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Friday, 15 September 2017

What causes the seasons?

Spring, summer, autumn, winter. What causes the seasons?

First, why do I need to explain this? Because it has come to my attention, that there are some people who don't understand the causes of the seasons.

The earth rotates on its axis, while it orbits in a near-circle around the sun. The rotation of the earth is the cause of there being night and day; the side facing away from the sun has night.

But the axis of rotation isn't upright with respect to the plane of the orbit. It's tilted, by about 23 degrees (the tilt wobbles; if you spin a top, you'll see the same thing, only the top wobbles faster). The period of the wobble is 41,000 years. Huh!

Some people have no idea what causes the seasons, and put it down to "it just happens", or "god did it", depending on their preferences. Some people think that the earth's orbit is elliptical (which it is, but only slightly so) and the difference between winter and summer is caused by the earth being closer to the sun, or further. A moment's thought dismisses that idea because if it were true, Australia and England would have summer at the same time, not six months apart.

No. The cause is the tilt of the axis.

Consider December and the North Pole. The sun never rises, and so cannot heat the Arctic. So it's colder. Now consider June - the sun never sets, so it's not so cold (still pretty cold, though).

But now move a bit south. To the UK, for example. In Winter, there are only eight hours of daylight, whereas in summer, there are sixteen hours of daylight! So it's not surprising that it's warmer in June than in December.

Also, in winter, the sun doesn't get so high in the sky (because of the axial tilt), so what sunlight does fall on the ground, is spread out more than it is in the summer.

And that's why summer is hotter than winter, and why summer is six months different in Australia.


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