Why is it colder in winter and warmer in summer?

If you walked up to people on the street and asked them why it’s colder in the winter, what do you think most of them would say? I bet they’d say that it’s colder because the sun is farther from the earth in the winter. I’ve never done an actual survey, but I’ve asked the question in my classes several times over the years, and that’s the most common answer I get.

In summer in the northern hemisphere, the north pole points 23 degrees towards the sun, while in winter it points 23 degrees away from the sun..

Actually, the distance to the sun has practically nothing to do with the change of seasons. The earth revolves around the sun in a very nearly circular orbit, which keeps it pretty much the same distance from the sun at all times. Yes, earth’s orbit is just a tiny bit elliptical, so the distance from the sun does vary by something like 3% plus or minus, but that’s not nearly enough to produce the dramatic changes in temperature between winter and summer. In fact, winter in the northern hemisphere actually happens when the earth is that little bit closer to the sun in its orbit, which is the opposite of what so many people think. This relationship does change very slowly over time, however, so in another 10,000 years we will indeed be just a little bit farther from the sun during the northern hemisphere winter. Something to look forward to.

What does cause the change in seasons? Even if you haven’t thought much about this, you’ve probably noticed that the sun is lower in the sky in the winter, which also makes the days shorter. This is caused by the tilt of the earth’s axis relative to its orbit around the sun. In the northern hemisphere summer, the north pole is tilted 23 degrees towards the sun, which causes the sun to be higher in the sky from our perspective here on earth’s surface. In the northern hemisphere winter, the north pole is titled 23 degrees away from the sun, so the sun appears lower in the sky.

On December 21 at noon, the sun is directly overhead at the Tropic of Capricorn (23 degrees south of the equator). As you go further north or south from that latitude, the sun’s rays strike the earth’s surface at a shallower angle and thus the the sun appears lower in the sky.

The light coming to us from the sun is a stream of photons. When those photons are absorbed by the material of the earth, their energy is transformed into thermal energy, which heats up the earth’s surface and atmosphere. Each photon contributes a bit of energy, so the earth heats up more when the density of photons striking its surface is greater.

When the sun is directly overhead, rays of sunlight strike the earth’s surface head-on, which means that the photons in a given patch of sunlight are spread over the smallest possible area of the earth’s surface. But the sun is only ever directly overhead if you’re in the tropics, and even then only at noon on two days of the year. The rest of the time and everywhere else, sunlight is always hitting the earth’s surface at an angle. The further that angle is from the perpendicular (i.e., the further the sun is down towards the horizon), the more the photons in sunlight are spread over a greater area of earth’s surface.

You see an example of this spreading out of the photons in a patch of sunlight in the lengthening of shadows towards the end of the day. A shadow represents the patch of sunlight that an object is intercepting. As the shadow lengthens, the area of earth’s surface that that patch of sunlight would have been spread across if the object hadn’t gotten in the way becomes larger and larger.

Of course, this illustration is only really accurate if the object is spherical—like a beach ball, for example—because only a sphere intercepts the same area of sunlight no matter what angle the sun is coming from. So if you have the time, you could set a beach ball on the ground in the late afternoon and watch the shadow gradually get larger as the sun gets closer to setting. But don’t do it on a windy day, because the beach ball might blow away. Or you could just imagine it.

Anyway, when the sun is lower in the sky in winter, the photons in sunlight are spread more thinly over a greater area of the earth’s surface, so there is a lower density of photons being absorbed by the material of the earth and its atmosphere. Fewer photons being absorbed means less thermal energy, so that part of the earth is colder. In short, the change of seasons is caused not by a change in the distance to the sun but by a change in the angle of the sun’s rays as they strike the earth’s surface.

© Joel Benington, 2012.

All images come from Wikimedia Commons, and are used under a  Creative Commons Attribution-ShareAlike 3.0 Unported license.