Mythconception: what causes the seasons?

28 February 2017

Interview with 

Tim Revell

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Tim Revell has been celebrating the seasons in this week’s mythconception…

As we begin to emerge from winter in the northern hemisphere and the southern hemisphere slides towards autumn, it's high time to debunk a common seasonal mythconception…

Which is that, because the Earth is tilted, when the northern hemisphere leans toward the sun it's a little bit closer to our heat source and so the northern hemisphere experiences summer. Meanwhile, the southern hemisphere is leaning away, so is a little bit farther off, and so it experiences winter. As the distance changes, so do the seasons.

The myth sounds good, intuitive even, but it's totally wrong! And here’s why.

The Earth's orbit around the sun is not a circle, it's actually a squashed circle known as an ellipse. This means that, at certain times of the year, our planet is quite a bit closer to the Sun than at others. In January for example, the Earth is 5 million kilometres closer to the Sun than in July.

Now this does cause some slight temperature differences, but it doesn’t produce the seasons. In fact, the northern hemisphere experiences summer when the Earth is farthest from the Sun.

So what about the tilt? Well Earth's tilt is about 23.5 degrees from the vertical and always points roughly in the direction of the North Star.

In July this means that the northern hemisphere points towards the sun, and the southern hemisphere points away. Six months later, when Earth has travelled to the other side of the Sun, the roles are reversed, with the northern hemisphere tilting away and the southern hemisphere tilting towards the sun.

The seasonal tilt makes a part of Earth only a teeny weeny bit closer to the sun compared to the millions of kilometres away we are from it. So it’s not the change in distance that causes the season, but it is the angle.

During summer, sunlight consistently hits that patch of the Earth at a more direct angle, and for longer, meaning that extra heat energy is transferred to the surface, warming things up more.

So as the northern hemisphere points towards the sun, sunlight hits it a more direct angle, and the days are longer, which all adds up to more heat arriving and warmer weather. Meanwhile the southern hemisphere points away, getting sunlight at a less direct angle over a shorter day, and so it experiences winter.

Interestingly over very long periods of time, Earth's tilt changes, moving between 21.4 degrees and 24.4 degrees. This means we're actually wobbling back and forth as we move through space, but this wobble is so slow that it takes 41,000 years to complete. This is far too gradual to affect the seasons in the short term, but over long time scales we think it plays a role in the formation of ice ages.

Currently we're slowly wobbling towards the upright position. This means that very, very gradually, the seasons will become a little less extreme. But this is over such a long period of time that the wobble is not going to have a noticeable effect on our climate over the next 100 years or so. There are plenty of other factors that will dominate those particular changes.

We're also not the only planet that has seasons. Uranus lies almost completely on its side, tilting at an angle of 97 degrees and so experiences extreme seasons. Whereas Venus has hardly any tilt at all so experiences very little in the way of seasonal differences.

Why on Earth the Earth actually tilts at all, we're not quite sure. It's thought that probably early on in its history the planet was hit by something sufficiently large to knock us slightly off kilter. It’s not a bad theory: Earth certainly took a lot of hits early on; in fact one of the collisions was so large that a left-over lump became the moon! But that’s another story…

Seasons really are nothing to do with our distance away from the sun, what really matters is the angle at which the sunlight hits us. If you’re heading to the beach to top up your tan, you better hope that your part of the planet is tilting towards the sun, for an awesome angular hit of sunbeams.

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