What really focused Galileo’s attention on the heavens, however, was the sudden appearance of a supernova in the night sky in October 1604. Such a thing was not supposed to happen. According to Aristotle, no change should ever occur in the heavens. Everything existing in the celestial spheres, like the sky and the planets, was made from an immaculate and unalterable substance called the quintessence. The heavens were, as an Aristotelian philosopher says in one of Galileo’s later dialogues, “ingenerable, incorruptible, inalterable, invariant, eternal”—indeed, as perfect and definitive as
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