There is a season Turn! Turn! Turn!

To everything there is a season and a time to every purpose under the heaven.

KABOOM! That is the sound of my head hitting my desk after reading Ken Parrot’s latest comment on my blog. What was this comment and why did I find it so mind shatteringly stupid? Before I can answer that we first have to look at what provoked his comment.

Commentator Neil Bates made a rather confused and meandering comment at the end of which he posed the question:

But weren’t there issue with explaining seasons, the whole axis thing? We hear so little about that. [He’s referring to the geocentric model of the world system]

Our courageous Ken P. never one to shirk a confrontation with the evils of geocentrism rushes in with the following answer:

The seasons are well explained by the earths tilt (or the tilt of the “sun’s path” – according to a Copernican model. Most schoolchildren have no problem. But not accfording to a geocentric one.

I am not familiar with how traditionalists explained the seasons at the time. (Like the phlogiston theory the geocentric model is not of scientific interest to science today – more of interest to historians and philosophers, who sometime get the science wrong). But it would have been simply explained by adding another movement to the sun which provided an annual movement with respect to the equatorial plane.

And that was the huge elephant in the room – the need for indulgence in a huge number of artificial ad hoc adjustments to retain geocentricism.


 Whatever the traditional explanation of seasons was I don’t think it followed in a natural way from the assumption of geocentricism as they do from a heliocentric model.

Why is all of this mega stupid? Well for a whole lot of reasons actually. Firstly if one knows nothing about a subject, which Ken P. quite happily admits is here the case, then one should do some research before opening ones mouth or one is liable to put ones foot in it. Ken P. actually manages to put both feet in with his boots on.

Secondly the correct answer to Neil’s question had already been given three comments above Ken’s by Rebekah Higgitt. However our hero isn’t interested in reading what other people have to say, it’s eyes closed and head first through the wall of his own prejudices.

Thirdly, and this is the really embarrassing part the geocentric explanation of the seasons far from being ad hoc and unscientific is mathematically identical to the heliocentric one! It’s only the frames of reference that differs. As Ken P. says himself it’s so simple a school kid can understand it. So for the benefit of Neil and Ken I will explain.

 Heliocentrism: Axial tilt.

The heliocentric model of the world system say that the season occur because the earth’s axis is tilted by approximately 23° to the plane of the earth’s orbit around the sun, see diagram below:



Geocentricism: Obliquity of the ecliptic.

The geocentric model of the world system says that the seasons occur because the ecliptic (the suns apparent annual orbit around the earth) is tilted by approximately 23° to the plane of the earth’s equator, see diagram below:


Notice something? They are to all intents and purposes exactly the same diagrams because they both refer to the same thing. Axial tilt is even, incorrectly, referred to as the obliquity of the ecliptic.

Even more embarrassing for Ken P. is the fact that in every day language when talking about the solar cause of the change of the seasons everybody, including scientists, actually describe the geocentric model and not the heliocentric one.

The sun’s annual journey (you see its already started geocentric not heliocentric), in the northern hemisphere, begins on about the 21st March when the sun is directly overhead at noon on the equator, the vernal equinox (from Latin equinoxium “equality of night (and day)”). It then travels northwards for approximately ninety days until it is directly overhead at noon on the Tropic of Cancer, summer solstice, whence it turns and starts heading southwards. Tropic comes from the Greek “tropikos” pertaining to a turn i.e. the turning point in the sun’s annual journey. After approximately another ninety days it is directly overhead at noon on the equator again, about the 21st September the autumnal equinox. From here it heads southwards until about the 21st December when it is directly overhead at the Tropic of Capricorn, winter solstice, whence it turns once more and heads back up north.

Now even Ken P. should be able to explain the turn of the seasons in the geocentric world system.

As a footnote Ken P. said in the middle of his answer.

the geocentric model is not of scientific interest to science today

Sorry to have to contradict you Ken but all astronomers when beginning their training first learn geocentric astronomy because that is the way that we perforce observe the universe. Also anybody learning stellar navigation, a very scientific discipline, learns geocentric astronomy again because that is perforce the way we observe the heavens.


Filed under History of Astronomy

21 responses to “There is a season Turn! Turn! Turn!

  1. Pingback: There is a season Turn! Turn! Turn! | Whewell's Ghost

  2. Anger Bear

    I guess I’m gonna have to call PWNED on that one.

  3. Heh, well I was just asking the first time around to get some answers. Maybe it depends on what you mean by “issues” – yeah of course you can just reframe the same motions in different relative terms. But then later I noticed that and was writing, not too clearly, about in the G-CT the orbit of the Sun itself revolves around the Earth like a hoop with elevation point turning round and round – just what the illustration here shows. (It isn’t so easy to describe such things well with words?) I just said it was “more complicated” to have changing planes of rotation, than a unique Earth axis going in a unique orbital plane for the relative effect.

    But also, did they already know in 16-17th C about the precession of Equinoxes? How about the slight inconsistencies from Earth’s elliptical orbit per Sun and Stars (and irregular annalemma, etc? Maybe some complications? (But not an “in principle” issue.) Thanks OP however for much needed graphical clarification.

    My other points stand: the low relevance of phases of Mercury and Venus (since already had to be orbiting the Sun directly), also then for Galilean satellites.

    I think however that Angry Bear provided the most “interesting” commentary, almost implying he/she really believes in the geocentric model or that it is just as simple, etc. What do you really think, AB?

  4. But also, did they already know in 16-17th C about the precession of Equinoxes?

    The precession of the equinoxes is thought to have been first discovered/recognised by the Greek astronomer Hipparchus in the 2nd century BCE. There are claims that it had been discovered earlier by the Babylonians or the Egyptians but none of them has been substantiated. There is a detailed discussion of the subject in Ptolemaeus’ Syntaxis Mathematiké in the 2nd century CE.

  5. Anger Bear

    If you think I’m believing in geocentrism based on what I wrote earlier, you’re a tool. So don’t think that. That said, with regards to the change of seasons geocentrism IS just as simple as heliocentrism, your constant protestations to the contrary notwithstanding. There is nothing complicated about the basic path of the sun in the geocentric model. The reason you think otherwise is because you dissociate the sun’s diurnal motion from the diurnal motion of the rest of the heavens and then construe it as weird that the sun performs two motions at the same time. Guess what, so does the earth. But when medieval astronomers saw the sun rise and set, they didn’t say that the sun by itself was orbiting round the earth within 24 hours, but the heavens to which it was affixed. In the classical geocentric model, the diurnal motion of all heavenly bodies is accounted for by a single rotation, namely that of the sphere of the fixed stars, which moves all the other spheres. The rotation of this one sphere accounts for exactly the same phenomena that the earth’s axial rotation accounts for in the heliocentric model. Both rotations are equally simple, there is no difference in explanatory economy involved in preferring one over the other, as Oresme well knew. Likewise, the sun’s path around the ecliptic is, in terms of explanatory economy, equivalent to the earth’s orbit around the sun. I hope I have made myself clear now. If not, read a book.

  6. No, AB, I had no reason to think you literally believed in the Geocentric model, esp. since you would know about the inertial frames issue. I was just amused/bemused by your defense of its simplicity etc. which is how an actual supporter would talk.

    However it is still not quite true that they are equally simple. Remember, the planets can’t just go around the Earth per se, because of their own orbits being connected to the Sun (however imagined in mere “relative” terms!) They have to effectively still go around the Sun even if the Sun is imagined to “go around the Earth”, or the equivalent “epicycle” for that purpose has to specifically correspond to the equivalent projection of the Earth’s actual orbit around the Sun. So, those extra circles are going to have this inexplicable, orderly connection to some common “hidden” process – unlike the “arbitrary” extra ones that simply model the elliptical motion around a focus. That is the main reason the H-CM is considered “simpler” in logical terms.

  7. Anger Bear

    That’s all very nice, but also a big change of subject, since the previous discussion was about the change of seasons. Of course the heliocentric model eventually wins in terms of simplicity if you invoke planetary orbits. Nobody on this blog every disputed that.

  8. AB you’ve got a point. I still think that part of the issue is simpler (even if part of the whole picture) because again, for an orbit to precess (in the sense of the normal to its plane sweeps out a cone) is “more complicated” IMHO than for orbits to remain in their planes. But I suppose it’s just no big deal, unlike for example the shocking pretense that the “many worlds” theory in quantum mechanics can make sense (see my name link if interested.)

  9. When did the idea that the earth has diurnal rotation first appear after Plato? Did Aristotle hold to it, or was his earth totally fixed?

  10. Good post, Thony, and adds an aspect often missing in this ongoing debate – clear explanations of the issues, with pix. Quite vital.

    @ Anger Bear – no idea who you are, but I like you, just on the basis of your replies to Ken on his blog.

    @Neil Bates — best to keep to the actual point, don’t you think?

  11. Anger Bear

    Heraclides of Pontus (4th cent. BCE). Aristotelian physics did not allow for a rotating earth.

  12. Gurdur – yeah, essentially but shouldn’t it be OK to make segues into issues that at least follow from related themes, like “simple theories are better” – so “how about the example of Y …” And sure, easy to abuse. Maybe not mention my own connection though, that’s annoying and already “implicit” in urling my name.

  13. seversky

    You know, in spite of their differences I bet all these strong personalities had one thing in common. They all believed the Universe revolved around them, They all believed in an egocentric cosmology.

  14. David

    A quick follow-up: does the model described above require that the earth rotate? I believe (based on another post on this blog) that that was not a characteristic of all geocentric systems.

    • No. Diurnal rotation is not part of the equation.

      • David

        So did the sun move in annual rotation around the earth? And if so, where did day and night come from. I’m obviously missing something here–although it would be good to know, since precisely this question came up in class recently. Thanks.

  15. Viewed from the earth the sun appears to go round the earth once a day producing day and night. However the path it follows is not constant but changes incrementally day for day throughout the year. This path is the so-called ecliptic (see diagram above) and takes one year to complete. There is a brief description of this annual journey in the third paragraph under the Geocentrism diagram above. If you take a photograph of the sun at the same time everyday from the same position at the end of one year you will have something that looks like a distorted figure of eight an analemma

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