The Geocentric Theory


Diagram taken from: http://www.astro.ucla.edu/~wright/ solar_system.html

The figure on the right shows the geocentric model, while the figure on the left is an enlargement of the Sun, Venus, Mercury, Moon orbits.

The early Greeks observed the sky and all that it contained. From their observations, the Greeks believed the Earth was the center of the moon, Sun, and the only known planets at that time, Mercury, Venus, Mars, and Jupiter. These planets were said to be moving around Earth in a clockwise direction. They believed the Earth was motionless, because no one felt the Earth moving. The stars appeared to move around the Earth daily, further convincing them of this theory, which became known as geocentric or Earth-centered. The Greeks had a basic understanding of geometry and trigonometry, which lead them to conclude that fast moving objects were closer to the Earth than slower moving objects.

Around 140 A.D., Claudius Ptolemy wrote thirteen volumes on the motion of the planets, and put the geocentric theory in its finest form. The models in Ptolemy's volumes became known as the Ptolemaic system. In these volumes, Ptolemy discusses epicycles, deferent, and retrograde. Epicycles are tiny circles the planets orbit on. The center of an epicycle would move along a large circle, known as the deferent. Ptolemy used epicycles in his models to make retrograde more visible. The combination of counter-clockwise motion, epicycles, and deferent create the motion of retrograde. Ptolemy explained retrograde motion, as an apparent westward drift. This westward drift of the planets would give one the impression that it was backing up.

The Roman Catholic church accepted the Ptolemaic theory, because biblical passages suggested the sun was in constant motion while Earth remained in one place. Since the Church was in control during this time period, anyone who did not believe in the Ptolemaic theory would be punished, possibly with house arrest.

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