Early Astronomy in the University of Michigan Collections


Against Grassi’s thesis, Galileo responded with a pamphlet published under the name of his disciple Mario Guiducci in 1619: Discorso delle comete di Mario Guiducci fatto da lui nell’Accademia Fiorentina nel suo medesimo consolato (Discourse on Comets by Mario Guiducci, delivered by him at the Florentine Academy during his very Consulship). Rather than being a systematic analysis of the nature of these three comets, which Galileo did not observe and wrongly described as atmospheric phenomena caused by vapor rising from the Earth and subsequently condensing in the cosmic sphere, the Discorso is in fact a comprehensive rebuttal of Grassi’s scientific method and of the Tychonian system at large.

On Comets

Discorso: Woodcut


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