It’s not one thing, but another

Our astronomer adds some complication to the season.

Our astronomer writes:

We are now a few days past the December solstice and a few days before Earth’s perihelion, where its orbit is closest to the Sun.  I’ve always enjoyed the incongruity of being closest to our heat source during our coldest season (in the northern hemisphere).  One can use it as an opening to teach a bit of basic astronomy: that how squarely we face the Sun is more important than how far we are away.  Here’s an analogy: you’re standing forty feet away from a bonfire at night.  (In Britain it might be a Guy Fawkes event, or if you prefer, a neo-pagan ritual anywhere).  You have one shoulder pointed toward the fire.  You’ll get warmer by turning to face the fire than you will by edging a step closer.

In the north, the two effects oppose each other and it turns out that the facing part wins.  (You can also think of it as the Sun being higher in the sky.  Astronomers seem to have no trouble switching at will between geocentric and heliocentric universes, but explaining things both ways at once can be confusing.)  That means that in the southern hemisphere they reinforce each other: the Earth is closest to the Sun in summer.  That must make the seasons in the south more extreme, right?  Hotter summers overall, and colder winters?

Well, no, it doesn’t turn out that way.  There is proportionately more ocean south of the Equator, and it takes more energy to heat water than it does to heat land; conversely, the oceans release heat more slowly during the winter.  Even though the Sun-distance effect and the facing-the-Sun effect reinforce in the south, the heat capacity of the oceans wins.  (On Mars, where there are no oceans and the distance to the Sun varies more, there are very strong differences between the seasons in each hemisphere, and in the way you’d expect.)

The poles themselves are the exceptions to each hemisphere’s rule: the North Pole is ocean, the South Pole land, and so there Antarctic winters are colder than in the Arctic.

If we look in more detail, of course, the average temperatures within each hemisphere work as we’d expect: though modified by local geography, equatorial temperatures are higher than mid-latitudes, and the poles are the coldest.  Even at the solstice a few days ago the Sun stood slightly over 23 degrees above the horizon at the South Pole, barely high enough to avoid shining directly into your eyes, while at the Equator it reached over 66 degrees–high enough that you’d consider it straight overhead if you didn’t actually measure it.  So even at the most favorable, the Pole must receive less energy than the tropics.

But it actually received more.  If you carry out the calculation, the altitude of the Sun is important, but at the Pole the Sun is above the horizon all day.  It turns out that for the twenty-four hours around the Solstice, the South Pole gets something like 25% more energy than the Equator.  The length of the day wins out over altitude of the Sun.

What should you take away from all this deliberate confusion?  Primarily, that a very plausible line of reasoning may turn out to be totally wrong.  Things that you may not think of may turn out to be more important.  And in the end you need to actually do the math: words by themselves don’t settle the matter.