Hellenistic Astronomy: The Science in Its Contexts edited by Alan C. Bowen and Francesca Rochberg

One of the daunting challenges involved in reviewing a 750-page standard tome on a subject like astronomy is being able to evaluate all aspects of the volume, covering technical data as well as any possible impact of subject matter on other disciplines. The editors, mindful of their readership consisting of both “insiders” and “outsiders”, have taken decisive steps towards making Hellenistic astronomy accessible and comprehensible, with an appropriate balance between complex graphs and arithmetic equations and more general topics, as well as a glossary of technical terminology. The present reviewer, an unrepentant “outsider”, will attempt to focus on some key issues involving the connections between Babylonian and Greek astronomy in the period in question, as well as the impact of astronomy as a whole. Reviewed by: M. J. Geller, Published Online (2021-08-31)Copyright © 2021 by M. J. GellerThis open access publication is distributed under a Creative Commons Attribution-NonCommercial-NoDerivatives License (CC BY-NC-ND) Article PDF Link: https://jps.library.utoronto.ca/index.php/aestimatio/article/view/37730/28731 Corresponding Author: M. J. Geller,University College LondonE-Mail: [email protected]

The true date of Meton’s observation of the summer solstice

The date assigned to Meton’s highly reputed observation of the summer solstice in the Almagest implies June 27, 432 BC. Since the solstice took actually place a day later, such an inaccuracy presents a puzzle. It can be demonstrated, however, that Meton’s observation was in fact accurate, for he made it on June 28, 433 BC. This follows from adequate interpretation of chronological indications in Thucydides and finds support in various data of the ancient sources. The mistaken date arose, and was maintained, because of the misleading assumption according to which the Athenian archon year invariably began after the summer solstice. It was wrongly decided that Meton had observed the summer solstice at the end of the year of the archon Apseudes and not at its initial part. The true date of Meton’s solstice presents in new light the observation of the summer solstice by Aristarchus of Samos, Eudoxus’ preoccupation with the octaeteris and early Greek astronomy in general. The beginning of the Peloponnesian War is now firmly established at early April (almost certainly, April 6), 431 BC.

The Moon and the Planets in Classical Greece and Rome

While the moon naturally featured in Mediterranean cultures from time immemorial, principally noted in the earliest literature as a marker of time, time-dependent constructs such as the calendar, and time-related activities, awareness and recognition of the five visible planets came relatively late to the Greeks and thence to the Romans. The moon underlies the local calendars of the Greeks, with documentary and literary evidence from the Late Bronze Age through the Imperial Roman period, and there are signs that the earliest Roman calendar also paid homage to the moon in its divisions of the month. However, although Homer in the 8th century BCE knows of a Morning and an Evening Star, he shows no indication of realizing that these are one and the same, the planet Venus. That particular identification may have come in the 6th century BCE, and it appears to have been not until the 4th century BCE that the Greeks recognized the other four planets visible to the naked eye—Saturn, Jupiter, Mars, and Mercury. This awareness probably came via contact with Babylonian astronomy and astrology, where identification and observations of the planets had figured from the 2nd millennium BCE and served as a basis for astrological prognostications. But it is time, not astrology, that lies at the heart of Greek and Roman concerns with the moon and the planets. Indeed, the need to tell time accurately has been regarded as the fundamental motivation of Greek astronomy. A major cultural issue that long engaged the Greeks was how to synchronize the incommensurate cycles of the moon and the sun for calendrical purposes. Given the apparent irregularities of their cycles, the planets might seem to offer no obvious help with regard to time measurement. Nonetheless they were included by Plato in the 4th century BCE in his cosmology, along with the sun and moon, as heavenly bodies created specifically to compute time. Astrology then provided a useful framework in which the sun, moon, planets, and stars all combined to enable the interpretation and forecasting of life events. It became necessary for the Greeks, and their successors the Romans, to be able to calculate as accurately as possible the positions of the heavenly bodies in order to determine readings of the past, present, and future. Greek astronomy had always had a speculative aspect, as philosophers strove to make sense of the visible cosmos. A deep-seated assumption held by Greek astronomers, that the heavenly bodies moved in uniform, circular orbits, lead to a desire over the centuries to account for or explain away the observed irregularities of planetary motions with their stations and retrogradations. This intention “to save the phenomena,”— that is, to preserve the fundamental circularity—was said to have originated with Plato. While arithmetical schemes had sufficed in Babylonia for such calculation, it was a Greek innovation to devise increasingly complex geometric theories of circular motions (eccentrics and epicycles) in an effort to understand how the sun, moon, and planets moved, so as to place them more precisely in time and space.

Babylonian and Indian Wisdoms in Islamicate Culture

The interaction of Islamicate civilization with those civilizations that preceded it or were contemporaneous with it has focused for the most part on Hellenistic civilization, and the huge body of scientific and philosophical literature which was translated and absorbed in the first centuries after the appearance of Islam. This paper aims to present two small but much needed correctives to this understanding. In the first section I argue that the “Greek” astronomy that was translated into Arabic ought more correctly to be described as Greco-Babylonian astronomy. In the second I turn to India: not only was a great deal of Indian knowledge absorbed at the time of the great translation movement, we must recall that the exchanges with India carried on well beyond the early Abbasids. I illustrate these points with some new materials in the fields of medicine, philosophy, and alchemy.

On the Equant Point for the Planets and the Moon

The introduction of the equant point signified a major improvement in the history of planetary models. Thanks to its incorporation in ancient planetary hypotheses, Greek astronomy reached a higher degree of accuracy in its predictive capabilities than any of its predecessors, an accuracy that would not be surpassed until as late as the seventieth century, when Kepler postulated his laws. In the Almagest, Ptolemy explains how he arrived at the necessity of the introduction of the equant point for Venus and Mercury. There is, however, some unsolved matter surrounding the argument Ptolemy gives and most scholars dispute that his observations of Venus were in fact the empirical basis from which Ptolemy built his model. These considerations led scholars to wonder about the real path Ptolemy took to discover the equant. In this paper, we present a simple argument through which Ptolemy might have arrived to the conclusion of the existence of an equant for the models of the planets, and to his conclusion of the bisection of the equant’s eccentricity by the centre of the deferent. Moreover, we show that this same argument could have been used to solve the problems posed by the second lunar anomaly.

Thales (fl. c.585 BC)

Known as the first Greek philosopher, Thales initiated a way of understanding the world that was based on reason and nature rather than tradition and mythology. He held that water is in some sense the basic material, that all things are full of gods and (purportedly) that all things possess soul. He predicted an eclipse of the sun and was considered the founder of Greek astronomy and mathematics.