Skeptical and Natural Philosophy

The Scientific Revolution

How did Ptolemy’s view of the solar system become the accepted theory?

Ptolemy of Alexandria (90–168 C.E.), using observations and existing written work between 127 and 151 C.E., codified the common sense of his time that the Sun and planets revolved around Earth. His work overthrew the more revolutionary writings of Aristarchus of Samos (c. 310–230 B.C.E.), who in On the Sizes and Distances of the Sun and Moon claimed that the Sun is much larger than Earth based on his observations of our Moon. According to Archimedes of Syracuse (287–212 B.C.E.), who combined mathematics with observations to found the science of mechanics, Aristarchus said “that the fixed stars and the Sun remain unmoved, that the Earth revolves around the Sun on the circumference of a circle, the Sun lying at the center of the orbit.” Aristarchus correctly surmised that to explain the apparent immobility of the fixed stars—and assuming Earth did move—the distances between the stars would have to be huge compared to the diameter of Earth’s orbit.

Aristarchus’ theory was defended by Seleucus of Babylonia in the second century B.C.E., but the consensus of educated opinion was that Earth was the center of the universe, either as a floating ball that the heavens revolved around, or a stable solid, which was how it appeared to humanity. Hipparchus of Nicaea (c. 190–c. 120 B.C.E.) in Bithynia, around 130 B.C.E., put forth a theory based on the work of Eudoxus of Cnidos (c. 409–350 B.C.E.). According to Eudoxus and Hipparchus, the apparent movement of the Sun, Moon and planets was the result of their presence in crystal spheres that were concentric in relation to Earth. It was this view that Ptolemy used as a basis for his mathematical calculations.


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