Born in Savona in 1590, Orazio Grassi entered the Jesuit Order at the age of eighteen, and soon became a professor of mathematics, first in Genoa and then in Rome. He was also an architect, and his was the project for the church of Sant'Ignazio adjacent to the Roman College, but his fame is associated mainly to the dispute on the comets in which he opposed Galileo. In 1618 the appearance of three comets aroused great interest in the scientific world, and Grassi was given the job by his fellow Jesuits of holding a public lecture on it at the Roman College, with a subsequent publication entitled De tribus cometis anni MDCXVIII disputatio astronomica (Rome, 1619). Galileo was urged on all sides to express himself on the matter, and he dictated his considerations to Mario Guiducci (1584-1646), one of his disciples, who in 1619 published the Discorso delle comete [Discourse on Comets] (Florence, 1619), which triggered the controversy that led to the composition of the Saggiatore [The Assayer] (Rome, 1623). Father Grassi in fact wrote a reply to the Discorso, entitled Libra astronomica ac philosophica (Perugia, 1619), in which he bypassed Guiducci entirely to argue directly with Galileo, who replied by writing what is considered one of his masterpieces, the Saggiat ore [The Assayer] (Rome, 1623). In 1626 Grassi, who meanwhile had begun to frequent Guiducci and to relax his polemical attitude, published his Ratio ponderum librae et simbellae: in qua quid e Lotharii Sarsii Libra astronomica, quidque e Galilei Galilei S imbellatore de cometis statuendum sit, collatis vtriusque rationum momentis, philosophorum arbitrio proponitur (Paris, 1626), in which he again attacked Galileo. This time Galileo preferred not to reply and let the matter play itself out over time. During the painful years of Galileo's trial Father Grassi displayed no acrimony toward him, even if their past controversy doubtless contributed to placing Galileo in an unfavorable light among the Jesuits.