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.