The Universe in the Classroom

To Every Season There is a Reason

Watching the Detective

Although a lot is happening during the seasons, the facts seem to fit together. The angle of 23.5 degrees pops up in different places: the latitudes, the ecliptic, the height of the Sun. And special things tend to happen on either a solstice or an equinox. In science as in murder mysteries, coincidences suggest there's an underlying cause. From these facts, we should be able to figure out the cause of the seasons. The usual approach scientists take is to come up with various ideas and try them out. Does the idea contradict a fact? If so, toss it out and try another one.

Let's begin with Fact 1. The change in temperature indicates that one of the Earth's heat sources is changing on a regular cycle. Which source might be involved?

  1. Volcanoes. Volcanoes are a heat source that changes. They are hot during eruptions and cooler in the meantime. But volcanoes erupt on their own erratic schedule, which contradicts fact 2.
  2. Human activity. People and their machines generate a lot of heat; cities are several degrees warmer than the countryside. But fact 2 seems to eliminate this as the cause of the seasons. People drive their cars and play their stereos all year round, and it has little effect on the seasons. Indeed, the seasons took place long before people affected the climate. Tree rings, for example, show a steady seasonal cycle for thousands of years.
  3. The Sun. Facts 4, 5, and 8 indicate that the seasons have something to do with the Sun. This would make sense, because the Sun is our main source of heat. If this heating changes, the whole world is affected.
Try coming up with other ideas and ask yourself, do they explain the facts? Scientists have thought about this for thousands of years, and concluded that the seasons are caused by changes in the way the Sun heats the Earth.

What is changing? Perhaps the Sun is getting bigger and smaller or, equivalently, closer and farther away. Summer is hotter because the Earth is closer to the Sun. This is the explanation that most Americans have adopted, according to surveys of scientific knowledge. The Quechua people of the Andes also favored this explanation. In their skylore, the Sun shrank when it became thirsty (during the dry season) and bloated when it gulped river water (during the rainy season).

But this idea contradicts facts 3, 6, and 7. If Earth moved toward the Sun enough to affect the weather, the Sun would look bigger. In Canada and northern Europe, for example, solar heating is four times stronger in June than in December. If size explained this increased strength, the Sun would have to appear four times bigger in area.

In fact, astronomers have found that the distance to the Sun varies by only 3 percent during the year. We are closest to the Sun in early January. This makes northern winter a few days shorter than northern summer -- the Earth moves fastest when it is closest to the Sun -- but affects temperature by 7 degrees Celsius (13 degrees Fahrenheit) at most. Besides, if fluctuating distance caused the seasons, both hemispheres would have summer at the same time. And how would a change in the distance to the Sun affect the constellations we see?

The difference between the hemispheres also eliminates the possibility that the Sun is getting brighter and dimmer. The total amount of heat coming from the Sun doesn't change. What's changing is how the heat is distributed. The seasons are a zero-sum game. When the Northern Hemisphere loses heat, the Southern Hemisphere gains heat. All's fair in love and seasons.

The Shadow Knows

The same sort of redistribution happens every day. The warmest time of day is early afternoon. But not every city can be the warmest at the same time. Each time zone has to take its turn. When New York is at its hottest, Chicago is still warming up; when Chicago is at its hottest, New York is cooling down. It depends on where the Sun is. When the Sun is overhead, it beats down on your forehead; at sunset, it doesn't feel nearly as hot.

If the changing angle of the Sun explains the daily redistribution of heat, maybe it also explains the seasonal redistribution of heat. After all, according to fact 4, the angle seems to have something to do with the seasons. It sounds plausible, but does it account for the other facts? For a start, fact 1 is satisfied. When the Sun climbs higher in the sky, its rays hit the ground more directly and heat the ground with greater intensity. When the Sun is lower in the sky, its rays hit the surface at an oblique angle, heating the ground with diminished intensity.

Think of shining a flashlight at the wall. When you shine the flashlight straight at the wall, the light is concentrated in a small, intense spot. When you tilt the flashlight, the light is diffused over a larger, dimmer area. In both cases, the amount of light remains the same; what changes is the area over which you're distributing the light.

Because the angle of the Sun changes, so does the length of the day (fact 5). If the Sun has to climb higher in the sky, it needs more time to move from east to west, so the day is longer. The length of the day, in turn, reinforces the redistribution of sunlight. The shorter days of winter reduce the total amount of heating, so it's colder.

The solar angle depends not only on the season, but also on your latitude. Although the angle varies by the same amount at every latitude (see figure 4), the effect of this variation on solar heating increases with latitude, a fact you can confirm with a little trigonometry (see Geometry of the Seasons). This explains why the seasonal swings in temperature are greater the further away you get from equator (see figure 1). Near the equator the swing is so small that you lose the transitions of spring and fall, and are left with a summer and winter that don't differ much in temperature.

What could cause the angle of the Sun to vary over the whole world? One possibility is that the Sun is sliding back and forth above a flat Earth (see figure 5). The Sun would be directly above the Tropic of Cancer on the vernal equinox, and above the Tropic of Capricorn on the autumnal equinox. Scientists are always brainstorming such scenarios, or models, and testing them out. This model would have seemed perfectly reasonable to the Babylonians, who thought the Earth was flat. But the flat-Earth model conflicts with figure 4, which shows that the solar angle oscillates by the same amount at every latitude. If the Earth were flat, the angle would oscillate less at high latitudes than at low latitudes.
sun and flat earth
Figure 5
The Sun above the Earth, if the Earth were flat. In this scenario -- or model, as scientists would call it -- the seasons occur because the Sun slides back and forth above the tropics; the variation of solar angle with latitude occurs because Sun is so close to the Earth that its rays splay out. The flat-Earth model accounts for many aspects of the seasons, but fails to predict the exact variation of the solar angle with latitude (see figure 4). On a flat Earth, the solar angle would not vary in direct proportion to latitude.

The variation in the angle of the Sun and stars with latitude allowed the ancient Greek astronomers to deduce that the Earth is round. Nowadays, everybody knows the Earth is round, but this wasn't so obvious 2,500 years ago. Scientists always insist on proof -- and figure 4 is pretty good proof that the Earth is round.

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