Ptolemaic system

The Ptolemaic system was a model to explain the motions of the heavens in which the earth was the centre of the universe and all other celestial bodies revolved around it, espoused by Claudius Ptolemaeus in his work, the Almagest some time around the 2nd century, C.E., and accepted for over 1,000 years by the vast majority of Europeans to be the correct cosmological model. It may be also called the geocentric model. It was overthrown by the Copernican revolution after Galileo Galilei and Copernicus discovered that the planets orbited the sun.

Problems with geocentricity

Unfortunately, the system still did not quite match observations. Sometimes the size of a planet's retrograde loop (most notably that of Mars) would be smaller, and sometimes larger. Ptolemy could not explain this even when he moved deferents off-center, for the change in loop size did not match with the change in speed. This prompted Ptolemy to come up with the idea of an equant. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed. Therefore, the planet actually moved at different speeds at different points in its orbit. By using an equant, Ptolemy claimed to keep motion which was uniform and circular, but a lot of people didn't like it because they didn't think it was true to Plato's dictum of "uniform, circular motion." The resultant system which eventually became Catholic dogma was an unwieldy one, using two sets of epicycles, revolving on a deferent, offset by an equant which was different for each retrograde planet (then known to be only Mars, Jupiter, and Saturn), but it predicted the beginnings and ends of retrograde motion far more accurately than either earlier Platonic spheres or early (and falsely perfect) Copernican systems.

Related Topics:
Observation - Mars - Ptolemy - Equant - Epicycle - Dogma - Deferent - Jupiter - Saturn - Retrograde motion - Copernican

~ ~ ~ ~ ~ ~ ~ ~ ~ ~