The comets of 1618
 
The three comets that appeared in 1618 were not only considered of bad omen (during that year a terrible war that would go on for thirty years broke out among the European powers) but they also arouse the interest of the scientific world.
There essentially were two different interpretations of that astronomic phenomenon:the first one was linked to the traditional Aristotelian thought which considered comets as atmospheric anomalies risen to the fire sphere;the second one,the most modern, was due to Tycho Brahe, who asserted that the comets were real heavenly bodies moving among the planets.
The Jesuit astronomers who had been influenced by the latest discoveries of Galileo (the Venus phases, the Medicean planets, the Mounts of the Moon) and who no longer believed in the Ptolemaic system shared the new ideas of Tycho, that is to say: the planets turned around the Sun and it turned around the Earth. The theory was consequently defended by the Jesuit Father Orazio Grassi, teacher of Mathematics at the Collegio Romano, in his discourse called ‘’Disputatio astronomica de tribus cometis anni MDCXVIII’’. This discourse, however, contained many old and mistaken scientific reflections.
Galileo who defended the Copernican assertions against the Tycho ones, replied to Grassi with the ‘’Discorso delle comete’’ written by his student Mario Guiducci who defined as false and vain the theories of the Jesuit Father.
Grassi answered with the ‘’Libra Astronomica ac Philosofica’’ by Lotario Sarsi Sigenzano (the anagram of the name of the author) in which he questioned the theories both of Guiducci and Galileo. Three years later Galileo replied with his work ‘’Saggiatore’’, that is considered a masterpiece of literary controversy. With this essay he clearly defined his theory about the comets and stated the basis of his scientific method. This controversy with the Jesuits, full of humour and bright ideas (the Libra,-‘balance’-by Grassi was opposed to the Saggiatore,-‘sensitive scales’ -by Galilei),in spite of the errors of Galileo (for him the comets were only optical illusions), was considered very witty but,later on, it would cost him dear.
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