In 1618, three comets appeared, visible to the unaided eye. These were the first comets to be observed with the telescope.
Grassi was the leading astronomer in Rome and a professor at the Rome College (Collegio Romano). As a Jesuit, Grassi was charged with teaching nothing in science contrary to Aristotle, who said that comets were vapors located beneath the Moon. Yet Grassi’s analysis demonstrated that these comets moved beyond the Moon!
This book therefore represents an interesting story: the leading Jesuit astronomer in Rome openly published an account of three well-known comets that was explicitly contrary to Aristotle. One might think that Galileo, whom Grassi admired, would have cheered him on. Yet in making this argument, Grassi unwittingly opened an episode that became known as the “controversy of the comets.” Tellingly, Galileo had been ill during 1618 and was not able to observe the comets himself.
Despite their avowed allegiance to Aristotle, Jesuits were more receptive than scholastics to heavenly comets, sunspots, new stars, fluid orbs, the corruptibility of the heavens and various non-Ptolemaic geocentric systems (see Introduction to the Jesuits).
Vigorous Jesuit participation in the mathematical sciences, consisting of sustained investigations in optics, geodesy, astronomy and cosmology, make it possible to regard the Jesuits as one of the earliest scientific societies.