When was Galileo found guilty of heresy?
Galileo affair
Galileo went on to propose a theory of tides in 1616, and of comets in 1619; he argued that the tides were evidence for the motion of the earth. In 1632 Galileo, now an old man, published his Dialogue Concerning the Two Chief World Systems, which implicitly defended heliocentrism, and was immensely popular. Responding to mounting controversy over theology, astronomy and philosophy, the Roman Inquisition tried Galileo in 1633 and found him "vehemently suspect of heresy", sentencing him to indefinite imprisonment. Galileo was kept under house arrest until his death in 1642.
Orazio Grassi
Grassi replied in turn with his treatise "Libra astronomica ac philosophica" published under the pseudonym of Lotario Sarsi Sigensano (an anagram of Horatius Grassius Salonensis, his name in Latin). In this Grassi both noted how close the ideas in Galileo's "Discourse" were to those advanced by Gerolamo Cardano and Bernardino Telesio (which the Church regarded as dangerous) and set out scientific arguments and experimental test results to show that comets were definitely not optical illusions. Grassi's work provoked another furious reaction from Galileo in the shape of his 1623 work "Il Saggiatore", in which he was savagely dismissive of Grassi's efforts.