Michelle Thaller
The reason for the seasons
Everyone loves to make fun of Harvard students. Extolled by many as the future leaders of our civilization, a few jabs here and there are a modest comeuppance for being the golden children of society. (Please note that this is said tongue-in-cheek, as a Harvard alum. I had a wonderful time there, and I can report that Harvard is not at all what its stereotype suggests. But they do take themselves a bit seriously sometimes, and a few jests wouldn't hurt.)
So, a few years ago, some reporters with a video camera attended a Harvard commencement ceremony and asked the cap-and-gowned graduates some "simple" general-knowledge questions. When the giddy and distracted young people got the answers wrong, people laughed. But one question that graduate after graduate missed was the deceptively simple "What causes the seasons?" Dozens of freshly minted Harvard grads confidently gave the wrong answer, or didn't have any idea at all.
At times like these, astronomers have to get down out of their Ivory Towers and actually talk to people, because it's important for people to understand what's going on around them in the sky. In truth, I don't ever remember being taught those things in any class I took at Harvard. So let's correct this oversight right now.
Probably the most common answer I get when I ask people what causes the seasons is "the Earth is closer to the Sun in the summer and farther away in winter." This makes a lot of sense. The Earth's orbit is not a perfect circle, but an ellipse with the Sun off-center. If our distance to the Sun changes, which is does, it must affect the amount of warmth we feel here on Earth.
For people who give this answer, I have only one reply: "Have you ever heard of the Southern Hemisphere?" You'd be surprised by the number of Northern Hemisphere chauvinists who aren't aware that the seasons are reversed Down Under. The Christmas/Hanukah holiday in Australia is very much like our 4th of July; folks grab a cooler, head to the beach, and throw a few prawns on the barbie. July and August, meanwhile, are the coldest months.
In fact, the Earth's proximity to the Sun has nothing to do with the seasons at all. For you trivia fans, the Earth's closest approach to the Sun occurs in December, coincidentally close to our winter solstice. The real reason many people think the seasons have to do with our distance from the Sun is that they've been deceived by astronomers. Not maliciously, of course, but many of the pictures and diagrams you see in astronomy textbooks describing the Earth's orbit are actually quite misleading.
I'm sure your science teacher wanted to make quite sure you remembered that the Earth's orbit was an ellipse, NOT a circle. The Sun is NOT at the center. And the diagrams showed you a sort of flattened, egg-shaped orbit to illustrate this point. In reality, the Earth's orbit is so close to a perfect circle that if you were shown a circle and our "elliptical" orbit side-by-side, you couldn't tell the difference.
In actual numbers, the Earth is 91 million miles away from the Sun at perihelion (the closest point to the Sun) and 95 million miles away during aphelion (farthest from the Sun). A difference of four million miles may seem like a lot, but in relative terms, it means that we are about 4% closer to the Sun during perihelion. Not enough to make any real difference.
But I would bet that you, right now, are well aware of the real reason we have seasonal changes, even if you never thought about it. During the winter months, I am profoundly annoyed by the fact that I spend almost all the daylight hours at work. It's dark when I get up (and I'm no early-bird), and dark when I leave my office around 5:00 p.m. In the summer I get at least another three hours of sunlight, when I can do a little shopping or take a nice walk around the park. The days are much shorter in the winter, and if you're especially canny, you've noticed that the Sun is a lot lower in the sky as well. During summer in Southern California, the Sun is very nearly overhead.
And that's the real kicker: the Sun is not only lower in the sky in winter, but the sunlight is weaker too. Combine those observations with the fact that the seasons are reversed in the Southern Hemisphere, and you're well on your way to figuring this mystery out -- a tilt in the Earth's axis.
Picture the Earth as having a large rod that passes straight through the center of the planet, coming out at the North and South poles. The rod represents our "axis", as we spin around it once a day. This spin is what causes our day and night, as each location on the Earth rotates towards, then away from the Sun.
Strangely, our axis is tilted compared to our orbit around the Sun. (This actually has to do with the gravitational tug of the Moon.) So instead of spinning around upright compared to the path we take around the Sun, the Earth spins at an angle. Our spin axis is tilted by about 23 degrees, compared to our orbit. And the axis always stays pointed in the same direction; it never changes orientation as we go around the Sun. So, because of this tilt, the Northern Hemisphere is angled toward the Sun for half the year and away from it for the other half.
Now comes another common misconception. Saying that the Northern Hemisphere is angled toward the Sun has nothing to do with the distance the Sun is away from us. The Sun is still the same distance away, and as we said above, that has nothing to do with the seasons anyway. Instead, tilting the Northern Hemisphere towards the Sun means that the sunlight hits the surface of the Earth more directly, which makes a huge difference in the warmth we get from the Sun.
During the long days of summer, the Sun is higher in the sky and imparts more heat to the atmosphere over the Northern Hemisphere. In the winter, the low Sun gives us much less heat, as well as annoyingly short days. In the Southern Hemisphere, everything is reversed. They get tilted toward the Sun (and get the benefit of more direct sunlight), when we're tilted away. That's why their summer takes place during our winter.
It's amazing that such a small tilt in the Earth's spin can have such huge effects on our weather, and I can completely understand why so many people don't find the reason for the seasons easy to grasp. A tilt in our axis seems vague and confusing compared with the wrong idea that our summer is caused by being closer to the Sun.
But take heart, now you can lord it over all those Harvard students.