It’s mid-January and you are probably contemplating whether we’ll get another snow day this week. Or maybe you are thinking about your upcoming ski trip? Or how you’ll pay your heating bills? any case, winter is probably on your mind.
Ah winter, that time of year when the Earth is farthest from the Sun. Right?
Wrong. The Earth is actually closest to the Sun in early January. It’s called the perihelion and this year it occurred on January 3rd.
My readers in the Southern Hemisphere are probably feeling pretty smug right now. After all, it’s pretty obvious that they are in the heat of summer because the Earth is just so darn close to the Sun this time of year.
OK, so maybe those Southern Hemispherians are a bit mistaken too. Yes, the Earth’s orbit is slightly elliptical, i.e., it has eccentricity. Right now the eccentricity is about 0.0167, where 0 is a perfect circle. Incidentally, the eccentricity varies on about a 100-thousand year cycle and is one of the reasons for glaciation and deglaciation patterns over the past 2 million years.
But eccentricity is not the reason for the seasons. The very fact that residents of the Northern Hemisphere and Southern Hemisphere experience different seasons at the same time should clue you in that it can’t be distance from the sun.
So what is it? It turns out that Earth’s axis is at a bit of a tilt: ~ 23.44 degrees to be exact. As the Earth revolves around the Sun over the course of a year, the axial tilt (called obliquity) means that different parts of the Earth’s surface receive direct sunlight at different times of the year. And it’s this receipt of varying intensities of solar radiation that drives temperature differences, and hence seasonality. There’s an animation here that steps you through it.
Interestingly, Earth’s tilt also varies over geologic time. It has a ~41-thousand year cycle, and right now we’re at about the middle of the range in variation of axial tilt. As tilt increases, seasonal contrasts over much of the world increase (hotter summers, colder winters), but decreased axial tilt is tied with the onset of continental glaciation. That’s because at high latitudes, when tilt is low, summers are even cooler, and more snow persists through the summer. That surviving snow forms the nucleus of glacial ice caps. We’re currently on the decreasing limb of the obliquity cycle, so human-influences aside, we should be slowly working our way towards another period of continental glaciation.
Oh, and did you know that the Earth also wobbles like a top (that’s called precession) and it varies on a 26,000 year cycle. The direction that the Earth’s axis is pointing when we we are at perhelion influences which hemisphere has stronger seasonal contrasts. But for now, let’s not confuse the issue.
The reason for the seasons is axial tilt, NOT distance from the sun.
(This post brought to you thanks to Yami’s call for our (un)favorite geologic misconceptions for the next Accretionary Wedge carnival. You have no idea how many of my students have this misconception. And then there are the ones that think global warming is occurring becuase the Earth is getting closer to the Sun, but that’s a whole ‘nother can of worms.)