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Why We Have Seasons

Science · Year 1–Year 5

Tap a month: see the tilt, the Sun's height and the day length.

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About the seasons

Two views of the same thing. On the left, the Earth at four points in its orbit with the tilt of its axis drawn the same way at every one of them: the line through each Earth is that axis, with N at the North Pole and S at the South, and it leans the same way all year. On the right, the same day from the ground: how high the midday Sun gets, how long the day lasts, and how long a shadow is. Tap a month, or tap the Earth itself, and both views change together.

IT IS THE TILT, NOT THE DISTANCE. Nearly every class will tell you that summer is when the Earth is closer to the Sun, and it is not: the orbit is very nearly a circle, and the Earth is actually closest to the Sun in early January, in the middle of a northern winter. What makes a season is that the axis leans, so sunlight hits one half of the Earth square on and the other at a glance, and the days are longer in the half that is leaning towards the Sun.

The orbit here is a circle seen from an angle, which is why it looks like an oval, and the far half of it is drawn paler than the near half to say so. Every one of the four Earths is exactly the same distance from the Sun, and the tool has no way to draw them otherwise.

Everything on the ground view is worked out from two numbers, the tilt of the axis and the latitude of the place, which is about 52 degrees north: the middle of England. The midday Sun is 61 degrees up in June and 15 degrees up in December, the day is 16 and a half hours long and then 7 and a half, and a shadow at noon in December is seven times the length of the same shadow in June. That last one is the shadows tool from the other end.

The Half buttons swap which hemisphere you are standing in, and the season names on the buttons change with them, because they have to: June is summer in the north and winter in the south. The two equinoxes, March and September, are identical from the ground, which is what the word means.

How to use it on your whiteboard

  1. 1Tap a season, or tap one of the four Earths, and watch both views change.
  2. 2Look at which half of the Earth is leaning towards the Sun.
  3. 3Compare the height of the midday Sun and the length of the day.
  4. 4Tap South to stand in the other hemisphere, and see the seasons swap.
  5. 5Tap Why? for the answer to “is it because we are closer to the Sun?”

Questions

Is it hotter in summer because the Earth is closer to the Sun?
No. The orbit is very nearly a circle, and the Earth is closest to the Sun in early January, which is the middle of winter in the north. Summer happens in the half of the Earth that is leaning towards the Sun, because the light hits it more directly and the days are longer.
Why does the orbit look like an oval?
Because it is a circle seen from an angle, the way a round plate looks like an oval when you tilt it. A circle drawn straight on cannot show the tilt at all, which is why every diagram of this is drawn from an angle.
What is the line through each Earth?
The axis it spins on, with N at the North Pole and S at the South. It points the same way in space all year, which is the whole reason we have seasons: it does not swing round to follow the Sun, so the northern half leans towards the Sun in June and away from it in December. The lit half of each Earth is the half facing the Sun, and that does swing round.
Why are the seasons the other way round in Australia?
Because the axis leans the same way all year. When the northern half is leaning towards the Sun, the southern half is leaning away from it, so June is summer in the north and winter in the south. Tap South to see it.
Where are the day lengths from?
They are worked out for about 52 degrees north, which is the middle of England, from the tilt of the axis. Day length depends on how far north or south you are, so the tool says where it means.