IntroductionPtolemaic system (tŏlˌəmāˈĭk) [key], historically the most influential of the geocentric cosmological theories, i.e., theories that placed the earth motionless at the center of the universe with all celestial bodies revolving around it (see cosmology). The system is named for the Greco-Egyptian astronomer Ptolemy (fl. 2d cent. A.D.); it dominated astronomy until the advent of the heliocentric Copernican system in the 16th cent.
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