Science · 3 min read
What Causes the Seasons? (It's Not How Close We Are to the Sun)
Most people think summer happens when Earth is closest to the Sun. It's actually closest in January. How the tilt of the Earth really causes the seasons.
By the Cognosc team ·
Ask people why it’s hotter in summer, and many will say the Earth is closer to the Sun then. It’s a sensible guess: stand closer to a fire and you feel more heat. It’s also wrong, and there’s a simple way to see it.
Try it
Why summer is warmer
Pick a month and a city. The yellow band is the same amount of sunlight each time.
June in London: the midday sun is 62° up and the day lasts 16.4 hours. The same sunlight is spread over 1.1 times the ground it would cover with the sun overhead, so each patch of ground gets 88% as much.
The clue that gives it away
When it’s summer in New York, London or Beijing, it’s winter in Sydney, Buenos Aires and Cape Town.
If distance from the Sun caused the seasons, the whole planet would have summer at the same time. It doesn’t. The two hemispheres have opposite seasons, so something else must be going on.
And in fact, the Earth is closest to the Sun in early January, in the middle of the northern winter. It’s furthest away in early July.
The real cause: the tilt
The Earth spins on an axis that’s tilted about 23.4 degrees compared with its path around the Sun. As the Earth orbits, that tilt keeps pointing the same way in space.
So for half the year, the northern hemisphere leans towards the Sun, and for the other half it leans away:
- Around June, the north leans towards the Sun. It’s summer in the north and winter in the south.
- Around December, the north leans away. It’s winter in the north and summer in the south.
- Around March and September, neither hemisphere leans towards the Sun. Those are the equinoxes, when day and night are roughly equal everywhere.
Why leaning towards the Sun makes it warmer
Two things change together.
The sun climbs higher
When your hemisphere leans towards the Sun, the midday sun is higher in the sky. Sunlight hits the ground more directly, so the same beam of light is concentrated on a smaller patch of ground.
Try it with a torch: shine it straight down at a table and you get a small, bright circle. Tilt it and the same light spreads into a larger, dimmer oval. Winter sunlight is the tilted torch.
The days are longer
Leaning towards the Sun also means more of each day’s spin happens in daylight. Longer days mean more hours of heating and fewer hours of cooling. Far north, summer days can last almost 24 hours.
Higher sun and longer days together make summer warm.
So does distance matter at all?
A little. The Earth’s orbit is only slightly stretched: the difference between its closest and furthest points is about 3%. That changes the sunlight we receive by about 7%, much less than the effect of the tilt. The tilt wins by a long way.
Why the hottest weather comes after the longest day
The longest day in the north is around 21 June, but the hottest weeks usually come in July and August. Land and especially oceans take time to warm up, the way a pan keeps heating after you turn the stove up. The same delay makes the coldest weather come after the shortest day.
Near the equator
Places near the equator get the sun high overhead all year, and day length barely changes. That’s why many tropical places have wet and dry seasons instead of hot and cold ones.
Test yourself
The seasons question is in the free physics misconceptions test, and the space test covers the Moon’s phases and other things we picture wrongly about the sky.