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.

Geocentricity

According to the Ptolemaic model, the spherical Earth is at the center of the universe. All heavenly bodies are attached to crystal spheres which rotate around Earth. The Moon is on the innermost sphere, and touches the realm of Earth, thereby contaminating it, and causing the light and dark spots and the ability to go through phases. It is not perfect like the other heavenly bodies, which shine by their own light. The planets are actually attached to 2 spheres: one sphere which is centered on Earth (the deferent), and another sphere (the epicycle) embedded within the deferent. 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 (in retrograde motion). (The earlier model based on concentric spheres explained retrograde motion, but did not explain the changes in brightness caused by the change in distance). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun (Mercury being closer to Earth), which explains why they are always near it in the sky. The order of spheres from Earth outward is: Earth, Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, Stars.

Related Topics:
Earth - Universe - Crystal - Sphere - Contaminating - Phase - Deferent - Epicycle - Retrograde motion - Moon - Mercury - Venus - Sun - Mars - Jupiter - Saturn

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