Alan C. Bowen

A New View Of Early Greek Astronomy

THE STANDARD HISTORY OF GREEK ASTRONOMY emphasizes the role of planetary theory in its earlier stages, by supposing that the early astronomers aimed primarily to explain planetary phenomena. This view derives from a passage in Simplicius (sixth century A.D.), based on the lost History of Astronomy by Eudemus (fourth century B.C.), in which Plato is said to have set astronomers the task of saving planetary phenomena by means of uniform circular motions.1 Many scholars doubt Simpliciuss authority and question Platos putative role in the development of astronomical theory.2 But their alternative accounts, while none too clear, likewise assume that the early period of Greek astronomy was concerned mainly with planetary motion. Some assign Platos role to the Pythagoreans who preceded him, following Geminus (first century A.D.), who reports:

The Demarcation of Physical Theory and Astronomy by Geminus and Ptolemy

The Hellenistic reception of Babylonian horoscopic astrology gave rise to the question of what the planets really do and whether astrology is a science. This question in turn became one of defining the Greco-Latin science of astronomy, a project that took Aristotles views as a starting-point. Thus, I concentrate on one aspect of the various definitions of astronomy proposed in Hellenistic times, their demarcation of astronomy and physical theory. I explicate the account offered by Geminus and its subordination of astronomy to arguments made in physical theory about what really is the case. I then show how Ptolemy treats the same topic but maintains that this science is sufficient on its own to determine the realia it studies. In this way, I identify two moments in an obvious process of intellectual change that had profound consequences for the history of astronomy and cosmology over the next 1500 years. My hope is that this will advance our understanding of the reception of horoscopic astrology in Hellenistic times and also serve to locate Ptolemy more fully in his intellectual context.

Geminus and the concept of mean motion in Greco-Latin astronomy

ConclusionGeminus account of lunar motion in chapter 18 of hisIntroductio astronomiae is, in our view, an important contribution to Greco-Latin astronomy because, in attempting to reconstruct arithmetically (the parameters of) the Moons motion in longitude, he undermines the task astronomers had hitherto set for themselves. This undermining of a commonly acknowledged view of the purpose of astronomy is articulated in a whole new set of questions concerning the nature and place of both observation and mathematical reasoning in the science of the heavens. Yet, one must not overlook the fact thatGeminus reconstruction also indicates resources for addressing these questions. Of these resources, the most powerful proved to be the idea that irregular motion could be quantified as a systematic departure from a mean motion, and the idea that observational data could be organized and structured by means of genetic arithmetical reconstructions.But, since we limit our attention to extant treatises and decline to speculate about works or parts of works that have not survived, we must say that it would takePtolemy to discern the new direction for astronomy thatGeminus opened up and to pursue it. In part, this involved straightening out the conflated conception of mean motion in chapter 18 — the μεση κίνησίςqua arithmetic mean daily displacement can only be anapparent lunar motion in longitude and not one the Moonreally makes, but the same need not be true of the μέση κίνησιςqua periodic mean daily displacement — and determining its proper relation to real and apparent planetary motion. Indeed,Ptolemys genius lay, we think, in seeing that even though, in assimilating Babylonian astronomy, earlier and contemporary Greco-Latin writers betrayed a confused, inconsistent, and unsophisticated grasp of the proper role of arithmetic, geometry, and observation in astronomical argument [seeBowen 1994], the solution lay in a mathematical reconstruction of the observed celestial motions, in which mean motion played an essential role.

Index Of Passages Cited

Aristotles great ambition in his De caelo was to determine the nature and structure of the geocentric universe. This collection of essays addresses key epistemological and methodological issues raised by the numerous arguments that Aristotle offers.