THE STUDY of SATURN's RINGS 1 Thesis Presented for the Degree Of

1 THE STUDY OF SATURN'S RINGS 1610-1675, Thesis presented for the Degree of Doctor of Philosophy in the Field of History of Science by Albert Van Haden Department of History of Science and Technology Imperial College of Science and Teohnology University of London May, 1970 2 ABSTRACT Shortly after the publication of his Starry Messenger, Galileo observed the planet Saturn for the first time through a telescope. To his surprise he discovered that the planet does.not exhibit a single disc, as all other planets do, but rather a central disc flanked by two smaller ones. In the following years, Galileo found that Sa- turn sometimes also appears without these lateral discs, and at other times with handle-like appendages istead of round discs. These appearances posed a great problem to scientists, and this problem was not solved until 1656, while the solution was not fully accepted until about 1670. This thesis traces the problem of Saturn, from its initial form- ulation, through the period of gathering information, to the final stage in which theories were proposed, ending with the acceptance of one of these theories: the ring-theory of Christiaan Huygens. Although the improvement of the telescope had great bearing on the problem of Saturn, and is dealt with to some extent, many other factors were in- volved in the solution of the problem. It was as much a perceptual problem as a technical problem of telescopes, and the mental processes that led Huygens to its solution were symptomatic of the state of science in the 1650's and would have been out of place and perhaps impossible before Descartes. But once the ring-hypothesis had been formulated, it was not im- mediately accepted. It was one of a number of hypotheses and a process of evaluation had to be gone through. The most interesting and enlightening aspect of this process of evaluation is the work done by the Acoademia del Cimento which anted as referee between Huygens and Honors Fabri in a controversy between two opposing theories. Not until several adjustments had been made to Huygens' theory was it completely accepted by all important astronomers in Europe. 3 ACKNOWLEDGMENTS I wish to thank Professor A. Rupert Hall, my supervisor, and Dr. Marie Boas Hall for their invaluable help and stimulation. I am indebted to the Royal Society for, the grant which helped make this research possible, and to the staff of the library of the Royal Society for their assistance. I also thank Dr. Angus Armitage for his helpful suggestions when I started my research and for allowing me to attend his lectures, Professor C.G. Wynne of the Physics De- partment at Imperial College for his help with problems in optics, and Professoresa Maria Luisa Righini Bonelli of the Istituto e Museo di Storia della Scienza in Florence for her help in locating Italian manuscript sources. Finally, I wish to express my gratitude to' my wife for making this research possible. 4 TABLE OF CONTENTS page Introduction 7 Section I Stating the Problem 10 Chapter 1 Galileo and Scheirer 11 Chapter 2 The Telescope 24 Chapter 3 Gathering Information, 1642-1655 43 Section II Theories 68 Introduction 69 Chapter 4 Hevelius 71 Chapter 5 Roberval and Hodierna 87 Chapter 6 Wren 98 Chapter 7 Huygens 110 Section III Evaluation 131 Chapter 8 The Immediate Reception of the Ring-Theory 132 Chapter 9 Divini and Fabri versus Huygens and the Judgment of the Accademia del Cimento 148 Chapter 10 The Controversy continues 168 Chapter 11 The Acceptance of the Ring-Theory: 1660-1675 179 5 Conclusion 194 Notes 202 Bibliography 228 Appendix A Major Developments in the Knowledge about Saturn's Rings, 1675-1895 239 Appendix B Hevelius to Bonilla% 9 December 1659 . 244 Appendix C Mustachio Divisii versus Christiaan Huygens: a Reappraisal 257 Appendix D Christopher Wren's De Corpora, Saturni 274 6 Altieeimtm planetam tergeminum obeervavi (Galileo, 1610) Annulo oingitur, tenui, piano, nuagnam oohaerente, ad eoliptioam inolinato (Huygena, 1659) 7, RODUCTION Seventeenth century observational astronomy has generally been considered as a convenient source of information and anecdotes bearing on the development of celestial mechanics and the synthesis of celestial and terrestrial mechanics. But one only has to open Newton's princinia to Book III to see how great a debt Newton owed to all the men who had spent countless nights looking through their telescopes and making painstaking measurements and estimates. The conception of the Universe which held sway until the beginning of the twentieth century is usually referred to as the Newtonian Universe, and quite rightly so, but Newton's Universe was constructed from the data made available by observers of the heavens, and their observations were not fortuitous: these observers were usually trying to answer the same questions with which Newton was occupied. This is of course amply illustrated by the fact that many of the. observers who made important astronomical discoveries were not merely astronomers. One has only to think of Galileo, Huygens, Borelli, and Hooke to realise this, and it is true for the vast majority of the observers of the 'heavens with whom we shall deal in this thesis. The first thirty years after the invention of the telescope, coinciding with the latter half of Galileo's life, naturally bear Galileo's stamp. Not only was he the discoverer or at least the oo- discoverer of all the wonderful new phenomena revealed by the telescope, but his attitude towards this instrument was characteristic of the general attitude towards it. All Galileo's discoveries, with the possible exception of the phases of Venus, were unexpected and therefore fortuitous. He never made systematic series of observations for their own sake. As a matter of fact, the only sustained series of observations made by Galileo was of the satellites of Jupiter, and his purpose here was a practical one: finding longitude at sea. He was content, (as well he might be!) with, so-to-speak, skimming the cream off the top, and after his brilliant discoveries, the telescope remained for a number of years a novelty with which to show one's friend or patron the Medician stars or the strange appearances of Saturn. 8 This situation did.not Change until the 1640's. Only in that- decade did the telescope become a bona fide part of the arsenal of the professional astronomer. At this time also, the so-called Keplerian telescope came into use. It is fruitless to argue about whether Fontana, Scheiner, Jansen, Rheita, or someone else first used the Keplerian configuration of lenses. The fact that the idea had been around since at least 1611, but did not find wide applicat- ion until the 1640's is ample proof that until that decade the telescope was indeed a toy in which an erect image was essential in order to show the writing on a far off church to some dignitary. The improvement of the Keplerian,telescope between 1645 and 1675 was indeed very great. But it is a very serious over-simplif- ication to ascribe all the great astronomical discoveries made between 1655 and 1685 solely to improvements in the instrument. The satellite discovered by Huygens had already been seen in Eng- land and Danzig, but had been considered a fixed star. Huygens did not discover or see a ring: Saturn's ring was invisible when.he ' formulated his ring-hypothesis. Solar parallax was not found fort- uitously; one does not send a man to Cayenne to make fortuitous observations! The fact is that what astronomers saw and discovered after Galileo's initial discoveries depended very much on what they were looking for. It is not a coincidence that the first published tables of Jupiter's satellites did not appear until 1652 (the first reliable tables did not appear until 1668), while mentions of Kep- ler's third law were exceedingly rare before 1650 and increased rapidly after that date. Observational astronomy, after about 1645, directed itself to the important questions of the age, and the discoveries made by men like Huygens and Cassini must not be viewed outside this context. For a greater part of the seventeenth century, the planet Saturn presented a great puzzle to observational astronomers. While today Saturn is still the most fascinating planet to look at through a telescope, it was no different in the seventeenth century, even before it was decided that its strange appearances were caused by a ring. Saturn presents a convenient opportunity for the study of seventeenth century observational astronomy because the problem of its appearances runs through the subject as a continuous thread. Interest in this planet was always high and if, for some reason, that interest temporarily flagged somewhat, it was always renewed 9 by one of the periodic disappearances of the mysterious anses. But besides this, the problem of Saturn sheds light on the apploach to problems and their solutions as practised by physical scientists in the seventeenth century. For this purpose it is a very convenient problem, with a definite starting date, a definite solution, and without any philosophical biases imposed by an earlier age. It therefore offers a unique opportunity to examine the method of approach employed by physical scientists in that century and how that method changed over the years. It is therefore with a dual purpose that I undertook the research for this thesis: an interest in observational astronomy during that vital period of exploration of the heavens-- the period in which our immediate corner of the Universe took on its modern shape and di- mensions - and an interest in the working of science in the seventeenth century as illustrated by this problem and its solution.

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