But it doesn’t move!

In the comments column to my last post Michael Fugate asked the following questions:

In 1610, what was the evidence that supported geocentrism over heliocentrism? 


Had the church proved that geocentricity was true?

Tim O’Neill and Anger Bear have already given good partial answers in the comments but I have decided to give a longer fuller answer here.

The answer to all of Michael’s questions can be summed up in four terms, tradition, common sense, physics and parallax, which I shall expand upon in that order.

Tradition plays a big role in the history of science and in particular in the transition from one major theory to a new one. An old established theory is not given up just because there is a new brash kid on the block. The situation is best summed up in a famous quote that is often falsely attributed to Thomas Kuhn but is in fact from Max Planck, who was, himself, famous for introducing a major new theory in physics:

A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.

This traditional resistance to change, a form of inherent conservatism, actually serves a useful purpose in the evolution of science. In championship boxing in a title match the challenger, if he wishes to win the title, is expected by the point judges to do more than the champion. He must be more aggressive, he must throw more punches and in general he must be significantly more active than the reigning champion. The theory is that the champion has already proved his worth, after all he is the champion, and the challenger must show that he is truly worthy to replace him. Being as good is not enough he must be better. The same is true of scientific theories an established theory has already shown that it can explain the phenomena covered by the theory and has over a given period of time proved its worth. A new theory must show that it can explain the phenomena better than the old theory and also stand up to a thorough critical examination, these things take their time and a change of theories does not take place overnight. The transition from geocentrism to heliocentrism took about one hundred and twenty years, which is about par for the course for such a large theory. When I was growing up in the 1950s and 60s relativity was well on the way to being established but it was still considered OK to cast doubts on its validity, quantum mechanics was definitely left field and very suspect. I can remember as the first tests of time dilatation were carried out using pairs of atomic clocks and a jet bomber. Nowadays both theories are completely accepted almost without question in both cases more than one hundred years after quantum theory and special relativity first saw the light of day.

In 1610 geocentrism had been the established theory of the universe in Europe since the Pythagoreans in the 6th century BCE. Eudoxus, Plato, Hipparcus, Aristotle and Ptolemaeus had all accepted that the earth was stationary and at the centre of the other celestial bodies that circled it. They all had different explanations of the mechanism of the system but the fundamentals were the same. In the period between the 8th and 15th centuries many Islamic astronomers and philosophers worked on those models but none of them questioned the basic facts. Also as a system for predicting the movements of the planets and the stars the Ptolemaic model had proved remarkably efficient for fourteen hundred years. If heliocentrism was going to usurp the throne it would have to be significantly superior. In fact it failed the first test as tables based on De revolutionibus proved to be no better than those based on the Syntaxis Mathematiké. In fact it was only when Kepler produced table based on the data of Tycho and his own planetary model, which were significantly superior to everything that had gone before that heliocentricity began to find widespread acceptance.

Tradition, in this case, was of course strongly supported by common sense. If you go out on a dark night somewhere where this is no light pollution and look up at the sky for a long period of time you should be able to note a series of things. Firstly the earth on which you are standing is rock solid and does not move. Secondly the stars all appear to be attached to a great sphere, which is slowly rotating about you at the centre. Thirdly the planets, a small number of especially bright and eye-catching stars, are also rotating around you at the centre of their apparent orbits. Congratulations you have just collected empirical evidence for geocentricity. Common sense, in this case literally that which we perceive with our senses tells us that the universe is geocentric, which brings us to physics.

Using the term physics here is not without problems. I am using it in the modern meaning of the term but at the beginning of the 17th century physics as we know it didn’t exist, which as we shall see is the whole problem, and the word physics meant something completely different in Aristotelian philosophy. Imagine you are sitting on a motorbike on a smooth straight road, you accelerate to 60 kph and then being a skilled and confident biker you take your hands from the handlebars and sit up straight. Now you take a newspaper out of your pocket and read it. This would of course be impossible, the headwind would blow the paper out of your hands and if you did manage to hold it tightly enough to spread it the wind would tear the paper in half. You were only travelling at 60 kph. If the world was spinning on its axis as dictated by heliocentrism someone standing on the equator would be whirling round not at 60 kph but in excess of 1600 kph! Even worse the whole world would be hurtling through space at over 100 000 kph! Now come on who are you kidding? Pull the other one it’s got bells on! Now of course people at the beginning of the 17th century did not have motorbikes, or newspapers for that matter, but they did have horses and carriages and knew all about headwinds. If the earth is moving at such frightening speeds why isn’t everything on it blown away? Copernicus already knew the correct answer to this problem. The earth and everything on it is contained in an envelope, we call it the atmosphere, which travels with the earth so there are no headwinds. However the physics necessary to explain this model didn’t exist in 1610. There was no concept of force or of mass, no correct definition of inertia let alone gravity how was this envelope supposed to work? What held it in place? Why wasn’t it blown away? The physics necessary to answer these questions was developed over the 17th century. Galileo in his Discorsi, who gave the laws of fall, was one of those along with Kepler, who gave the first primitive definition of force and the concept of gravity, Stevin, who first described the vector parallelogram of force before there were vectors, Beeckman, who gave the correct definition of inertia, and others who laid the foundation of physics on which Newton would build the theory of universal gravity and thus give scientific substance to Copernicus’ envelope. There are other physical problem thrown up by heliocentrism that had to be and were solved in the course of time but I think the one example is enough to show that heliocentricity was not ready to become champion in 1610. Interestingly the Jesuit astronomer Riccioli realised that if the earth rotated on its axis then, that which we call, the Coriolis effect must exist. He set up experiments to detect and because he failed to do so he concluded that the earth does not move.

Astronomically the big bummer was stellar parallax. If the earth rotated around the sun then it must be possible to detect stellar parallax when viewing the stars from opposite extremes of the earths orbit. All attempts to do so failed miserably up to the 1820s a very black mark against heliocentricity. In fact heliocentricity was accepted long before stellar parallax was finally detected. However at the beginning of the 17th century this failure weighed heavy in the arguments against heliocentricity. The alternative and true explanation that the nearest stars were so far away that the parallax was too small to be detected with the available instruments meant distances so great as to be literally inconceivable at that time. Only over time as the accepted dimensions of the universe grew larger and larger did this alternative become plausible.

All in all to believe in heliocentricity at the beginning of the 17th century was literally an act of blind faith and those that opposed it did so on solid scientific grounds and not purely out of some sense of religious bigotry as is often claimed by those who don’t know their history of science.



Filed under History of Astronomy, History of science, Myths of Science, Renaissance Science

43 responses to “But it doesn’t move!

  1. Michael Fugate

    To believe in geocentricity at the time was also a matter of blind faith. All we really know is that evidence as we know it today did not play a strong role in deciding this issue. Stellar parallax is a red herring; it doesn’t support the geocentric model either. Some people surely opposed heliocentrism purely on religious grounds – just as some Greeks were said to have opposed it on religious grounds. Admittedly, given the entrenchment of geocentrism at the time, one would need some very strong evidence to overcome it and it is to be expected that Copernicus and Galileo would be viewed with scepticism. It is difficult for any of us to get into the mind of a 17th c. European, let alone that of an ancient Greek, to understand their thought processes on this issue. To say that science was the deciding factor over religion (or vice versa) in any single person’s mind is just speculation.

    • As soon as you claimed that stellar parallax is a red herring you disqualified yourself as a rationalo discussion partner. Stellar parallax was a central dominating concern for European astronomers from the publication of De revolutionibus until its final discovery in the 19th century. To dismiss it as you do is a sign of pure ignorance.

      Geocentricity was never a matter of blind faith, all the available empirical evidence pointed very clearly and unequivocally to the fact that the earth did not move. If you continue to deny this then I challenge you to demonstrate that the earth moves using only the knowledge and instruments available at the beginning of the 17th century.

      I’ll give you a little hint to help you on your way, don’t bother to try because you can’t!

  2. Anger Bear

    Blind faith, eh? I don’t know about you, but it doesn’t occur to me that moving I’m through space faster than a sportscar while I’m sitting here. The fact that the data doesn’t support geocentrism in *hindsight* is completely irrelevant to the issue. Let’s be historians (who are boring, but intellectually honest) and not a sneering bunch of presentists (who are annoying).

  3. Ah yes, there is an obsession with suggesting that ‘x’ was wrong, and that the ‘truth’ came along and bulldozed all previous lies.

    ‘Science’ is not that simple. Many of the greatest natural philosophers found no great merit in Copernicus – as Bacon wrote: in the system of Copernicus we find many great difficulties.

    They considered, investigated, and found him wanting. Geocentrism wasn’t a matter of blind faith. It was investigated and found to be the most plausible explanation, given the evidence. So they were wrong. Big deal. The important thing is that they saw no reason to replace the then-current view – and Bacon, of all people, despised the passing down of knowledge via authority.

  4. Michael Fugate

    No it wasn’t – the only systematic evidence available in the 17th c. was the astronomical data and it was better supported by heliocentrism. This was known over 2000 years before by the Greeks. It was our esteemed historian who claimed heliocentrism was based on blind faith – I was only returning the favor. The geocentrists had blind faith in Aristotlean physics which was pretty much evidence-free. Common sense is not science.

  5. Tim O'Neill

    “All we really know is that evidence as we know it today did not play a strong role in deciding this issue.”

    You “know” this? How? Sounds to me and to others here that you are making statements about what you’d *like* to be true. It also seems to have an agenda which is warping your objectivity.

    “Stellar parallax is a red herring; it doesn’t support the geocentric model either. ”

    It does. If the earth isn’t at the centre of the system then the earth is moving around the sun. The (seeming) lack of stellar parallax indicated that this wasn’t happening. Therefore the earth wasn’t moving and it was the other bodies that were in motion. That supported geocentrism, as the Greeks and Romans concluded quite rationally.

    “To say that science was the deciding factor over religion (or vice versa) in any single person’s mind is just speculation.”

    To calmly and objectively note that science was very much part of the reasoning behind what was believed, however, isn’t “just speculation”. That’s historical fact.

    But it doesn’t fit some peoples’ modern agendas. Thus the shrieking every time those of us without agendas state this unremarkable thing …

  6. “Blind faith” seems like a much too strong claim. Once one had Kepler’s model, it just fit the astronomical data much better. Moreover, some of the traditional distinctions between the heavens and earth that were philosophically connected to geocentrism (e.g. immutability of heaven) had been shown to be wrong. Sure there were deep issues with heliocentrism but saying that it took blind faith seems to be an exaggeration.

    • Immutability of the heavens is not and never was a prerequisite for geocentricity and the Stoics for example rejected immutability but accepted geocentricity.

      Yes, things did start to change with Kepler’s work, as I have already stated above, but that was only after about 1630 after he had punlished his Epitome between 1617 and 1621 and the Rudolphine Tables in 1627, the latter being the more important of the two. The question that I answered referred specifically to 1610. I stand by the statement that in 1610 to accept helicentricity with the necessary double movement of the earth was an act of blind faith, as all the empirical evidence pointed clearly and unequivocally to a stationary earth.

  7. Anger Bear

    “No it wasn’t – the only systematic evidence available in the 17th c. was the astronomical data and it was better supported by heliocentrism. This was known over 2000 years before by the Greeks.”

    This is simply false. “The Greeks” never “knew” anything like that. Aristarchus’s theory was widely considered absurd, especially since neither he nor any of his followers (there weren’t many) ever came up with a workable mathematical model to support his claim (this is why Copernicus is not just a rehash of Aristarchus). Aristarchus belongs to an earlier age of Greek astronomy that was effectively eclipsed when people like Hipparchus hit the scene, who could account for the movements of the “wandering stars” so well by geometrical means that heliocentrism became passé. No-one after Hipparchus seems to have seriously entertained this idea. Also, Aristotelian physics is not evidence-free (the last time I lit a match, the flame went “up”) and to make a big distinction between common sense and science is again a bunch of presentist hogwash. Common sense creates default positions, which in turn impose a burden of proof on anyone who claims otherwise. Thony’s example regarding quantum physics should have made this clear to you.

  8. Ken

    This is weird. Thony C claims: “geocentrism had been the established theory of the universe in Europe since the Pythagoreans in the 6th century BCE.”

    Yet in 1616 the Catholic Decree of the Index suspending or banning books describing evidence for heliocentricism stated:

    “This Holy Congregation has also learned about the spreading and acceptance by many of the false Pythagoran doctrine, altogether contrary to the Holy Scripture, that the earth moves and the sum is motionless. . . ”

    Thony C seems to be misrepresenting Pythogorus as well as the complexity of the evolution of scientific ideas.

    Incidentally, Kepler and Galileo had no trouble explaining the lack of parallax and quantum mechanics was well established by the 1960s when I taught it in Chemistry classes. Thony C is confusing arguments of philosophical interpretation of QM (which still linger) with the fact that theory was amazingly successful and accurate by that time.

    • It is generally agreed, in fact I know of no one who decents, that the so-called Greek two sphere model of the universe i.e. the earth is a stationary sphere at the centre of the sphere of the fixed stars which rotate originated with the Pythagoreans in the 6th century BCE. The spheres of the planets inbetween came later.

      The passage you quote has an interesting history as it compounds an error made by Copernicus in the De revolutionibus. A work attributed falsely to Plutarch describes the Pythagorean Philolaus as believing that the earth at the centre of the universe revolved around a central fire on the opposite side in the orbit to an anti-earth. Copernicus quotes this pasage to as a precedent to justify his own attribution of movement to the earth. The system so described is of course not geostatic but is geocentric. The anti-earth was added in order to explaim solar eclipse and to raise the number of objects in the universe to ten, the earth plus seven planets plus the fixed star sphere plus anti-earth, a number of special sacred meaning to the Pythagoreans.
      This highly unorthodox geocentric model does not appear to have been propagated by anyone else.

      This led some people in the 16th and 17th centuries to refer incorrectly to the Copernican hypothesis as the Pythagorean hypothesis an error compounded, as I have already said, by the Church.

      You obviously don’t read what I write otherwise you would have noticed that the lack of parallax could be explained by increasing the distance to the stars an ad hoc solution that found little favour at the beginnig of the 17th century although it became the accepted explanation with time. Galileo may have used this explanation to avoid the lack of paralllax but it didn’t stop him working feverishly to develop new methods of discovering parallax because he was well aware that this was a potentially fatal flaw in the arguments for heliocentricity.

      If QM was so accepted as you claim why did John Bell bother to develop his inequality? I have a feeling with lived in parallel universes in the 1960s!

      • Ken

        I agree about the parallel universes. Modern chemistry us (and was in the 60s) firmly resting on a QM base. It is extrely successful and accurate. Your reference to Bell confirms my thought that you had been referring to the inevitable and lingering disputes over philosophical interpretation.

        As for Pythagorus – you may be right about interpretation there. The fact is that the church, the inquisition, Galileo and Kepler themselves did describe the heliocentric model as Pythagorean.

        Reading both Galileo and Kepler it is clear they had considered the problem of parallax and dud not see it as a problem. They advanced arguments against the closeness if the fixed stars. It was hardly ad hoc as it did make clear what was necessary to actually obtain the measurements eventually.

        We do that all the time. A challenging prediction like this is welcomed in scientific research and is not a reason to throw away a perfectly good idea. You have disingenuously misrepresented Galileo’s “feverish” research. The point is such an explanation is a reason to do more work in science, not stop and rest on the suggestion alone.

  9. Anger Bear

    So we’re using the Holy Congregation’s promulgations as our source on ancient Greek astronomy now, do we?

  10. Tim O'Neill

    “Kepler and Galileo had no trouble explaining the lack of parallax ”

    Proposing a hypothetical explanation that could not be (at that stage) demonstrated to be true was not enough. You can’t produce one unfounded hypothesis to prop up another one. Kepler and Galileo had to do a lot more than propose that the stars *might* be far enough away to explain the lack of an observable stellar parallax. As everyone at the time, including Kepler and Galileo, knew.

    You seem to be indulging in some desperate scrabbling around to find some way to cling to your ideological and emotionally-based position.

  11. Anger Bear

    Oh, and if heliocentrism is so blatantly obvious and only the stupid Catholics didn’t get it because of their “blind faith”, why did the Chinese, who were unencumbered by both Aristotle and the Bible, have to wait until the Jesuits – of all people – came, rather than vice versa? It’s funny how all these clamorous indictments against the Catholic Church for holding back science somehow amount to a big insult to non-European civilizations, who didn’t have the evil Church to hold them back and somehow still didn’t make it, despite being given a 1000-year headstart.

  12. Would it count as an ad hoc way of saving the phenomena to point out that you don’t need to accuse the church of scientific imbecility to find reasons to deplore the regime of Popes like Urban? However you judge Barberini’s relationship with Galileo, the man himself was absurdly vain, madly ambitious, and an undisputed master of nepotism. I suggest we consider what else was going on in those times. For example, while Galileo was going through his ordeal, which was pretty mild as those things went, a more serious ideological war was going on between Rome and its Venetian critic, Paolo Sarpi, who was a churchman himself but highly critical of church policy. Sarpi wrote a devastating history of the Council of Trent that pointed out Rome’s corruption and venality and was the target of an assassination attempt that probably occurred because Rome had trouble enforcing its will legally in Venice. Incidentally, Sarpi was a friend of Galileo and corresponded with him knowledgeably about scientific matters–you sometimes get the feeling that everybody was a renaissance man in those days, but hey, it was the renaissance.

    My point is that it is an error of perspective to think that the Galileo affair was a central issue at the time. Since Galileo was, if nothing else, a supremely gifted self promoter, he certainly wasn’t obscure; but the religious wars of the age weren’t contested over astronomy–both Protestants and Catholics were mostly on the same side of the debate over Copernicus. You can find plenty to deplore about these guys—I have an extremely low opinion of the late Renaissance/Baroque church myself—but their occasional interference with the sciences is the least of it.

    • Ken

      Thanks for the calmness and pointing out the obvious, James. I am rather shocked, as a scientist without any ideological axe to grind, to find such a hostile attitude in some of the comments towards someone who is accepted as a hero in scientific history.

      I probably diasagree with you on estimation of significance of the Galileo affair. From a scientific perspective one can see in his approach, and in his discussion of theological issues or interpretation, a key and necessary adjustment in philosophical outlook required to enable the scientific revolution to occur. I am not laying it all on him, but his writings certianly illustrate that point.

      You are probably right about the low religious significance – but I actually have no interest in that field so won’t comment on that.

      • Galileo was who he was. It is assuming facts not in evidence to assume that people who point out his sometimes dubious behavior are claiming he wasn’t a significant figure because he wasn’t a.) a Saint or b.) a positivist. I’m not a scientist, though as a technical writer and editor I’m some thing of a scientific fellow traveler and yet a guy with an essentially humanist education, which is perhaps why I get a kick out of Galileo who was simultaneous a con man and a very great scientist. Galileo, who, by the way, was an not insignificant literary critic, famously favored the ironic and not very pious Ariosto over Tasso, a poet who was much more in tune with Counter Reformation sensibilities but had the sense of humor a chunk of gneiss. Me too.

        Folks, these guys have been dead for almost 400 years. You can’t harm ’em by recognizing their foibles. You might even get a kick of their foibles.

  13. Ken

    Jim, I can’t understand why you direct your comment at me. I don’t for a minute “assume that people who point out his sometimes dubious behavior are claiming he wasn’t a significant figure because he wasn’t a.) a Saint or b.) a positivist. ” Not for a minute. I probably agree with you about his personal character. I also agree that he was an significant scientist who played an important role in the scientific revoltuion. To call him a scientific hero says exactly nothing about personal character. Nothing.

    Thie hostility I refer to has nothing to do with personal character – i don’t know that anyone has actually referred to Galileo’s character. I am referring to the hostily in the misrepresentation of the science. To claim as “Ye Olde Statistician” did that the panel of assessor theologians were a committee of scientists and that this casts Galileo as someone who was attempting to use his faith against science! To refer to Galileo’s work as frenzied becaue he recognised measurement of parallax was an important test. And what is behind the anger of “Anger Bear.”

    In recognising our scientific heroes we have absolutely no problem recognising and acknowledging negative character features. But lets not try to avoid or misrepresent the significance of people like Galileo because of some current sensitivities quite unconnected with science.

    • Anger Bear

      Dude, I would be immensely grateful if you could enlighten me as to what is behind the anger of “Anger Bear”. I’m sure it would help me coping with my condition. Wait, what?

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  15. Right. Also, there’s the point that with GR and Machian perspective there really isn’t supposed to be a “who is really moving” – ! (Yes, even with circular motion.) Sure, the Earth is “the one moving” in a simplest approximation to an inertial frame. But still, confident assertion that Earth moves instead of the Sun is flabby. That’s something Galileo should get … (he understood the sort of inertial relativity bearing his name – anyone REM more on his thoughts about circular motion, c.f. the Newton bucket etc?)

    BTW, Fourier analysis means that “in principle” we can indeed construct orbits out of epicycles, albeit clumsy and perhaps requiring infinite series for perfect match.

    Well, it makes me wonder: what apparently “justified” intuitions of today’s scientists, are misleading them now?

    • You ask: What apparently “justified” intuitions of today’s scientists, are misleading them now?

      The Geocentric model had the “advantage” that it put humanity at the centre of everything – and we all “know” that we are very very very special – so it must be true

      May I suggest that much research into the brain and human intelligence is based on the idea that we are very very very special. To confirm we are so special we have created the word “intelligent” to prove how intelligent we are.

      I am currently just setting up a Brain Storm on my blog (www.trapped-by-the-box.blogspot.com) which is hoping to look at this very issue and might interest you. Ideas that I think will come out of it could be

      (1) The basic information processing mechanisms of the human brain are no more special than the giraffe’s neck. Some parts of it have been significantly expanded to cope with the demand for more processing power to cope with an increasing cultural load.

      (2) The big difference between and the Great Apes (apart form extra processing capacity in a larger brain) is virtually all cultural – and this includes what we chose to call intelligence – which depends on what we learn – and not the basic biological properties of the neurons in our brain.

      (3) The widely used “stored program computer” model has led to the mad search for the right “intelligence algorithms”. No-one seems to have stopped ask the question “Which came first, the program or the data?”

      • Andrew Brew

        Except that the Geocentric model was not regarded as supporting a notion of humanity’s “specialness”. The centre of the universe was the bottom of the universe – everywhere you looked was up. That we were regarded as occupying the physical place of least dignity in the entire universe was a commonplace of both classical and medieval poetry.

      • Andrew, that is somewhat true but note that the part of the celestial sphere you doesn’t see at the time (including the Sun, planets etc.) is directionally “below” you. But I think it’s true, so many people overestimate the “importance” issue when it’s more a “appearance” issue.

    • isomorphismes

      So according to Sean Carroll’s GR notes (on the arXiv), Galileo’s great discovery was that of independent vector dimensions. I.e. motion ẋ[t] is independent of motion ẏ[t].

      I’ve already attributed the idea to him twice http://isomorphismes.tumblr.com/post/20541715959/categorial-decomposition-galileo Was I wrong to do so?

  16. Also, note that Mercury and Venus have to be going around the Sun anyway to explain their not circling all around our apparent sky! So people already had to think that *something* went around the Sun. Furthermore, people say the phases of Venus noted by Galileo supported heliocentrism, but those phases would be seen regardless of whether the Sun+Mercury+Venus revolved together around the Earth, nor just the latter two around the Sun along with the Earth (remember, my point about relative motion!)

    But weren’t there issue with explaining seasons, the whole axis thing? We hear so little about that.

    • Rebekah Higgitt

      The seasons were easily explicable by the notion that the sun’s path (ie the ecliptic) was at an angle to the celestial equator.

      • Thanks for saving me the trouble 😉

      • The exact value of the obliquity of the ecliptic (the tilt of the earths axis) was one of the most discussed subjects in the history of astronomy going back to at least Hipparchus. It actually varies with time.

      • Ken

        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.

        Philosophers of science often use the geocentric model as an example of bad science because of the indulgence in ad hoc solutions and lack of novel predictions. Alan Chamber, for example, writes in “What is this thing called science”:

        “The Ptolemaic explanation of retrograde motion did not constitute significant support for the program because it was artificially fixed up to fit the observable data by adding epicycles especially designed for the purpose. By contrast, the observable phenomena followed a natural way from the fundamentals of the Copernican theory without any artificial adjustment. The predictions of a theory or progam that count are those that are natural rather than contrived.”

        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.

      • isomorphismes

        How were the seasons known to be a celestial phenomenon? My common sense guess would probably be “Seasons just happen” — no relation to the sky other than the length of day maybe?

  17. Anger Bear

    Ken, I don’t get it, you admit to being ignorant of ancient astronomy but nevertheless you’re willing to make judgments regarding its reasonableness in account for the seasons. Why don’t you just educate yourself (for starters: James Evans, The History & Practice of Ancient Astronomy, New York/Oxford: Oxford University Press, 1998)? The fact of the matter is, the geocentric account of seasons is just as economical as the heliocentric one.

  18. Just a two-cents here, based on what I recall and not having access yet to the Evans book. Yes you can get seasons out of the geocentric model, but note that just having a tilted Solar sphere doesn’t cut it: you need to explain the change in relative angle: the orbit of the Sun has to change relative to the Earth’s axis (and for simplicity they sometimes allowed for rotation of Earth, just not revolution.) That means for example, circuit holding the Sun’s orbit to revolve itself over the period of a year. (Like a tilted hula hoop, the tilt diameter revolving round and round.) The heliocentric is simpler because once the Earth’s axis is considered “fixed,” then it naturally points differently relative to the Sun at different times.

    As a commenter at http://boards.straightdope.com/sdmb/showthread.php?t=501901 said:
    “The only difference was in how the Sun got north or south. We say that the Earth rotates once each day, and revolves around the Sun once per year at an angle of 23.5 degrees relative to the axis of rotation. They said that the Sun revolved around the Earth once per day, and superimposed upon that a secondary revolution, once per year, at an angle of 23.5 degrees relative to the daily revolution.”

  19. Anger Bear

    So in effect both models postulate two separate motions. The geocentrists attribute an annual movement along the ecliptic to the sun (or its sphere) and a diurnal rotation to the heavens. The heliocentrists impute both motions to the earth: an axial diurnal rotation and an annual orbit around the sun. But that doesn’t change that the earth has to perform two separate motions, neither of which is self-explanatory. How does it make their model simpler then?

  20. Anger Bear, as best I gather the seasonal motion of the Sun would be rather odd: like I said, a loop that itself turns around. With the Earth revolving, we just have straight up rotation and revolution. But removing the biggest retrograde complications was the big deal AFAICT. In any case, making the whole thing simpler in terms of inertial frames – the mechanical physics, not mere graphical relations – was the ultimate big deal. Anyway, kind of moot now isn’t it?

  21. Anger Bear

    It’s not all that moot, considering that the presentist refusal to see the merits of the geocentric model while downplaying the oddness of the geokinetic one (as it presented itself to observers back then) is one of factors that fuel this discussion. To describe the sun’s path as a “loop that itself turns around” is a serious distortion of what medieval astronomers actually had in mind when they thought about these issues in terms of celestial mechanics. For them, the solar sphere was only involved in the annual motion, while the diurnal motion was brought about by the sphere of the fixed stars, which carried over to all the lower spheres (lunar and planetary), not just the sun. This combination of two different motions is exactly what we would directly perceive when we’d look at the celestial theater in fast forward. To belief that all of this is the result of the globe on which we’re standing both rotating and orbiting at breakneck pace is where the oddity beginns. The important, point, however, is that in both the geokinetic and geostatic models you have one rotational movement that can account for the diurnal motion of *all* heavenly bodies (not just the sun). As far as the sun goes, they are exactly equivalent in the number of unproven assumptions that have to be made to make the thing work. If astronomy in the sixteenth century had only dealt with the moon and the sun, the Copernican revolution may have never happened.

    • Since nobody but nobody reads the last comment on one of these threads maybe I can safely make a screamingly obvious point here. Back in the 16th and 17th Centuries, there was a dispute between reasonable people about the motion of the heavenly bodies. What really ended the debate was not a decision on which system had fewer epicycles and assumptions, but the emergence of a persuasive dynamic explanation. So long as it’s just a matter of geometry, Tycho’s system works as well as a heliocentric system even if you consider the phases of Venus. Once you develop laws of motion and a theory of gravity, however, it becomes pretty hard to think that the itty bitty Earth is lording it over the enormous sun. It seems there was this guy named Newton…

  22. Pingback: On Galileo | My Blog

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