difference there is runs the wrong way for the northern hemisphere, which is why the picture misleads so reliably.

What actually causes the seasons

Earth's axis is tilted 23,5° and stays pointing the same way all year. That is the entire answer.

For half the orbit the southern hemisphere leans towards the Sun. For the other half it leans away. Leaning towards the Sun does two things at once, and a learner needs both:

  1. The Sun climbs higher in the sky, so the same sunlight is squeezed onto a smaller patch of ground. More energy per square metre. That is intensity
  2. The days get longer, so that more intense sunlight also arrives for more hours

Both together make summer. Tilt away and you get a low Sun spread thinly over the ground, for fewer hours: winter.

Date What is happening In South Africa
21 December Southern hemisphere tilted towards the Sun Summer solstice. Longest day
21 March Neither hemisphere tilted towards the Sun Autumn equinox. Day and night nearly equal
21 June Southern hemisphere tilted away Winter solstice. Shortest day
21 September Neither hemisphere tilted towards the Sun Spring equinox

Notice that the solstices are the other way round from the northern hemisphere. December is midsummer here and midwinter in Europe, from the same orbit and the same tilt. That swap is the proof that distance is not the cause: if it were, both hemispheres would have summer at the same time.

Where this fits in the curriculum

Subject Natural Sciences
Grade 7
Term 4
Strand Planet Earth and Beyond
Topic The relationship of the Sun to the Earth
Status Class activities. Term 4 carries no formal assessment
Marks The topic revision is 25. Our worksheet is 40

The orbit drawing, and how to fix it in one minute

The problem is not what the page says. It is what the page draws.

The words usually read that the orbit is almost circular, and the key concepts usually credit the seasons to the tilt and to the intensity of sunlight. Both correct. Then the diagram shows a long oval.

The fix: draw it yourself with a loop of string and two pins. Put the pins 2 mm apart on a page, loop a 30 cm string round them, and trace the ellipse with a pencil held against the loop. You will draw something indistinguishable from a circle, and that is Earth's orbit at the correct shape.

Then say the number that settles it. Earth is about 147 million km from the Sun in early January and about 152 million km in early July. That is a difference of roughly 3 per cent, and we are nearest in our summer and furthest in our winter. If distance drove the seasons, it would make our summers slightly hotter and Europe's slightly milder, which is a small real effect and nothing like the seasons themselves.

The Sun is not 6 000 °C, and it is not a ball of gas

What you will read What is actually the case
"The surface temperature of the Sun is about 6 000 °C" The photosphere is about 5 500 °C. 6 000 is the figure in kelvin, relabelled as Celsius. The two scales differ by 273, and somewhere along the line the unit was swapped and the number kept
"A huge ball of very hot gas" It is plasma. At those temperatures the electrons are stripped off the atoms, which makes it a fourth state of matter that conducts electricity and responds to magnetic fields. That is why the Sun has sunspots and flares, and a ball of gas would not
"Eventually it will burn out" It fuses hydrogen into helium. Burning needs oxygen and there is none in space

Our own Grade 8 solar system page already publishes 5 500 °C, so our two pages agree with each other and disagree with the printed figure. That is worth knowing before a learner brings both to class.

Day length, and the part that surprises a class

The circle of illumination is the line around Earth separating the lit half from the dark half. Because the axis is tilted, that circle does not pass through the poles, and that is what makes days longer in one hemisphere than the other.

Where What happens over the year
The equator Day and night are about equal all year. Twelve hours each, every month
Johannesburg, 26° S About 14 hours of daylight in December, about 10 in June
66½° S, the Antarctic Circle On 21 June the Sun does not rise at all. The whole line of latitude stays inside the dark half

The equator having no real seasons is the fact that convinces a class. It is the same distance from the Sun as everywhere else, it gets the most intense sunlight of anywhere, and it has no summer and no winter, because its tilt towards the Sun barely changes.

Solar energy and life, and one absolute statement that fails

You will read that the energy Earth receives from the Sun sustains ALL life on Earth.

It does not quite. Deep-sea hydrothermal vent communities live kilometres below any sunlight, on chemical energy from the vents. So do bacteria living in rock far underground. The process is called chemosynthesis, and it is the one real exception.

Say "almost all life" and the sentence becomes true, and considerably more interesting.

And one claim to soften for a different reason: you will read that one reason there is no life on other planets is that they are too hot or too cold. No life has been found elsewhere, which is a different statement from there being none. Stating it as settled fact closes off the most interesting open question in the topic, and a Grade 7 is perfectly able to hold "we have not found any yet".

Where the Sun's energy genuinely does end up is worth drawing as a chain: sunlight into plants by photosynthesis, plants into animals, and some of both buried and compressed over millions of years into coal, oil and gas. A tank of petrol is stored sunlight. Our Grade 8 photosynthesis page does the first step properly, and the sources of energy topic does the last one.

One small thing on fossil fuel formation: it is usually attributed to pressure alone. Heat, time and the absence of oxygen matter at least as much. Without the missing oxygen the plants would simply have rotted.

Running the globe and torch model so it works

  1. Tilt the globe and keep it tilted. The commonest mistake is straightening it up as you carry it round. The axis points the same way all year
  2. Keep the torch at the same height as the equator, not above it
  3. Walk the globe anticlockwise seen from above the north pole, and spin it west to east
  4. Stop at each quarter and look at where the light falls, not at how far away you are
  5. Mark your own town with a sticker before you start. The whole point is what happens to that dot

Darken the room properly. The model fails in daylight because the shadow line is what you are trying to see.

If it does not work

Problem Cause Fix
The class concludes summer is when we are closest The orbit diagram Draw the orbit with string and two pins 2 mm apart. Then give them the January and July distances
The model shows no difference between the seasons The globe is being straightened up as it moves Tilt it once and never touch the tilt again
Both hemispheres seem to have summer together The torch is above the globe rather than level with it Torch at equator height
A learner writes the Sun is 6 000 °C Copied off the page About 5 500 °C. 6 000 is the kelvin figure
A learner writes that sunlight sustains all life The absolute wording Almost all. Hydrothermal vents run on chemosynthesis
Nothing is visible at all The room is too bright Block the windows. The shadow line is the experiment

How the 40 marks are made up

Part Marks
What causes the seasons, and what does not 12
Solstices and equinoxes, southern hemisphere 8
Day length and the circle of illumination 8
The Sun itself: temperature, state and fusion 6
Solar energy and life on Earth 6

Term 4 carries no formal assessment. The topic revision is 25 marks and it adds up correctly. Our worksheet is 40 and the split above is ours.

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

Related practicals

What you need to run it

Item Price What it is for
World globe, 30 cm R400 The one thing that makes this topic work. It comes already tilted, which is the whole lesson
Latitude and Longitude wall chart R85 For the circle of illumination and the 66½° line

Why a bought globe beats a ball. The activity has learners make a globe by drawing on a ball, and that is a fine craft exercise. But the tilt is the entire explanation of the seasons, and a ball has no axis to tilt. A globe on a stand is mounted at 23,5° permanently, so a class cannot accidentally straighten it up, which is the commonest way this model fails.

We hold five. It is not a deep line, so ask before planning a purchase order around it.

And the honest note on the rest: you need a torch and a dark room, and we do not sell either. A phone torch works. The globe is the purchase; everything else is the curtains.

Charts are in educational wall charts and the rest of the astronomy range is in astronomy.