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Ptolemy
The philosopher and driving force behind Ptolemy astronomy was Clausius Ptolemy. He lived from 90 A.D until 168 A.D. During his lifetime he was a mathematician, astrologer(mixed astronomy with astrology), and geographer. His theories dominated the world’s understanding of astronomy for over a thousand years.
While it is known that many astronomers published works, Ptolemy’s work The Almagest is the only tome known to have survived. In it he outlined his geometrical reasoning for a geocentric view of the Universe. As outlined in the books of the Almagest, the cosmos according to Ptolemy astronomy was based on five main points: the celestial realm is spherical, and moves as a sphere, the Earth is a sphere, the Earth is at the center of the cosmos, in relation to the distance of the fixed stars, Earth has no appreciable size and must be treated as a mathematical point, and Earth does not move. He also theorized eight ‘spheres’ surrounding Earth where the other planets existed. In order they were: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the sphere of fixed stars. Much of his work was based upon, but an improvement of, the work of his contemporaries.
In the Ptolemaic system, each planet is moved by two or more spheres: one sphere is its deferent. The deferent was a circle centered on a point halfway between the equant(directly opposite the Earth from the center of the deferent) and Earth. Another sphere is the epicycle which is embedded in the deferent. The planet is embedded in the epicycle sphere. The deferent rotates around the Earth while the epicycle rotates within the deferent, causing the planet to move closer to and farther from Earth at different points in its orbit, and even to slow down, stop, and move backward(retrograde motion). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun, which explains why they are always near it in the sky.
It was not until 1543, when Copernicus introduced heliocentrism, that Ptolemy astronomy was seriously challenged and eventually overthrown. Another contribution that Ptolemy made was his catalog of constellations. He only listed 48 constellations. Most of these are still included in the list of 88 known constellations that are taught today. Claudius Ptolemy will remain one of the most influential astronomers in history.
We have written many articles about Ptolemy Astronomy for Universe Today. Here’s an article about the geocentric model, and here’s an article about ancient astronomy.
If you’d like more info on Ptolemy Astronomy, check out NASA’s article about Ancient Astronomers, and here’s a link to an article about the Discovery of the Solar System.
We’ve also recorded an entire episode of Astronomy Cast all about the History of Astronomy. Listen here, Episode 184: History of Astronomy, Part 2: The Greeks.
Sources:
http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html
http://microcosmos.uchicago.edu/ptolemy/astronomy.html
http://galileo.rice.edu/sci/theories/ptolemaic_system.html
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