Copernican hypotheticals!

Alun Salt is doing what historians are not supposed to do and indulging in counterfactual speculations. He asks the question, “Would Copernicus have been more convincing if he’d been more accurate?” He continues:

[…] I was wondering if Copernicus would have been more convincing if he’d used ellipses in his model instead of circles. By using circles Copernicus had to use epicycles like Ptolemy, though not so many1. Still, it gave the impression that epicycles were necessary. If that’s the case then why not have a stationary Earth as well? The discovery that planetary motion would be better described by ellipses didn’t come about till Kepler’s work almost a century later.

His own answer is in the negative as he argues that it would have been too much change at one go and even less people would have accepted his work than the handful, who did in reality in the first decades after publication. I think he is wrong and I would like to explain why.

First of all in reality Copernicus could not have used elliptical orbits as the data that was available to him was too inaccurate for him or anybody else to calculate the very small eccentricity that differentiates the elliptical planetary orbits from circles. However to pursue Alun’s hypothetical we will assume that a Tycho had lived before Copernicus and he had had the necessary data available to carry out Kepler’s calculations and deduce the existence of the elliptical orbits; if this had indeed been the case then I’m relative certain that his system would have found substantially more acceptance. Why?

To understand we first have to examine the function of astronomical systems both in antiquity and in the Renaissance. Astronomical systems were expected to deliver accurate prognoses of the positions of celestial objects and the occurrences of celestial occurrences such as eclipses. A system was judged by its accuracy in this regard. The Ptolemaic system was actually quite good in this respect but over the centuries the fundamental data delivered by Ptolemaeus became more and more corrupt due to constant copying of manuscripts and so the accuracy of the prognoses was below that wished for. When Copernicus originally published the De revolutionibus it was greeted very warmly by the astronomical community because they hoped it would deliver more accurate prognoses than the Ptolemaic system. Unfortunately being based on the sane degenerate data it could not deliver, so initial enthusiasm turned into disappointment. As a historical footnote Tycho was one of those disappointed by the equal inaccuracy of both systems and this is why he set out on his amazing programme of observational astronomy to deliver new, accurate basic data on which to model astronomical systems.

Kepler’s system based on Tycho’s data delivered the required accuracy and was therefore accepted despite the fact that it had abandoned the 2000-year-old tradition of the Platonic axioms that required circular orbits with uniform motion for celestial objects. If Copernicus had been able to calculate and use the accurate elliptical orbits then his system would also have delivered accurate prognoses and have found widespread acceptance within the astronomical community.

1) I should point out that Copernicus wanted to use circles because he wished to restore the purity of the Platonic axioms that he saw as violated by Ptolemaeus’ use of the equant point. Secondly his system actually required more circles than Peuerbach’s version of Ptolemaeus’ geocentric system in use at the time.


Filed under History of Astronomy

7 responses to “Copernican hypotheticals!

  1. Pingback: Copernican hypotheticals! | Whewell's Ghost

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  3. Laszlo

    That’s interesting that you bring up counterfactuals in the history of science. I can see the difficulties with that kind of speculation when a historian is talking about political phenomena, like the rise of the Bolsheviks or the fall of the Berlin Wall.

    Copernicus, though, seems to be different. I understand there is a social or political side of science and mathematics, but I want to think that theory is more influential on how science moves along than academic cliques or wider social phenomena.

    Anyone here have any thoughts about the influence of “social” vs. theoretical factors? Maybe that distinction is the wrong one in the first place.

  4. Laszlo, first I should point out that I didn’t introduce the counterfactuals but Alun Salt did and I just followed his lead because I thought that what he had written was interesting.

    On the question of “social” versus theoretical you are of course jumping with both feet into the middle of the internal contra external debate that has been central to the historiography of science since the late 1950s and which reached a climax in the 1990s with the so-called science wars with the post modernist claims that scientific truth is only relative.

    It is a debate that to a certain extent I avoid but I have to state that I’M a social historian of science i.e. an externalist so I too am very wary of counterfactuals in the history of science.

    How theories are viewed, understood, accepted or developed is very much a product of the times in which they are created and the social, economic and political climates in which they exist.

    ‘What ifs’ tend to ignore one or more of those factors thus making the speculations invalid.

    • There are surely some ridiculous exceptions, but most of the people I read who are interested in the social history of the sciences aren’t really skeptics. The truth or falsity of scientific results just isn’t their department. That’s what scientists do and properly so. Meanwhile, science is obviously a social process: them fruitflies aren’t likely to figure out their own genetics and one has to be more Hegelian than Hegel to think that the inner logic of nature is the entelechy that works itself out more or less automatically as the secret script of intellectual history. At times I’ve been accused of post modernism myself; but far from celebrating social relativism, the interesting questions to me and many others of my despised tribe are how, under what circumstances, and to what extent people get around to giving a voice to the things. The regularities underlying the periodic table may be why chemistry is valid, but they can’t explain chemistry as a human activity. For that one needs knowledge of a historical or sociological kind.

      My problem with counterfactual history is that so many of its votaries cheat. They get a kick out of the idea that small events have huge consequences; but in making this point, they violate their own premises. Once William of Orange gets shot by a stray cannon ball at the Battle of the Boyne, the contingencies promptly cease so that the creator of the counterfactual can project an alternative history that then runs on rails without further accidents. But if things balance on a knife point at all, they presumably balance on a knife point at every moment. As a rhetorical device, counterfactuals are irresistible; and I, for one, don’t try very hard to resist ’em. But I know the difference between striving for an effect and seriously investigating the nonlinear dynamics of history.

  5. j mct

    This question doesn’t exactly concern the subjext of this post, but it is something I’ve wondered about and I think it would something that’s right up your alley.

    One thing that I never read about, but always thought mattered, as to why geocentrism was kind of the default view was that it yielded a stationary earth. The stationary earth would be a good explanation for the feeling of non-movement that a man feels when standing on terra firma, the reason one doesn’t feel like one is moving while standing on terra firma, is because one isn’t moving. Until Newton comes along, and can calculate that the force due to gravity dwarfs the pseudo-centripital force from the spinning earth, and one can compare numbers as it were, it would seem to me that one would sort of consider being on a Copernican earth, with his epicycles, as something of a carnival ride, one is moving in an orbit, one is spinning around the earth’s axis (at 1000 mph at the equator), and the earth is also doing epicycles, all that ‘movement’ at high speeds should get one a bit sick to one’s stomach.

    I never see this come up in anything I read about geo vs helio, is it because it never did? If not, how does one account for not feeling like one is moving while one is moving at rather fantastically high speeds, and not in a straight line, per the early modern and earlier experience, that one cannot feel oneself moving, prior to Newton.

    Love your site. Thanks.

  6. Pingback: More on Copernicus | AlunSalt

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