Final answer:
Galileo was required to teach the geocentric theory at the University of Pisa, which was supported by the Roman Catholic Church and based on the ideas of Aristotle and Ptolemy.
Step-by-step explanation:
At the University of Pisa, Galileo was required to teach the geocentric theory, which posited that Earth was the immutable center of the universe. This theory was aligned with the long-standing beliefs of Aristotle and Ptolemy and was backed by the powerful Roman Catholic Church. Despite his personal support for the Copernican heliocentric model, which placed the Sun at the center of the solar system, Galileo faced substantial opposition and risk when promoting Copernicanism against the accepted views of his time.