Volume 7, No. 1

Galileo as an Astrologer

Nick Kollerstrom

Abstract
Galileo cast horoscopes as part of his job as a mathematicus. He sometimes wrote out character-interpretations of these nativities, for example, for his close friend Sagredo in Venice. In 1604 he was summoned and tried by the Inquisition for being too fatalistic in his astrological forecasts for his clients. His public lectures on the New Star of 1604 argued that it had germinated out of a conjunction between Mars and Jupiter. When in 1610 he dedicated the moons of Jupiter to Cosimo II de Medici he used the latter's horoscope to argue for it. He wrote a letter to Dini explaining how the new moons of Jupiter might affect its 'influence.' His Dialogues of 1630 criticised astrologers who were only wise in retrospect, but his writings contain no scepticism as such towards astrology. Science historians have greatly ignored this side of Galileo's life.