The concept of a spherical Earth was introduced by the ancient Greeks in the 6th century BCE. Hypotheses about its place in the universe soon followed. Ancient Greek philosophers imagined our motionless planet at the center of the universe, with all the celestial bodies revolving around it in a series of nested, concentric spheres. That theory dominated astronomy until the advent of the Sun-centered Copernican system in the 16th century. According to Aristotle, what moved each planet’s sphere? Discuss