The word "ge" in Greek means "earth." Geocentric means that the earth is in the center. In the geocentric system, the earth is statically located at the center and the rest of the planets revolve around it, including the sun, which was also considered a planet.

The planet nearest to earth is the moon, which completes a single revolution about the earth in one month. Further out are the rest of the planets: Mercury, Venus, the Sun, Mars, Jupiter and Saturn.

The geocentric system is often called the Ptolemaic system after Ptolemy. Ptolemy lived in Alexandria in Egypt during the second century A.C. and is considered the most important astronomer of antiquity. In his book "The Mathematical Collection," which later came to be called the Almagest, Ptolemy described the structure of the planetary system and the location of each star within it. In his book, Ptolemy summarizes the activities of centuries of ancient Greek astronomy and also adds a number of new concepts.

For 1,400 years, up to the 16th century, all astronomers - Greeks, Muslims and Christians - worked within the Ptolemaic system, with additions and corrections to the basic structure created by Ptolemy.

Ptolemy's astronomy is based on the Physics of Aristotle which separates the activities of the sub-lunar world (i.e., on the earth) and the supra-lunar world. According to Aristotle the supra-lunar world is made up of a special material which differs from the material on earth. This material is called ether. Ether is a very hard material, but lighter than anything known on earth. Ether's main feature is its circular motion, and thus all the stars, by their nature, move in circles.

In Ptolemaic astronomy all stars move along spheres. These spheres revolve about the earth, but the earth is not always precisely at their center. Each planet has a system of spheres along which it moves. The merging of the circular motions of the planets produces non-circular motion. Beyond the spheres of the planets, all the stars are located within a single sphere. These stars have fixed places in the sphere.

The stars are all located within a single sphere. This sphere revolves around the earth and completes an entire revolution every 24 hours. This movement around the earth is also common to the planets, including the sun. The daily revolution around the earth is added to the various movements of the different planets, each in relation to its own spheres.

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