Regiomontanus on Ptolemy, Physical Orbs, and Astronomical Fictionalism: Goldsteinian Themes in the “Defense of Theon

Regiomontanus on Ptolemy, Physical Orbs, and Astronomical Fictionalism: Goldsteinian Themes in the “Defense of Theon against George of Downloaded from http://direct.mit.edu/posc/article-pdf/10/2/179/1789149/106361402321147522.pdf by guest on 01 October 2021 Trebizond” Michael H. Shank University of Wisconsin-Madison To honor Bernard Goldstein, this article highlights in the “Defense of Theon against George of Trebizond” by Regiomontanus (1436–1476) themes that resonate with leading strands of Goldstein’s scholarship. I argue that, in this poorly-known work, Regiomontanus’s mastery of Ptolemy’s mathematical astronomy, his interest in making astronomy physical, and his homocentric ideals stand in unresolved tension. Each of these themes resonates with Gold- stein’s fundamental work on the Almagest, the Planetary Hypotheses, and al-Bitruji’s Principles of Astronomy. I flesh out these tensions with an intriguing interpretation of the history of astronomy, in which Regiomontanus contrasts the two-dimensional “eccentric astronomy” attributed to the Almagest and the Arabs with the three- dimensional spheres of the “later” astronomers. A similar contrast reappears when Regiomontanus portrays as a “ªctitious art” an astronomy that does not go beyond the accommodation of computations to the appearances. To con- clude, I use Regiomontanus’s expression to reinstate (pace Goldstein and Barker in this journal, 1998) ªctionalism as an actor’s category in Osiander and sixteenth-century astronomy. I. Introduction Sooner or later, historians of science who work in ancient, medieval, or early modern astronomy must come to terms with the work of Bernard I thank Dr. V. Soboliev and the Archive of the Russian Academy of Sciences (St. Petersburg branch), without whose hospitality and permission to use their holdings, this research would not have been possible. I also wish to thank Richard L. Kremer and David Lindberg for their criticisms and comments; the International Research and Exchanges Commission, the National Science Foundation (grant SBR-9729712), the Graduate School of the Uni- versity of Wisconsin-Madison, and especially the UW’s Institute for Research in the Hu- manities for their support. Perspectives on Science 2002, vol. 10, no. 2 ©2003 by The Massachusetts Institute of Technology 179 180 Regiomontanus on Ptolemy, Physical Orbs, and Astronomical Fictionalism Goldstein. His contributions span not only an enormous chronological range, but also multiple languages, even more cultures, and often the in- teractions between them. His work in almost every case has opened up new vistas on the ªeld. His research has frequently taken a thorough look at neglected ªgures, works, or problems, exploiting his knack for demon- strating that some marginalized topics were in fact central episodes of the history of astronomy. Downloaded from http://direct.mit.edu/posc/article-pdf/10/2/179/1789149/106361402321147522.pdf by guest on 01 October 2021 His recovery of the missing half of Book I of Ptolemy’s Planetary Hy- potheses is no doubt his most famous ªnd (Goldstein 1967), but many other important examples abound. He has brought out of the shadows several crucial issues and ªgures in the history of astronomy and its cultural pere- grinations. Consider Ibn al-Muthanna, whose tenth-century commentary on the zij of al-Khwarizmi is now available in a Goldstein edition and translation. Preserved in Hebrew, this text offers clues about the fate of Greek astronomy at the interface between India and the Arabic world. Or consider the original homocentric astronomy of the twelfth-century Arabic-writing Andalusian al-Bitruji (º. 1200), the edition and translation of which we also owe to Goldstein’s scholarship. The Latin world im- mediately picked up al-Bitruji’s Principles of Astronomy, which became grist for the cosmological mill down to the time of Copernicus. The list must be cut short, but not without mentioning a ªnal, signally important ex- ample. Consider Levi ben Gerson (d. 1344), the polymath rabbi from Bagnols in Provence, who worked at the interface between Hellenistic, Hebrew, Arabic, and Latin science. Thanks to Goldstein’s analyses, Levi has become the most original astronomer of fourteenth-century Europe (Goldstein 1985, and most recently 2001). To gauge where this scholarship has taken us, one need only notice how impoverished the ªeld looks if the insights gained from these ªgures, works, and analyses drop out of our historical consciousness. A second striking feature of Goldstein’s work is its conceptual range. He has dealt with a spectrum of problems in the history of astronomy that stretches from the geometrical foundations of theoretical models, through problems of parameters, observations, instrumentation, and the computa- tion of tables, to issues of cosmology including, in each case, the philo- sophical underpinnings and cross-cultural diffusion. He has proved himself a rare specimen of scholar—a hybrid between a hedgehog, a fox, and a gadºy, who does not shrink from asking unsettling critical questions about our favorite narratives or sources. What did Aristotle and his con- temporaries really know about planetary motion anyway? Just what was the relation between theory and observation in medieval astronomy? What difference would it make if historians of astronomy took more seri- ously Pliny’s clues about the state of the ªeld in the ªrst century? (for ex- Perspectives on Science 181 ample, Goldstein 1972 and 1997b). The list of probing questions could go on. Finally, Bernie Goldstein is one of the most cooperative scholars in the ªeld. The fact that he has co-authored articles with more than half a dozen other scholars with very diverse interests says much about the liveliness of his curiosity. In light of this short bio-bibliography, the title of my paper may seem Downloaded from http://direct.mit.edu/posc/article-pdf/10/2/179/1789149/106361402321147522.pdf by guest on 01 October 2021 tautological. If Goldstein had touched only the strands of the ªeld men- tioned above, almost any work in medieval astronomy contains some Goldsteinian themes. My title, however, aspires to be an informative. As we shall see, speciªc and strong resonances between Goldstein’s work and that of Johannes Regiomontanus (1436–1476) create challenging problems for our interpretation of the latter’s work. As historians, we can coun- tenance simultaneously such diverse, even contradictory, approaches as the mathematical framework of the Almagest, a physical world picture that models its theories in three dimensions, and a homocentric ideal. Regiomontanus, however, was trying hard to bring these disparate tradi- tions together as a working astronomer. These tensions in his outlook emerge most clearly in a long and long-neglected work that he called “The Defense of Theon against George of Trebizond.”1 This text brings together several puzzling aspects of Regiomontanus’s thought and practice and forces us to deal with them at the core of his work, instead of letting them fester on the fringes. After a brief overview of Regiomontanus’s life and the growing interest in his work during the last generation, I sketch the usual image of him as a mathematical astronomer in the mold of Ptolemy’s Almagest. While this characterization deserves a permanent place in all subsequent evaluations of his work, it conveys a surprisingly incomplete picture of his concerns and of the tensions between them. While we know about some of Regiomontanus’s answers and see how they ªt our own historical problems, the “Defense of Theon” can take us much farther in the direction of understanding what problems he had set for himself. Second, I summarize the relatively recent puzzle of Regiomontanus’s abiding interest in homocentric astronomy, that is, in an astronomy that dispenses with such fundamental Ptolemaic tools as eccentrics and epicycles. Third, I intro- duce brieºy Regiomontanus’s “Defense of Theon,” in which his mastery of traditional mathematical astronomy, his interest in making astronomy physical, and his homocentric ideals stand in direct tension. By coinci- dence, each of these themes recalls the Goldsteinian Leitmotive of the Alma- gest, the Planetary Hypotheses, and al-Bitruji’s Principles of Astronomy. 1. Richard Kremer and I are bringing out the text and collaborating on several studies. 182 Regiomontanus on Ptolemy, Physical Orbs, and Astronomical Fictionalism Finally, I lift out of the “Defense of Theon” two interrelated themes that further resonate with Goldstein’s scholarship. The ªrst is Regiomon- tanus’s intriguing interpretation of the history of astronomy, which shows how surprised, and perhaps pleased, he would have been to come across the complete version of Ptolemy’s Planetary Hypotheses. The second theme is Regiomontanus’s reºection on the status of astronomical models, which offers new insights on the much vexed question of realism and ªctionalism Downloaded from http://direct.mit.edu/posc/article-pdf/10/2/179/1789149/106361402321147522.pdf by guest on 01 October 2021 (or instrumentalism) in the astronomy of the ªfteenth and sixteenth cen- turies. Goldstein, in collaboration with Peter Barker, has in this journal made a recent contribution to the problem of Osiander’s preface in partic- ular (Barker and Goldstein 1998). I will use the “Defense of Theon” to of- fer a brief challenge to their interpretation. II. Brief biography Johannes Regiomontanus (1436–1476) ªrst appears in the university re- cords of Leipzig as Johannes molitoris (“John of the

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