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Aphelion: shouldn't earth be at its coldest today?

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Stephen Marsh Stephen Marsh | 15:30 UK time, Monday, 4 July 2011

Distance travelled ~ 475'968'000 km: day 185

Today July 4 is a rather special day in our annual journey around the Sun. At 15.00 GMT Earth will be at the furthest point away from the Sun it will reach all year. I know it sounds bizarre considering how warm it is today but it's true, and it's all down to our orbit.

The Earth's orbit around the Sun is not a perfect circle. It's an ellipse and the Sun does not sit at the centre, it's offset to one side. So today we are 5 million kilometres further away from the Sun than when we were closest to the Sun six months ago in January during Perihelion. And if you remember it was pretty cold in January so you might ask why isn't our proximity to the Sun in winter warming us and our increased distance away from the Sun in summer cooling us down?

sun at aphelion and perihelion

Image © 2001-2011, Anthony Ayiomamitis

The truth is our elliptical orbit and our distance from the Sun is not the primary driving force behind our climate. So what is?

Well number one is the that fact that the Earth is tilted at 23.4 degrees from vertical. During Perihelion the northern hemisphere is tilted away from the Sun, so received less solar radiation and we get winter. In our summer we are tilted towards the Sun so despite being farther away on our orbit we get increased solar radiation. Which is why today in July it's lovely and warm.

But what about the southern hemisphere? Well it's a bit more complex down there. Earth is closest to the Sun during their summer when they are tilted towards the Sun and this means they get 7% more solar radiation. Therefore you'd expect the southern hemisphere summer to be a lot warmer than the northern summer. But it's not, in fact it's actually colder.

And now at Aphelion it's winter down south, and the hemisphere is both furthest away and tilted away from the Sun. So you'd think it's winter would be a lot colder, what with the tilt pushing the hemisphere away from the Sun. Well in fact it's warmer than our winters. And that's all because of the ratio of land to oceans in the hemispheres. The northern hemisphere has a lot of land but the southern hemisphere is predominantly water with very little land and it's this that is powering the climate.

While land reacts very fast to solar heating, it warms up and cools down very quickly. The oceans react very slowly to solar energy. They take a long time to warm up and a long time to cool down. This means that at perihelion in their summer the oceans haven't absorbed enough energy to warm up the atmosphere, they are still cool from the previous winter, so they keep the temperatures cool. In the southern hemisphere winter it's the opposite. The oceans have held onto some of the heat they absorbed during the summer and are keeping the air above warm.

So even though our elliptical orbit takes us at this point of the year further away from the Sun than it will for the rest of the year, it's warm in the north because we are tilting towards the Sun and a milder winter in the south because of the actions of the oceans.


  • Comment number 1.

    When I started reading this I thought it was going to give an answer to a question that had puzzled me. A couple of years ago I was in Tierra del Fuego. The latitude south was about the same as that of Galloway is north; and it was late March, the equivalent of late September in the northern hemisphere. Yet when Dumfries would have been enjoying its usual early autumn warm spell, and when both towns were subject to the general oceanic warming effect, Ushuaia was bitterly cold with frequent snow showers which lay on the mountains down to about 50 metres. Could this difference really be all down to the North Atlantic Drift? It seems that perhaps the answer is yes, after all.


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