Conflicts with philosophers
In October 1604, when Galileo was writing to Sarpi about his law of falling bodies, a supernova appeared in the evening sky. Galileo was told about it a few days after a medical student named Baldessar Capra and his mathematics tutor Simon Mayr had observed and confirmed it. A new star had been seen in 1572 and had been proved by Tycho Brahe to be among the fixed stars. According to Aristotle's fundamental principles, no change could ever take place in the heavens, because everything in them was made of a perfect and unalterable substance called the ‘quintessence’. Change occurred only in the ‘elemental’ materials of earth, water, air, and fire. Natural philosophers accordingly taught that comets were not astronomical events, but meteorological phenomena situated in the elemental sphere beneath the moon. New stars could be explained as some kind of tailless and motionless comets, but not as bodies actually in the heavens.
Galileo wrote to astronomers in other cities and compared their observations with his own. Like Tycho's star, this new star exhibited no detectable parallax; no matter where it was observed from, it was seen in the same place with respect to nearby fixed stars. That cannot happen for things as close as the moon. Since people were always excited by unusual appearances in the sky, Galileo gave three public lectures on the new star, explaining how astronomical observations and