Horoscopes of the Moon: Weather Prediction as Astrology in Ptolemy’s Tetrabiblos

Ptolemy’s Tetrabiblos is not usually considered as a meteorological text. I examine the sections of the Tetrabiblos that deal with weather prediction and argue that Ptolemy considers it part of astrology. I show that the way Ptolemy categorises weather prediction differs significantly from other Greco-Roman writers, including Cicero (On Divination) and Sextus Empiricus (Against the Astrologers). I argue for these conclusions: first, that Ptolemy considers weather prediction to be part of astrology – i.e., a scientific practice which includes giving an account of the causes of phenomena with reference to the stars; to give such an account for the weather requires a practitioner to go beyond the observation and cataloguing of weather signs. Secondly, I argue that understanding weather prediction as astrology means that the distinction between horoscopic and general astrology is more fluid than is sometimes thought. Finally, I will show that Ptolemy’s views on weather prediction provide a case study for his view of astral causation.