Why do we have seasons?

A cosmic ‘short story’

Even in the best universities students arrive with misconceptions, and because many engage primarily in rote learning, these misconceptions do not get remediated. Joe Novak (in his “Learning, Creating, and Using Knowledge: concept maps as facilitative tools in schools and corporations”, 2010, p. 69) gives an example: in a video developed at Harvard University, 21 out of 23 graduates and faculty members knew that the earth’s orbit was not a perfect circle (actually, they thought it was much more elliptical than is the case), but erroneously believed that the seasons on the earth (at least in the Boston area) were caused by the earth’s proximity to the sun. In the same book Novak uses a simple concept map (click here) to show the key ideas needed to understand why we have seasons. There is nothing wrong with Novak’s concept map (how could there be, he invented them), but for a long time I wanted to do a slightly different concept map of my own on the same subject (seasons). This is it. The main point is to stress yet again the importance of concept mapping to meaningful learning and structured writing. As such, it is a useful complement to the systems approach for designing effective action (see previous post). And it is fun, too!

Here comes the sun       Without the sun there can be no seasons or life, so many civilizations regarded the sun as a deity, called Helios by the Greeks. The element helium is named after this Helios. On March 7, 321, Sunday, already sacred to Christians and to the Roman Sun God Sol Invictus, was declared the day of rest by Constantine the Great. The colossal statue of Nero in Rome was transformed into a statue of Sol Invictus after his death, some two-and-a-half centuries earlier. The colosseum was named after this statue (the ‘Colossus’), of which only the huge, yet inconspicuous bottom part of the pedestal has remained. You can find it between the colosseum and the remnants of the large temple of Venus Felix and Roma Aeterna.

the earth's seasons

The Big Bang      … took place 13.8 billion years ago and produced mostly hydrogen and helium. The sun was formed only 4.6 billion years ago. Nuclear fusion produces the sun’s energy. This happens in a reactive core zone, where hydrogen is converted into helium at a temperature of 14 million Kelvin. This process is extremely violent, yet stable and ‘slow’ from our Earth point of view, with only relatively small changes in the amounts of hydrogen and helium: it has been going on for more than 4 billion years and will continue to do so for another 5 billion years, a clear testimony to the ‘efficiency’ of nuclear fusion and the gigantic size of the sun (333,000 times the weight of Earth). Much of the sun’s energy escapes in the form of light, which is composed of 10% ultraviolet, 40% visible light and 50% infrared (thermal) radiation. Only the minutest fraction of a fraction of the sun’s light reaches the Earth, fortunately.

Heating the Earth     … is a fairly complex affair, in which the earth surface plays an important role. Not to mention the greenhouse gases, of course, because there is a very important back-and-forth radiation between the earth’s surface and the greenhouse gases. This is best  treated as another subject, in another concept map (You may like to know that it is possible to combine different concept maps in a so-called knowledge model. For more information, I refer to Novak’s book, see above.) I think it is good to mention that certain bands of visible light are essential for photosynthesis, which is key to life on Earth and also produced the crude oil, natural gas and coal that supply most of our energy. And of course, the sun’s light also drives the ocean currents and wind circulation and has powered evolution over the past 3.5 billion years or more.

Wobbly Earth    Like a top (you know, the child’s toy that spins around on a point), the Earth wobbles a bit, be it in very, very slow motion. If we want to know why there are seasons, the wobbly part can be ignored as can the slightly less than perfect circle of the Earth’s orbit around the Sun. One thing is important, though: the tilted axis of rotation. This is where a picture says more than a thousand words:

summer and winter solstice

The angle of (daily) rotation is indicated by the letter ε (epsilon). The direction of axial inclination remains the same during the annual rotation of the earth around the sun. As a result the northern hemisphere, including the North Pole, receives much more solar radiation during the northern, June solstice than during the southern, December solstice, when the entire area North of the Arctic Circle doesn’t receive any direct sunlight at all for at least a full, 24-hour day. This is the main cause of seasonal change (at least in the parts away from the equator).

Angles of exposure       Another factor of importance is the angle of exposure of the Earth’s surface to the sunlight, which leads to lower average light intensity, the further one goes away from the equator. There is also a secondary, slightly compensatory effect in the form of longer day-lengths during summer, the further one goes away from the equator (If you can’t ‘see’ that, you may have a slight problem with your spatial ‘intelligence’). As a result the sun, bleak as it may be, no longer sets in the entire area North of the Arctic Circle during the summer solstice. This effect, in combination with clear skies, also results in very hot summers in many sub-tropical areas such as the Mediterranean, California and parts of Chile and the East Coast of Australia, where – during summer, that is – it is often hotter than in the tropics.

Meaningful learning       You may have liked what you read, but in fact I hope you liked the concept map even more. Perhaps you think I cluttered up this ‘cosmic short story’ with too many ideas. On the other hand, the extra ideas (e.g. about Helios) may help some of you to remember stuff better (e.g. that nuclear fusion of hydrogen – you know, the H in H-bomb – produces helium, so you will never forget what the sun is made of). No matter what you think, I remain convinced that when you look at the concept map, you are far more likely to remember and understand the contents of this entire post and will do so for a much longer time than otherwise would be the case. That’s what we call meaningful learning: understanding the linkages between daily life (‘Sunday’), history (Greeks and Romans, the H-bomb of the Cold War), evolution (sun), physics (seasons, nuclear fission), mathematics (the scales of it all), and cosmology (Big Bang).

About csl4d

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