Did Ptolemy make novel predictions? Launching Ptolemaic astronomy into the scientific realism debate

doi:10.1016/j.shpsa.2015.04.002

Studies in History and Philosophy of Science Part A

Volume 52, August 2015, Pages 20-34

https://doi.org/10.1016/j.shpsa.2015.04.002 Get rights and content

Highlights

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We present four novel, successful predictions of Ptolemaic astronomy.
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We launch Ptolemaic astronomy into the scientific realism debate.
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We challenge selective scientific realism with new counterexamples.

Abstract

The goal of this paper, both historical and philosophical, is to launch a new case into the scientific realism debate: geocentric astronomy. Scientific realism about unobservables claims that the non-observational content of our successful/justified empirical theories is true, or approximately true. The argument that is currently considered the best in favor of scientific realism is the No Miracles Argument: the predictive success of a theory that makes (novel) observational predictions while making use of non-observational content would be inexplicable unless such non-observational content approximately corresponds to the world “out there”. Laudan's pessimistic meta-induction challenged this argument, and realists reacted by moving to a “selective” version of realism: the approximately true part of the theory is not its full non-observational content but only the part of it that is responsible for the novel, successful observational predictions. Selective scientific realism has been tested against some of the theories in Laudan's list, but the first member of this list, geocentric astronomy, has been traditionally ignored. Our goal here is to defend that Ptolemy's Geocentrism deserves attention and poses a prima facie strong case against selective realism, since it made several successful, novel predictions based on theoretical hypotheses that do not seem to be retained, not even approximately, by posterior theories. Here, though, we confine our work just to the detailed reconstruction of what we take to be the main novel, successful Ptolemaic predictions, leaving the full analysis and assessment of their significance for the realist thesis to future works.

Section snippets

Introduction: selective scientific realism as a meta-empirical, testable thesis

Scientific realism (SR) about unobservables claims that the non-observational content of our successful/justified empirical theories is true, or approximately true. As is well known, the argument that is currently considered the best in favor of SR is a kind of abduction or inference to the best explanation, dubbed the No Miracles Argument (NMA). NMA states that the predictive success of a theory that makes (novel) observational predictions while making use of non-observational content/posits

Venus and Mercury, and only they, produce transits¹

The successful and novel prediction involved in this first case asserts that Mars, Jupiter and Saturn are always beyond the Sun (i.e. they are the “outer planets”). This does not mean that you would never find any outer planet closer to the Earth than the Sun. That is also implied by Ptolemy's theory but it is not true; during retrograde motion, Mars is closer to the Earth than the Sun. It means that whenever Mars, Jupiter or Saturn are in conjunction with the Sun, they are beyond the Sun. This

Outer planets are never eclipsed by the Earth's shadow

In this brief section we will present other successful novel prediction that follows from the same set of theoretical claims that implies the prediction of Section 2. Therefore, it could be understood as a kind of corollary of the previous section. As we noted above, a lunar eclipse is produced when the Earth passes between the Sun and the Moon and therefore the Earth's shadow obscures the Moon; in Fig. 1, Fig. 2 (right) the Moon (C) is eclipsed because it is inside the Earth's shadow cone. For

The changing phases of outer planets

It is well known that, contrary to Heliocentrism and Brahe's Geocentrism, Ptolemy's Geocentrism wrongly predicts that Venus does not show strong change in phases. The story is somewhat more complicated though, for although it is true that Ptolemy's system was not capable of predicting some of Venus's phases, it is also true that it does correctly predict other phases of the inner planets and all the phases of the outer ones. In particular, what constitutes a novel prediction is the fact that

The increasing brightness during the retrograde motion for Mars

It is usually claimed that the main reason for the rejection of Eudoxus' homocentric spheres in favor of the epicycle and deferent system was that the former cannot explain the patent increase in planetary brightness during retrograde motion. If the brightness of the source is taken as constant and variation in brightness depends only on distance, then an increase in brightness is obviously interpreted as the planet approaching the Earth. In the Eudoxian proposal, all the spheres are centered

Some objections

Ancient geocentric astronomy has generally been ignored in the ongoing scientific realism debate. We have argued here that Ptolemaic astronomy presents at least four predictions that are both successful and novel, in the sense that the theory was not designed to accommodate them, thus qualifying them as cases that deserve attention in the SR debate. Moreover, the fact that in these cases there seems to be almost nothing preserved at the theoretical level by the superseding heliocentric theory

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So far, no convincing, principled reason for only going back so far in the derivation of predictions has been proposed. Authors involved in the realism debate (e.g., Lyons, 2002, p. 68; 2006, p. 557; Carman & Díez, 2015, p. 32) often talk as if each individual counterexample to the no-miracles argument is, by realist lights, a miracle. That is, they seem to interpret the argument as stating that any individual instance of novel success brought about by a false constituent of any given theory would constitute a miracle.
In this paper, I introduce a new historical case study into the scientific realism debate. During the late-eighteenth century, the Scottish natural philosopher James Hutton made two important successful novel predictions. The first concerned granitic veins intruding from granite masses into strata. The second concerned what geologists now term “angular unconformities”: older sections of strata overlain by younger sections, the two resting at different angles, the former typically more inclined than the latter. These predictions, I argue, are potentially problematic for selective scientific realism in that constituents of Hutton's theory that would not be considered even approximately true today played various roles in generating them. The aim here is not to provide a full philosophical analysis but to introduce the case into the debate by detailing the history and showing why, at least prima facie, it presents a problem for selective realism. First, I explicate Hutton's theory. I then give an account of Hutton's predictions and their confirmations. Next, I explain why these predictions are relevant to the realism debate. Finally, I consider which constituents of Hutton's theory are, according to current beliefs, true (or approximately true), which are not (even approximately) true, and which were responsible for these successes.
The planetary increase of brightness during retrograde motion: An explanandum constructed ad explanantem
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Therefore, we have here a successful and novel prediction of a clearly false theory that the realists should face (Carman & Díez, 2015).
In Ancient Greek two models were proposed for explaining the planetary motion: the homocentric spheres of Eudoxus and the Epicycle and Deferent System. At least in a qualitative way, both models could explain the retrograde motion, the most challenging phenomenon to be explained using circular motions. Nevertheless, there is another explanandum: during retrograde motion the planets increase their brightness. It is natural to interpret a change of brightness, i.e., of apparent size, as a change in distance. Now, while according to the Eudoxian model the planet is always equidistant from the earth, according to the epicycle and deferent system, the planet changes its distance from the earth, approaching to it during retrograde motion, just as observed. So, it is usually affirmed that the main reason for the rejection of Eudoxus' homocentric spheres in favor of the epicycle and deferent system was that the first cannot explain the manifest planetary increase of brightness during retrograde motion, while the second can. In this paper I will show that this historical hypothesis is not as firmly founded as it is usually believed to be.
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Scientific Progress and Scientific Realism
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^☆: Research for this work has been funded by the research projects FFI2012-37354/CONSOLIDER INGENIO CSD2009-0056 (Spain), and PICT2010 No. 319 (ANPCyT, Argentina)/PIP No. 114-201101-00112 (CONICET, Argentina). We want to thank L. Laudan, S. French, B. van Fraassen and two anonimous referees for comments and criticisms to previous versions of this paper.

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Did Ptolemy make novel predictions? Launching Ptolemaic astronomy into the scientific realism debate☆

Highlights

Abstract

Section snippets

Introduction: selective scientific realism as a meta-empirical, testable thesis

Venus and Mercury, and only they, produce transits1

Outer planets are never eclipsed by the Earth's shadow

The changing phases of outer planets

The increasing brightness during the retrograde motion for Mars

Some objections

Studies in History and Philosophy of Science

On a Greek qualitative planetary model of the epicyclical variety

Centaurs

Simplicius on the planets and their motions

Rounding numbers: Ptolemy's calculation of the Earth-Sun's distance

Archive for History of Exact Sciences

Preservative realism and its discontents: Revisiting caloric

Philosophy of Science

De revolutionibus orbium cœlestium

The 2012 transit of Venus

Mechanical astronomy: a route to the ancient discovery of epicycles and eccentrics

The Arabic version of Ptolemy's planetary hypotheses

Transactions of the American Philosophical Society, New Series

The pre-telescopic treatment of the phases and apparent size of Venus

Journal for History of Astronomy