What Causes Change of Seasons?

In the academic year 2010/11, I conducted a poll among 74 students at the Faculty of Humanities and Social Sciences in Zagreb, examining their knowledge of basic scientific facts. A half of the population were freshers, the other half were students of the second, third, fourth and fifth years of their studies, with about an equal distribution within that group. One of the questions was this:

Change of seasons is caused by:

(a) variation in the angle of the Sun’s radiation due to the tilt of the Earth’s axis of rotation

(b) variation of distance of the Earth from the Sun due to the elliptical orbit of the Earth

(c) periodic variations in Solar activity

59% of the students did not know that (a) is the correct answer. A large majority of them (55% of the total population) thought that (b) was the correct answer. To many, (b) may seem an obvious answer. Of course, people who hold that belief, and who live in the Northern hemisphere, are regularly surprised to learn that Earth is closest to the Sun (perihelion) in early January, when they typically experience winter cold. Also, they immediately see that their belief is contrary to the fact that winter cold in the Northern hemisphere coincides with summer warmth in the Southern hemisphere. For if the proximity to the Sun were the cause of summer warmth, it would be so on the whole plant.

In the formulation of answer (b) I added a hint about the elliptical orbit of the Earth because I supposed that the common knowledge of the ellipticity of planetary orbits (Kepler’s First Law) would suggest an explanation for variations in the Earth’s distance from the Sun, and thus subtly endorse the false answer. However, the supposition of the ellipticity of planetary orbits is not necessary for the false belief that the change of seasons is caused by variation in distance between the Earth and the Sun.

Here is an example from Aristotle:

For this reason it is not the primary locomotion which is the cause of generation and corruption, but that in the inclined circle. For in this latter there is both continuity and being moved with two movements; for, if there is always to be continuous generation and corruption, there has always to be, on the one hand, something being moved so that these changes may not fail, and, on the other hand, two movements, to prevent there being only one of the two results. So the locomotion of the whole is the cause of the continuity, whilst the inclination is the cause of the approach and retreat. For this results in its coming to be further away at one time and nearer at another, and since the distance is unequal the movement will be irregular. So, if it generates by approaching and being near, this same thing destroys by retreating and coming to be further away. (De generatione et corruptione II.10 336a31-b7; tr. Williams)

In other words, the Sun’s annual motion along the ecliptic, which is inclined, is the cause of the Sun’s approaching and retreating from the Earth, thereby causing the seasons, in particular spring and summer, which bring about heat conducive to generation of living beings, and then also autumn and winter, which bring about cold conducive to destruction of living beings. This is very clear from another passage:

The efficient, controlling and first cause is the circle of the Sun’s revolution. For it is evident that as it approaches or recedes the Sun produces dissolution and composition and is thus the cause of generation and destruction. (Meteorologica I.9 346b22-24; tr. Lee)

Now this is puzzling for two reasons. First, Aristotle did not think that the Sun is made of fire or that it produces any heat by itself. Rather, he thought that it is the friction between the uppermost sublunary (fiery) layer and the lowermost superlunary (ethereal) layer that generates heat, but that was a subject of controversy even in the Peripatetic school, so we may put that aside.

Second, Aristotle subscribed to a Eudoxan theory of the heavens, which involves a number of nested concentric spheres with the Earth in the middle. But if the Earth is in the middle, and the Sun is carried by several concentric spheres (3 according to Eudoxus, 5 according to Callipus, 9 according to Aristotle; cf. Metaph. XII.8), how can he account for the supposed variation in the distance between the Sun and the Earth, that is for the Sun’s ‘approach’ and ‘retreat’?

One may try to solve this problem by supposing that by ‘approach’ (prosienai) and ‘retreat’ (apienai) he means merely approach to it highest point at summer solstice and retreat to its lowest point at winter solstice. But that won’t do, since Aristotle in the first passage above explicitly connects ‘approaching’ with ‘being near’ (engus) and ‘generating’ (presumably, through heat), and on the other hand ‘retreating’ with ‘being farther away’ (porrō) and ‘destroying’ (presumably, through cold). How can he have variation in ‘being near’ and ‘being farther away’ if the Sun is carried by a set of concentric spheres? The suggestions that the spheres are not perfectly regular, or that the Earth is not in the centre of these spheres, can be rejected straight away in light of Aristotle’s statements in the De caelo.

I am not aware that this problem has been raised in scholaraly literature (though my search hasn’t been thorough at all), and at the moment I do not have an answer to it. But I would welcome any hints and suggestions.

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10 Responses to What Causes Change of Seasons?

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  2. István Baksa says:

    As for the second issue, there may be a way to save Aristotle by saying that this problem only arises if we take the Earth to be a point in the middle and not a sphere that has parts. It’s true that Aristotle repeatedly says that the Earth has a point-like ratio to the surrounding universe (Meteorology, I,3. 340 a 6-8; I,14. 352 a 26-28) but he also speaks of the latitudinal differences in climate in Meteorology II,5; this shows that the spherical shape of Earth (or rather its extended nature) did have meteorological implications for him. So perhaps what he means is that the Sun gets nearer to and farther from a certain part of Earth; and if we take Earth to be a sphere in the middle, and not a point, the Sun’s distance will indeed change over time with respect to a given point on the Earth’s surface.

    • Thanks for your valiant attempt to save Aristotle’s grace, Istvan.
      I wish to say two things in reply. First, the size of the Earth is indeed said to be negligible in relation to the size of the whole universe, but a good deal of the universe are the heavens, and the shperes that carry the Sun are among the closest to the Earth. Hence, the fact that there is a point-like ration between the Earth and the rest of the universe, does not, by itself, exclude the possibility that the Sun is relatively close to the Earth.
      Which brings me to my second point. The solution you propose would work only if the Sun were very close to the earth, so that the variation in the Sun’s distance from a particular spot on the surface of the Earth would have a noticeable meteorological effect. However, there are two problems I see for this your solution. One is that Aristotle claims that the Sun is larger than the inhabited world (whatever that would amount to), so it must be pretty far away from the Earth, and certainly farther away than the Moon. I don’t think that Aristotle ever ventures a guess as to how far the Sun is from the Earth, but my feeling is that the distance would be too large to make a plausible case for the claim that the differences in distance due to the Sun’s motion along the ecliptic could have meteorological effects on a given spot on the surface of the earth. The other problem is the following. We have daily variations in distance between the Sun and any given spot on the Earth. Presumably, it is cold in the morning because the sun at the eastern horizon is further away from a given spot on the Earth than at noon. All good and well. But then how come that summer mornings are warmer than winter noons, at least in the zone that Aristotle inhabited?

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  4. Dahri says:

    I think we are dealing with that thing that Dawkins called the “Northern hemisphere chauvinism” in his “God Delusion”. I am guessing that most of the students did not remember that, if the B was the only cause of the change of seasons, that we would be seeing same weather on both hemispheres (and they have obviously forgot about that). But, it is strange, since in elementary school, in the geography classes, we were all thought about what the middle latitudes represent, and that the Sun’s radiation angle varies by each new day, and other things that influence seasonal weather. I only wonder wheter this represents a simple flaw in logic, or a lack of common knowledge.

    • Thanks for your comment, and apologies for replying belatedly.
      I do not think that we are in fact dealing with what Dawkins called the “Northern hemisphere chauvinism” for the simple reason that the ancients did not know whether anyone lived on the southern hemisphere. To be sure, some ancients supposed that people populated the other hemisphere (the antipodes), but they had no confirmation. In fact, some believed that the region around the equator was so scorching hot, that it would be impossible to get through to the temperate habitable zone of the southern hemisphere and thus obtain a confirmation. I suppose you can’t be chauvinist about X if you are unaware of the alternative Y, and hence I think it would be unfair to call the ancients chauvinists about the northern hemisphere.

      • Dahri says:

        Oh, I see I mislead you a bit with my first sentence in the commentary. I explicitly referred to the subjects of your poll as being affected with the “Northern Hemisphere chauvinism”. And as for Aristotel’s framework on variations in Sun’s movement and change of seasons, I am left clueless, but it reminds me of the same conclusion he drew on the multiformity of life that differs between the high tide and reflux. He observed the distinction claiming that some of the life adapted to the high and other to the ebb tide, but did not continue theorizing further than this simple induction, and made no more comments about it, which is a shame since we could have had a theory of evolution, at least as a hypothesis, before Lamarck and Darwin.

      • Yes, I should have read the first and the second sentence in your original comment in conjunction. Thanks for the clarification.
        There is no way Aristotel could have formulated a theory of evolution, since he believed all forms (here: natural kinds of living beings) to be immutable and eternal. That was a major metaphysical committment of Aristotle’s, and one whose influence has justly been decried as a long-standing obstacle to the discovery of evolution.
        By the way, can you give a reference for Aristotle’s views on the animals adapted to high or low tides?

  5. Dahri says:

    It was mentioned (this observation by Aristotle) in a discussion during a lecture in Ancient Greek philosophy by my professor some time ago, and he pointed out the same thing as you did, that Aristotel believed all forms to be eternal, and any idea of transmutation of life would greatly distort the equilibrium of his worldview. I’ll try to ask my professor for the reference, since I am not sure myself as to where it might be. Forgive me for inconvenience of not providing reference to my claim, but I promise to give it to you by next monday.

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