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Ptolemaic system


In astronomy, the geocentric model (also known as geocentrism, or the Ptolemaic system) is a superseded description of the universe with the Earth at the center. Under the geocentric model, the Sun, Moon, stars, and planets all circled Earth. The geocentric model served as the predominant description of the cosmos in many ancient civilizations, such as those of Aristotle and Ptolemy.

Two observations supported the idea that the Earth was the center of the Universe. First, the Sun appears to revolve around the Earth once per day. While the Moon and the planets have their own motions, they also appear to revolve around the Earth about once per day. The stars appeared to be on a celestial sphere, rotating once each day along an axis through the north and south geographic poles of the Earth. Second, the Earth does not seem to move from the perspective of an Earth-bound observer; it appears to be solid, stable, and unmoving.

Ancient Greek, ancient Roman and medieval philosophers usually combined the geocentric model with a spherical Earth. It is not the same as the older flat Earth model implied in some mythology. The ancient Jewish Babylonian uranography pictured a flat Earth with a dome-shaped rigid canopy named firmament placed over it. (רקיע- rāqîa'). However, the ancient Greeks believed that the motions of the planets were circular and not elliptical, a view that was not challenged in Western culture until the 17th century through the synthesis of theories by Copernicus and Kepler.

The astronomical predictions of Ptolemy's geocentric model were used to prepare astrological and astronomical charts for over 1500 years. The geocentric model held sway into the early modern age, but from the late 16th century onward, it was gradually superseded by the Heliocentric model of Copernicus, Galileo and Kepler. There was much resistance to the transition between these two theories. Christian theologians were reluctant to reject a theory that agreed with Bible passages (e.g. "Sun, stand you still upon Gibeon", Joshua 10:12). Others felt a new, unknown theory could not subvert an accepted consensus for geocentrism.


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