Conservative astronomers of Galileo’s time, working without telescopes, ascribed without deviation to the ancient theory of egocentricity. This theory of astronomy held that the earth (“geo,” as in “geography” or “geology”) lay at the center of the solar system, orbited by both the sun and the other planets. Indeed, to the casual observer, it seemed common sense that since the sun “rose” in the morning and “set” at night, it must have circled around the earth. Ancient authorities like Aristotle and the Roman astronomer Ptolemy had championed this viewpoint, and the notion also coincided with the Catholi Church’s view of the universe, which placed mankind, God’s principal creation, at the center of the cosmos. Buttressed by common sense, the ancient philosophers, and the church, the geocentric model of the universe
seemed secure in its authority. The Ptolemaic theory, however, was not impervious to attack. In the 16th century, astronomers strained to make modern observations fit Ptolemy’s geocentric model of the universe.