THE HISTORY PRACTICE OF ANCIEN T ASTRONOMY he History e^ Practice of Ancien t Astronomy J A M ES E V A N S New York Oxford Oxford University Press 1998 Oxford University Press Oxford Ne w York Athens Aucklan d Bangko k Bogota Bueno s Aires Calcutt a Cape Town Chenna i Da r es Salaam Delh i Florenc e Hon g Kong Istanbu l Karachi Kual a Lumpur Madri d Melbourn e Mexic o City Mumba i Nairobi Pari s Sa o Paulo Singapor e Taipe i Toky o Toront o Warsa w and associated companies in Berlin Ibada n Copyright © 1998 by Oxford University Press, Inc. Published by Oxford University Press, Inc. 198 Madison Avenue, New York, New York 10016 Oxford is a registered trademark of Oxford University Press All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press. Library of Congress Cataloging-in-Publication Data Evans, James, 1948- The history and practice of ancient astronomy / James Evans. p. cm . Includes bibliographical references and index. ISBN 978-0-19-509539-5 I. Astronomy, Ancient. I . Title. QBI6.E93 199 8 5io'.938—dc21 97-1653 9 For permission to reprint, I gratefully acknowledge the following: Archiv für Orientforschung, for permission to quote from Hermann Hunger and David Pingree, MUL.APIN: An Astronomical Compendium in Cuneiform. Gerald Duckworth & Co., for permission to quote from G. J. Toomer, Ptolemy's Algamest. Harvard University Press, for permission to quote from the following volumes in the Loeb Classical Library Aristotle: On the Heavens, W. K. C. Guthrie, trans. Strabo: Geography, Horace Leonard Jones, trans. Cicero: De re publica, Clinton W. Keys, trans. Pliny: Natural History, H. Rackham, trans. The University of Chicago Press, for permission to quote from Richmond Lattimore's translation, The Iliad of Homer. The University of Wisconsin Press, for permission to reproduce data from W. D. Stahlman and O. Gingerich, Solar and Planetary Longitudes for Years —2500 to +2000 by 10-day intervals. 13 1 5 1 7 1 9 2 0 1 8 1 6 1 4 1 2 Printed in the United States of America on acid-free paper Being asked to what end he had been born, he replied, "To study the Sun and Moon and the heavens." Diogenes Laërtius, speaking of Anaxagoras. Lives and Opinions of Eminent Philosophers II, 10. I know that my day's life is marked for death. But when I search into the close, revolving spirals of stars, my feet no longer touch the Earth. Then, by the side of Zeus himself, I take my share of immortality. Epigram attributed to Ptolemy. Palatine Anthology IX, 577. This page intentionally left blank The ancient Western astronomica l tradition i s one of great richness and impressive duration. It begins with records of planet observations made by the Babylonians in the second millennium B.C. It includes the development of an astronomy based on geometrical methods and philosophical principles by the Greeks between the time of Aristotle (fourth century B.C.) and the time of Ptolemy (second century A.D.). After a period of decline, or at least of quiescence, astronomy underwent a renaissance in the Islamic Middle East in th e nint h centur y A.D. For th e nex t severa l centuries the languag e of astronomical learning was Arabic, as Greek had been before, and as Akkadian reface had been before that. This astronomical tradition culminated with the astro- nomical revolution of the sixteenth century in central Europe, where Latin was the language of scientific discourse. This history of nearly 3,000 years therefore involves contributions by the Babylonian, Greek, Arabic, and medi- eval Latin cultures. But it was the Greek period that determined the fundamen- tal character of this endeavor. This book is called The History and Practice of Ancient Astronomy. In the largest sense, its subject is the ancient astronomical tradition o f the West, which I take to encompass the period and the cultures named. But the focus of this book is the Greek period. One cannot really understand what medieval Arabic and Latin astronomy were about, nor can one understand what Coper- nicus and Kepler did in the Renaissance, without understanding Ptolemy. Of course, Greek astronomy did not develo p in a vacuum. Indeed, in our century scholars have come to appreciate how important a n influence Babylonian astronomical practice exerted on the Greeks of the Hellenistic and Roman periods. Babylonian astronomy is a complex subject, intellectually and historically rich, and fully worthy of study in its own right. I have not been able to devote space to Babylonian astronomy that would be commensu- rate with its intrinsic significance. However, I have tried to include enough to give the reader an insight into the essential character of Babylonian astron- omy, its historical development, and the nature of its influence on the Greeks. In the same way, I have not attempted to write a history of medieval Arabic astronomy or of medieval or Renaissance European astronomy. Each of these subjects, i f treated i n adequat e detail, would requir e a book o f its own . However, I have often illustrated the continuity of the Western astronomical tradition by showing what becomes of some aspect of Greek astronomy (e.g., astronomical tables) in the Middle Ages. Some subjects, such as the astrolabe, that show a rich development in the Middle Ages are treated in considerable detail. And, o f course, n o treatmen t o f Greek planetary theory coul d b e considered adequate if it omitted a discussion of its radical transformation by Copernicus in the sixteenth century. In calling this book History and Practice I pledged to stay as close and as true as possible to both. Staying close to history means bringing the reader into direct contact with the ancient sources. I have tried always to tell not only what but also how we know about the astronomy of the ancient past. Throughout the book, many extracts from ancient writers are reproduced, to allow the reader to form his or her own impression of the ancient astronomical discourse. While scholars can agree about the main outlines of the history of Western astronomy, opinion is often divided on details, and occasionally even on issues of major importance. Where the evidence is conflicting, I have not tried to hide our ignorance but have presented the case as I see it. The materia l culture of ancient astronomy is an important part of its history. The instruments used by the ancient astronomers are a part of the story, no less than the texts they wrote and studied. Many illustrations are reproduced here to provide a visual impression of the nature of the evidence on which our reconstruction of the past must be based. In our time, knowledge i s fragmented into hundreds of specialities and subspecialities. No one science occupies a central place. But in ancient Greece Vlll PREFAC E and medieval Islam, as well as in medieval Europe, astronomy held a privileged place, with important connections to philosophy and religion, as well as to art and literature. For the ancient Pythagoreans, astronomy was one of the four chief branches of mathematics, along with arithmetic (i.e., number theory), geometry, and music theory. In th e medieval universities these same four arts became the quadrivium—the upper-level sequence of courses in the arts curriculum. Thus, an introduction to astronomy remained a central part of the experience deemed essential for an adequate education. A complete history of the astronomical tradition certainly cannot leave out of account the relation of astronomy to the broader culture. Staying close to the practice of astronomy means explaining a subject in enough detai l for the reader to understand wha t the ancient astronomer s actually did. Nearly every subject that is treated in this book is treated in enough detail to permit the reader to practice the art of astronomy as it was practiced in antiquity. After working through chapter 3, the reader should be able to make a sundial by methods approximating those used by Greek and Roman astronomers. After working through chapter 7, the reader should be able to predict the next retrogradation of Jupiter, either by the methods of the Babylonian scribes or by the methods of Ptolemy. The decision to focus on astronomical practice entailed a number of com- promises. Fo r example, topics that seeme d to o comple x t o b e treated i n adequate detail without extravagant demands for space and on the reader's patience have been omitted. The bes t example of such an omission is the ancient lunar theory. Thus, while both the Babylonian and Greek planetary theories are discussed in detail, I have chosen to let the Moon go. But I am confident that the reader who has mastered Ptolemy's theories of the Sun and of Mars in this book will have no trouble with the lunar theory if he or she should pursue it elsewhere. In focusing on practice, the question naturally arises of what astronomical knowledge the reader can be assumed already to possess. I have not assumed that the reader knows any astronomy. The basic astronomical facts required for understanding the ancient texts are developed as the book progresses. But perhaps the most serious choice to be made in writing a book about astronomical practice is the selection of the appropriate level of mathematics. For, i n both Greec e and Babylonia , astronomy was already a thoroughl y mathematical subject. My goal has been to treat the astronomical concept s rigorously and accurately, but to minimize the mathematical tedium as much as possible. This is done by several different methods . First, I have followed the ancient and medieval practice of emphasizing astronomical tables. Already in Ptolemy's day handy tables were produced to make astronomy more user-friendly. These tables (for problems associated with the daily revolution of the celestial sphere and for the more complex motions of the planets) in fact served to define the practice of astronomy. Wherever in the medieval world there were tables, real astronomy was prac- ticed; where tables were lacking there were only dilettantes and dabblers. So the reader of this book will learn to use tables. And thus the reader will be prepared for any further study of Greek, Arabic, or medieval or Renaissance Latin astronomy. A second way I have found of minimizing the mathematical labor is to rely on graphical methods and on models (such as the astrolabe) whenever possible. So, for example, the reader can construct a sundial purely by graphical methods, without any computation at all. The reader can predict the position of Mars according to Ptolemy's theory by manipulating an instrument (the Ptolemaic slats) rather than by performing a tedious trigonometrical calcula- tion. Some of the necessary models can be assembled from the patterns found in the appendix to this book. PREFACE I X When a more detailed mathematical treatment of some topic seems desir- able, I usually place it in a special section or separate it off in a Mathematical Postscript, after a less mathematical treatment. This will allow readers who are on friendly terms with trigonometry to pursue a subject in more detail, without subjecting other readers to unnecessary abuse. Those who wish to skip the mathematical postscripts can do so without fear that they are missing concepts essential to later developments. In the sciences, it is common t o encounter monograph s in which th e author interrupts the development from time to time by posing problems and exercises for the reader. This is the author's way of saying, You can't be sure you understand this material unless you can use it. But the exercises and suggestions for observations that are interspersed throughout this book are unusual features for a historical work. These are meant to give the reader the chance to practice the art of the ancient astronomer. Any attempt at a grand historical synthesis or a philosophical analysis of the Greek view of nature that is not underpinned with a sound understanding of how Greek astronomy actually worked is headed for trouble. I hope that the attention to detail and the provision of exercises will also make the book useful for teaching. But every reader of the book—the general reader, the classicist who wants to know more about Greek planetary theory, the astronomer who wants to understand the early history of his or her field—is urged to work as many of the exercises as possible. There is all the difference in the world between knowing about and knowing how to do. In translations from ancient writers, pointed brackets < > enclose conjec- tural restorations to the text. Square brackets [ ] enclose words added for the sake of clarity but that have no counterparts in the original text. When the translator is not identified, the translation is my own. ACKNOWLEDGMENTS Over the years, many people helped in many ways to make my work easier and more enjoyable. In the United States: Arnold Arons, Bill Barry, Bernard Bates, Alan Bowen, H. James Clifford, Michae l Crowe, Thatcher Deane , Owen Gingerich, Thomas L. Hankins, John Heilbron, Ronald Lawson, Paul Loeb, Lilian McDermott, Rober t Mitchell, Matthew Moelter, Brian Popp, Jamil Ragep, Mark Rosenquist, Patricia Sperry, Noel Swerdlow, and Alan Thorndike. In Canada: J. L. Berggren, Hugh Thurston, and Alexander Jones. In Grea t Britain : Richard Evan s and Michael Hoskin . I n Sweden : Jöran Friberg. In Denmark: Kristian Peder Moesgaard. In France: Suzanne Debarbat, François De Gandt, Libby Grenet and Franz Grenet, Michel Lerner, Henri Hugonnnard-Roche, Alai n Segonds, Rene Taton, Georges Telier and Renée Telier, Jean-Pierre Verdet, and Christiane Vilain. In Germany: Gabi Hansen and Klaus Hansen. My warmest thanks to them all. This book is dedicated to Sharon, Elizabeth, and Virginia. Seattle, Washington J . E. September 1997