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.)
Thanks. I love geology. It's always good to put this depressing, cold weather back into perspective. I hadn't even thought about why my weather was cold but I was just recently thinking that it had no purpose. But ah yes, cycles. The world is full of cycles. I just happen to currently be in the wrong hemisphere.
I knew the seasons were caused by the tilt of the earth.
Can the eliptical effect be expressed as a distance from perfect circle ?
Does the tilt not place part of the earth closer to the sun during that cycle ? Is that tilt "distance" greater than the elliptal "offset".
Are the temperature differences tween the seasons caused by day length or "positioning".
The reason for the seasons is axial tilt, NOT distance from the sun.
Right! And since the earth varies as to which way it is pointed, either towards the sun which is 27 million degrees at it's center. Or towards outer space which has spots that over 400 degrees below zero, there is plenty of temperature room in-between to make us feel too hot or too cold! LOL! Thanks for the post!
Dave Briggs :~)
Thanks for writing this post. I'm non-earth-science enough to be delighted by your eye-opening explanation of something I just barely remember from fourth grade.
You'll have to let me know if you ever find a carnival on misconceptions in biology.
It seems every year I teach primate sexuality, there are a dozen guys in class who think ovulation is that time of the month when women bleed.
It never ceases to amaze me how little students know about basic science when they get to us!
I recall taking a class on atmospheric science and the lecturer told of how he'd been at some event in the USA attended by many TV weather presenters, one of whom asked him whether it was true that in Australia the seasons were different! (For the record I can assure everyone that it is certainly not Winter here right now)
I taught that last week! I love asking the students when they think perihelion is and how they look puzzled that it isn't in the summer! Were you the one that had that "Axial Tilt: It's the reason for the season" logo? I love that.
Perhaps your students are from Mars, where eccentricity seasonality is significant.
Stu: That's unbelievable. I thought TV weather presenters mostly had meteorology degrees.
Addy N: The post you are referring to is this one, but I'm not the one who designed the logo.
Lab Lemming: That made me snort. It would explain some other things too.
I think this is an issue for so many people because they have seen an image in a textbook somewhere of the earth's orbit around the sun and in the image, it is grossly elliptical (possibly to convey depth?). These images often have a tilting earth, too, but what people remember is the most obvious thing and that's the elliptical orbit. And they know from their own experiences that when you are further from a heat source, you feel colder. Often, these same people know that the seasons are reversed in the southern hemisphere, but don't make the connection between that and reason we experience winter.
And then there is the matter that some people do not realize that all parts of the earth recieve the same amount of daylight in a year, it's just partitioned out differently.
True... depending on how you define 'same amount of daylight'. Obviously annual solar energy intensity at ground level is location dependent.
Wayfarer and JasonE: Were you in my lecture yesterday?
I'd never heard that there are people who think that global warming is occurring because the earth are getting closer to the sun. People will think anything so it makes sense, but wow, where have they been?!
Most interesting and important. Not mentioned, but of course the dynamics suggested, its role in "global warming vs. glaciation". Indeed the "green house" contribution is put into a more realistic perspective in relationship to the massive, but subtle forces described. If indeed axial tilt is moving us inexorably towards "glaciation" again, maybe the silver-lining in controlling greenhouse gases, is to intelligently use it like a "thermostat" someday-- again, careful what you wish for Mr. Gore---it would have advantages of visiting Chicago some day without it being under a mile of ice!
why does the northern and southern hemisphere experience diffrent seasons at the same time