David Zax in Smithsonian.com:
Certain pursuits were not in an astronomer’s job description, says Dava Sobel, author of the bestselling historical memoir Galileo’s Daughter (1999). “You didn’t talk about what the planets were made of,” she says. “It was a foregone conclusion that they were made of the fifth essence, celestial material that never changed.” Astronomers might make astrological predictions, but they weren’t expected to discover anything new.
So when Galileo, then 45 years old, turned his telescope to the heavens in the fall of 1609, it was a small act of dissent. He saw that the Milky Way was in fact “a congeries of innumberable stars,” more even than his tired hand could draw. He saw the pockmarked surface of the moon, which, far from being perfectly spherical, was in fact “full of cavities and prominences, being not unlike the face of the Earth.” Soon he would note that Jupiter had four moons of its own and that Venus had moonlike phases, sometimes waxing to a disk, sometimes waning to a crescent. He later saw imperfections in the sun. Each discovery drew Aristotle’s system further into question and lent ever more support to the dangerously revolutionary view that Galileo had privately come to hold—set out just a half-century earlier by a Polish astronomer named Nicolaus Copernicus—that the Earth traveled around the sun.
“I give infinite thanks to God,” Galileo wrote to powerful Florentine statesman Belisario Vinta in January of 1610, “who has been pleased to make me the first observer of marvelous things.”