Discourse on Comets
[3][4][5] In 1616 the heliocentric views of Copernicus were declared formally heretical and Galileo was warned by Cardinal Bellarmine to neither teach nor defend them.
[8]: 233–6 In March 1619, Galileo received a letter from Giovanni Battista Rinuccini alerting him to the fact that some people outside the Jesuit order were claiming that Grassi's lecture on comets provided a definitive proof that Copernicus' ideas were wrong.
[9] The evident threat to Copernicus, whom Galileo could no longer defend, prompted him to attack the Tychonic ideas now popular among the Jesuits with particular force.
Rather, as in his Letter to the Grand Duchess Christina four years previously, he was insisting that the burden of proof lay with those who had ideas that did not accord with his own.
[15] He intended to criticise pedantic thinkers who believed they had easily found a definitive answer to something, disregarding the fact that nature may have many possible ways of producing the same effect.
It advanced the proposition that the absence of parallax observable with comets was due not to their great distance from the Earth, but to the fact that they were not real objects; they were probably atmospheric effects.
'[19] Against classical authority: The Discourse opens with a review of the opinions on comets of Aristotle, Anaxagoras, Democritus, Hippocrates of Chios and Seneca the Younger.
[8]: 239 The pamphlet was a major factor in the alienation of the Jesuits from Galileo, who had previously been broadly supportive of his ideas, even despite his attacks on Christoph Scheiner.
[10] While Guiducci and Galileo were working in the Discourse, a second anonymous Jesuit pamphlet appeared in Milan - Assemblea Celeste Radunata Nuovamente in Parnasso Sopra la Nuova Cometa.
The debate continued when, in Perugia later in 1619, Grassi published a reply to the Discourse in La Libra Astronomica ac Philosophica under the pen-name Lotario Sarsi Sigensano.
Guiducci concluded with an attempt to reconcile experimental evidence with theological arguments, but firmly asserted the primacy of data gathered through observation.
