MICHAEL MAESTLIN'S ACCOUNT OF COPERNICAN PLANETARY THEORY ANTHONY GRAFTON Department of History, University of Chicago THE PUBLICATION of Kepler's Mysterium cosmographicum (1596) was supervised by Kepler's teacher, Michael Maestlin, who added to it an appendix on Copernican planetary theory with parameters extracted from Erasmus Rein- hold's Prutenic Tables. A consideration of the appendix, here translated, and of the correspon- dence between Kepler and Maestlin during the writing of the Mysterium puts the relationship between the teacher and student in a new light. For it is evident that after Kepler had left the Academy at Tiibingen, Maestlin continued to direct his astronomical studies, providing him with advice, corrections, and, in one letter, a full and much needed exposition of Copernican theory. The appendix, which Maestlin adapted from that letter, shows the most advanced state of planetary theory at the end of the sixteenth century, and is even today lucid and penetrating. I. INTRODUCTION The Mysterium cosmographicum of Johann Kepler has never lacked students. Historians of astronomy have long realized that, as Kepler's first editor, Christian Frisch, wrote, the Mys- terium was "like a well, from which Kepler's later works were drawn.T"' For in the Mysterium Kepler began the work which led to his discovery of the true nature of planetary motion; it was there that he first posed the questions to which his "three laws" were the answers.2 Yet no one seems to have discussed the treatise which Kepler's teacher, Michael Maestlin, added to the Mysterium when he edited his student's work for publication-namely, Maestlin's appendix 1 Kepler, Opera Omnia, ed. Chr. Frisch, 1 (Frankfurt a. M. and Erlangen, 1858): p. 1. 2 J. L. E. Dreyer, A History of Astronomy from Thales to Kepler (paperback reprint; New York, 1953), pp. 373- 379, still provides by far the best analysis of the Mys- terium. J. B. J. Delambre, Histoire de l'astronomie moderne (reprinted as The Sources of Science, No. 25; New York, 1969) 1: pp. 314-324, 344-351, is also valu- able. And Frisch's Prooemium (loc. cit.) is of great use, especially for analyzing the correspondence between Kepler and Maestlin which accompanied the writing and publishingof the Mysterium. "On the Dimensions of the Heavenly Circles and Spheres, according to the Prutenic Tables, after the theory of Nicolaus Copernicus."3 Maestlin himself is largely responsible for this neglect. For in his introduction to the appendix he states the purpose of his work in a way which seems very clear, and makes his work appear somewhat trivial. Maestlin says that he thinks it useful to add to Kepler's Mysterium and Rheticus's Narratio Prima ". . . the dimensions of the spheres of the world, by which the planets are moved, according to the hypotheses of Copernicus."4 Instead of using the values given by Copernicus, Maestlin has accepted those used by Erasmus Reinhold in his Prutenic Tables (1551). For Reinhold, ... as he states in the preface of the Prutenic Tables-and as the numbersin those tables attest- . . . analysed the observations and demonstrations set out by Copernicus much more precisely and more accurately than did Copernicushimself.5 Unfortunately, however, Reinhold recorded his values for the dimensions of the planetary spheres not in the Prutenic Tables which he com- puted from them but in his Commentary on Copernicus, which, as Maestlin notes regretfully, remained unpublished because of Reinhold's early death.6 Maestlin has therefore found it necessary . . . to analyse the numbers in . . . [Reinhold's] tables, and to extract from them the values which Reinhold determined by careful analysis of observa- tions and demonstrations in order to compose those tables.7 3 The appendix is edited in Kepler, Gesammelte Werke 1, ed. Max Caspar (Munich, 1938): pp. 132-145 (hereafter this edition is cited as "Kepler, Werke"). I have also used Caspar's editions of the first edition of the Myste- rium (ibid., pp. 3-80) and of the NarratioPrima of Rhe- ticus, which Maestlin also added to the Mysterium (ibid., pp. 86-131). Caspar's notes in this volume are very helpful. 4 Kepler, Werke 1: p. 132, 11. 8-10. 6 Kepler, Werke 1: p. 132, 11. 12-15. 6 Kepler, Werke 1: p. 132, 11. 17-24. 7 Kepler, Werke 1: p. 132, 11. 25-27. PROCEEDINGS OF THE AMERICAN PHILOSOPHICAL SOCIETY, VOL. 117, NO. 6, DECEMBER 1973 523