The reasons for the seasons can be summarized in two words: tilt and revolution. That is, the tilt of the earth in space, and the fact that it revolves (orbits) around the sun. This is one of the most difficult concepts in earth science to grasp. Why? In part because of the basic science we were taught years ago. Nearly everyone learns at an early age that the closer you are to a heat source, the warmer it gets. For many of us, this was our unfortunate introduction to lit candles and matches. This experience leads us to the first misconception about the seasons, which is that the distance from the earth to the sun controls the seasons.
To compound the problem, most two-dimensional drawings show the earth in an exaggerated elliptical orbit around the sun, to drive home the point that the orbit is not completely circular. Therefore, students assume a change in distance to the heat source is why the seasons have different temperatures (Figure 1.).
In reality, the earth’s orbit is very close to being circular, and students must learn that the changes in the earth-sun distances throughout the year are, in fact, insignificant with regard to the amount of the sun’s energy received at the earth’s surface. At its furthest point (aphelion), the earth – sun distance is about 95 million miles. At its closest point (perihelion), the earth – sun distance is about 92 million miles (Figure 2). This is only a 3% change in distance to an energy source over 90 million miles away…..certainly not enough to provide seasonal temperature changes.
The earth is actually further away from the sun in July, and closer to it in January (Figure 2). Given that July is the summer in the northern hemisphere, and January is winter, students will realize the earth – sun distance does not account for the seasons.
The other major problem occurs when students are told that the earth is tilted in space, and that it tilts towards the sun in summer, and away from the sun in winter. While this is a true fact, students then mistakenly conclude that the tilt of the earth physically changes during different seasons, wobbling on its axis and tilting towards the sun in the summer, and away from the sun in the winter. That, however, is not true.
The earth’s tilt is fixed, with the axes pointing in the same direction throughout the year. The North Pole always points towards Polaris (the North Star). It is because of the revolution of the earth around the sun, along with the fixed tilt, that the tilt is towards the sun in the northern summer, and away from the sun in the northern winter (Figure 3).
The tilt causes the sun’s rays to be more focused, or concentrated, on the northern hemisphere in the summer months, and more spread out in the northern hemisphere in the winter months. An analogy would be pointing a flashlight beam at a high angle to a surface (summer), and then changing the angle of the beam to a lesser angle (winter) where the beam is more spread out.
Modeling appears to be the best way to drive the tilt and revolution concept home to students. If given models of the earth with colored dowels for the North Pole, and a central sun, students can usually determine the correct model through trial and error (Figure 4). If the students practice constructing the model several times, it’s highly likely they will have learnt it.
Once the concept of tilt and revolution is understood, other aspects of the seasons, such as what happens celestially on the solstices and equinoxes, the length of day changing with the seasons, and why certain lines of latitude (Tropic of Cancer, Capricorn, and more) are important are far easier to learn and understand, especially with a correct physical model in front of them.
To summarize, understanding the meaning of those two little words - tilt and revolution - unlocks the door to a true understanding of the reasons for the seasons.
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