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The geocentric cosmos was readily appropriated by Christians, many of whom believed that God existed outside the stellar sphere and (as evidenced by Dante's fourteenth-century poem Divine Comedy ) that angels turned the epicycles and spheres in the intervening spaces.Īstronomy had a primarily practical function, and by the thirteenth century it played a central role in determining the dates of religious festivals. These approaches, and arguments over the reality of the celestial orbs, continued to be refined in the following centuries by first Muslim and then Christian astronomers. After Ptolemy's composition of the Almagest in the second century c.e., the movements of the planets could be accurately represented by means of techniques involving the use of epicycles, deferents, eccentrics (whereby planetary motion is conceived as circular with respect to a point displaced from Earth), and equants (a device that posits a constant angular rate of rotation with respect to a point displaced from Earth). This accounted for the apparent motions of the planets, including their retrograde motion, although others followed Aristotle in positing the existence of homocentric spheres. From the third century b.c.e., astronomers began to conceive of planets rotating in small circles (epicycles) around a point that moved on a carrier orbit (or deferent). The retrograde motion of the planets offended against the apparent simplicity of heavenly motions, as well as the dictum that their orbits were circular. scholar Eudoxus, astronomers and natural philosophers became increasingly committed to the idea that the motions of each planet could be accounted for by means of their own specific, homocentric spheres. Following the theories of the fourth-century-b.c.e. Many Greek astronomers suggested that the Sun could be placed on the equator of an inner sphere that revolved once a year, hence constituting a second sphere in addition to the stellar orb. These usually moved eastward, though occasionally they moved in the opposite direction, thus exhibiting retrograde Against the backdrop of this outer sphere, the Sun was seen to move along a path termed the ecliptic, while the planets moved within eight degrees of this path. Most envisaged these revolving stars to be located on a sphere, composing an incorruptible, celestial orb that could easily be contrasted with the world of change, generation, and corruption on Earth (to which irregular meteorological phenomena such as comets were also understood to belong). The ancient Greeks observed that the stars revolved westward around the north celestial pole every twenty-three hours and fifty-six minutes, thus constituting a kind of objective clock. For the vast majority of ancient astronomers, the stars seemed to be equally distant from Earth in what was an Earth-centered (or "geocentric") cosmos. The movement of the stars and planets has fascinated humans for thousands of years.