3 January, 2008
Today the Earth is at perihelion – that is, we are the closest we get to the Sun during our annual orbit. The planets do not travel round the Sun in perfect circles, but have elliptical orbits where the Sun is at one focal point. This means that we vary our distance from the Sun through the year and indeed the Sun is slightly bigger in the sky just now, although like the changing size of the Moon the change is so small and over so many months that you wouldn’t notice it just by looking at it. But this closeness of the Sun does bring more light and heat to the Earth. In fact overall there is about 5°C increase in temperature from aphelion (when the Sun is furthest away in July) to perihelion just now. This should mean that the summers and winters in the northern hemisphere are milder than those in the south.
And it gets worse for those in the southern hemisphere. The Earth speeds up it’s orbit the closer it gets to the Sun. Not just the Earth, but all planets do this, it’s part of the same reason that Mercury and Venus have shorter years than the Earth – they are closer to the Sun. What this means is that we are moving at our fastest in our orbit just now so the northern hemisphere has a shorter winter than the south, and a longer summer. It’s looking good for anyone up here above the tropics!
But wait, the differences don’t seem that much in reality – the Australians seem to have quite a nice summer, is there something else going on? There is in fact another major factor which affects climate: the oceans. Land heats up quicker and cools down faster than the oceans and since the southern hemisphere is mainly made up of ocean, it has a delayed reaction to the heating of the Sun, evening out the temperature changes.
It is indeed just a coincidence that perihelion occurs so close to where we have chosen to have new year, but regardless, for the next six months the Earth will be slowing down slowing down as we edge our way back out a few more million kilometres from the Sun.