Works of Galileo Galilei, Part 3, Volume 12, Astronomy: Discourse on the Comets Produced by him at the Florentine Academy During his Very Consulship


Three comets appeared in the skies over Europe in 1618, a phenomenal series of events that ignited a debate about the nature of these celestial bodies and the implications of their appearance for the Aristotelian theory that celestial bodies were unchanging and “incorruptible.” In 1619, the Jesuit astronomer and mathematician Orazio Grassi published under a pseudonym his treatise on the comets, in which he upheld the established view of celestial bodies as unchangeable and orbiting the Earth. Already under attack for his defense of the theories of Copernicus, Galileo Galilei (1564–1642) did not issue a direct reply to Grassi, but worked through his student and follower Mario Guiducci, who in 1619 published his Discorso delle comete (Discourse on comets). The text of Guiducci’s work is in large part attributable to Galileo. Underlying the discussion about the comets, although not made explicit, was the debate about the motion of the earth and the validity of the Copernican view that the Earth moves around the sun.

Last updated: September 18, 2015