The Ptolemaic system is geocentric cosmological model that was widely believed for over a millennia. In the 2nd century CE Claudius Ptolemaeus expounded on Aristotle’s geocentric system. His system attempted to maintain the basic Greek geocentric model devised by Aristotle and centuries of Greek astronomers, while at the same time predicting the celestial motions.
Today’s cosmological models are based on concepts of space; however, the Ptolemaic system was based on the concept of place. Everything within the system has a set place and is mostly unchanging. The system consists of two basic parts the earthly and the heavenly. The earthly half is the “abode of change and corruption” and was made up of four elements, earth, water, air. The later three formed concentric spherical shells around the earth element. It was reasoned that because earth is the heaviest element that it must be at the center of the cosmos.
The second half of the cosmos, the heavens, consists of aether, an immutable substance in which the sun, planets, and stars were fixed in. To explain the seemingly independent motions of the celestial bodies it was rationalized that each body is fixed in an individual aether concentric spherical shell that rotates independently of the others. The order of the solar system was, Earth, Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the fixed stars. Because the stars seemed to be unchanging they were all placed together in the last aether shell.
Beyond the last concentric spherical shell, which contained the stars, was the Prime Mover who set the motion of the cosmos from the outside. Due to the belief that the “natural motions of heavenly bodies and their spheres was perfectly circular, that is, circular and neither speeding up or slowing down,” an outside being was formulated to explain this perfect motion.
Due the lack of the concept of space, and that the Ptolemaic system is based on the idea that everything has a set and orderly place, beyond the last aether shell containing the stars there is nothing except for the Prime Mover. There is not even empty space as the astronomers at the time thought that the cosmos was completely filled up by these tightly fitting concentric spherical shells composed of the four elements or aether. At the edge of the cosmos there was nothing.
Today it is known the solar system is actually heliocentric, or sun based; for Ptolemaeus to have expounded upon a geocentric system that proposed the perfect circular motion he had to create a complicated system of orbital in order to explain the observed celestial motions. This is one of the biggest weakness of the theory, because the heliocentric theory is eloquent and simple it is inevitably more accurate. “We are to admit no more causes of natural thing than such as are both true and sufficient to explain their appearances,” Isaac Newton.
