Celestial Models by Ptolemy and Copernicus

Currently, most people consider it obvious that the sun is at the center of the solar system, but the sun-centered (heliocentric) concept was slow to evolve. In the 2nd century ad, Claudius Ptolemy proposed a model of the universe with the earth at the center (geocentric). His model (shown left) depicts the earth as stationary with the planets, moon, and sun moving around it in small, circular orbits called epicycles. Ptolemy’s system was accepted by astronomers and religious thinkers alike for several hundred years. It was not until the 16th century that Nicolaus Copernicus developed a model for the universe in which the sun was at the center instead of the earth. The new model was rejected by the church, but it gradually gained popular acceptance because it provided better explanations for observed phenomena. Ironically, Copernicus’ initial measurements were no more accurate than Ptolemy’s, they just made more sense.

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