Next Page | Contents | Previous Page
The geocentric school of astronomy began with Eudoxus of Cnidus (409-356 B.C.), an eminent resident at the academy of Plato (427-347 B.C.), several decades after Philolaus had postulated his distinctively non-geocentric theory. Eudoxus' theory was further developed by Callipus (c. 325 B.C.), Aristotle (384-322 B.C.), Apollonius (c. 220 B.C.), Hipparchus (190-120 B.C.), and finally Claudius Ptolemy of Alexandria (A.D. 85-165).
Eudoxus, after developing a certain mathematical procedure, evolved the first geocentric model of the celestial system wherein he assigned a spherical shell to every periodic movement that centered upon the Earth, a combination of such spheres describing reasonably well the complex periodic movement of a particular celestial body. All of the spheres were fixedly embedded in the surfaces of the spheres farther out. In this manner he explained the motions of the celestial system by using twenty-seven spheres, one for the fixed stars, three each for the Sun and the Moon, and four each for the five known planets -- Venus, Mercury, Mars, Jupiter, and Saturn.
As new periodic phenomena were identified, the system had to be expanded. Callipus gave each celestial body an extra sphere, bringing the total up to thirty-four, while Aristotle added a further twenty-two spheres. In the field of astronomy Aristotle was responsible for the idea that the spheres which carried the celestial bodies along their paths were real physical entities, not mere geometrical constructions as Eudoxus had previously supposed and Ptolemy would later postulate. (His strong adherence to objective reality did not allow Aristotle to view his theory as a mere geometrical construction without any corresponding existence in the universe.) He also believed that the outermost sphere of the fixed stars was moved by the Primum Mobile (the Prime Mover) which governed the entire universe.
Unlike the heliocentric system which originally came from the ancient navigator-businessmen's business-based integration of reality, i.e., the sphericity of the Earth, the geocentric theory was purely an academic enterprise and entailed many difficulties from the very beginning. Furthermore, it not only was an extremely complex theory technically but also became progressively more complex throughout its development. Heraclides' model of the spinning Earth was one of the attempts made to overcome those difficulties within the context of the geocentric universe. During the Alexandrian period, Apollonius, Hipparchus, and Ptolemy also tried to overcome the limitations inherent in the geocentric system, again within the conceptual framework of the geocentric model. They failed to take into account the validity of the heliocentric concept developed by Aristarchus through his remarkable hypotheses.
Apollonius and Hipparchus both developed the system of eccentrics and epicycles. Apollonius suggested that if a planet moved in a circle, the epicycle, the center of which moved upon another circle, the deferent, which centered upon the Earth, then the motions of the planets could be quantitatively accounted for. He further suggested that the celestial bodies moved in circles eccentric to the Earth, the center of their orbits lying at some distance from the center of the Earth. Hipparchus further developed Apollonius' concept. Ptolemy adopted and evolved the system of eccentrics and epicycles used by Hipparchus to explain the apparent motions of the celestial system.
Next Page | Contents | Previous Page
Disclaimer - Copyright - Contact
Online: buildfreedom.org - terrorcrat.com - mind-trek.com