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Ptolemy and Ptolemaic Astronomy
This is when a kind of Dark Ages developed and when few other theories came forth until the time of the Renaissance.
PTOLEMY AND PTOLEMAIC ASTRONOMY
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Ptolemy was considered an ancient astronomer, living in Alexandria in Egypt at around 200 AD. His main contribution to astronomy was to provide a mathematical basis for the geocentric theory of the solar system. The basis of his initial work came from the ideas of Aristotle. His body of work included a thirteen-book series called the Almagest, which were designed to explain the different astronomical concepts known about at the time. Some of it was also based on the star charts of Hipparchus, who also named the different constellations.
The first argument that Ptolemy made was that the earth itself must be an immovable object. He said that, because all things must fall toward the center of the universe and because on earth, all things fall to the ground, the earth must be the center of the universe. He also argued that, if the Earth spun, you would throw something upward and it would fall in a different spot, which was not observed.
According to Ptolemy, the Earth was the center, followed outward by the Moon, Mercury, Venus, Sun, Mars, Jupiter, and finally Saturn. There was, however, the observation that some planets appeared to go backwards some of the time—a phenomenon called retrograde motion. This was explained using the idea that there were circles upon circles with planets undergoing small circles called epicycles within a larger circular orbit. Figure 3 shows these epicycles on a Ptolemaic model of the solar system:
Figure 3.
In this model, the larger circles around the earth were called deferents, while the small circles or epicycles moved around these deferents. He still believed that there were crystalline spheres that were where the different celestial bodies, including planets, existed as attached objects. He required the presence of 28 total epicycles to explain the way that the planets moved in his geocentric system. He also invented the “equant”, which was the center of a planet’s epicycle that moved with some degree of uniform angular velocity.
