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Epicycle in Ptolemaic Astronomy: Definition & Overview

Instructor: Cassie Beyer

Cassie holds a master's degree in history and has spent five years teaching history and the humanities from ancient times to the Renaissance.

Ptolemaic astronomy was an earth-centric view of the universe that envisioned that all of the planets orbited around the earth. Learn how epicycles were used to address discrepancies in the observed motions of the planets.

Background

Planetary motions as observed in the night sky.
Image of Observed Planetary Motions

Claudius Ptolemy was a Roman astronomer who lived in Alexandria, Egypt, in the 2nd century AD. His attempt to explain the motions of the celestial bodies became the accepted theory throughout Europe and the Middle East for almost 2000 years. Only during the Scientific Revolution of the 16th and 17th centuries did people such as Nicolaus Copernicus and Galileo Galilei call the Ptolemaic model into question.

Ptolemy's Observations

According to Ptolemy, the earth sat in the center of the universe with all of the planets (including the sun and the moon, which were also described as planets during this time) sitting within nested crystalline spheres surrounding the earth. They were ordered according to the amount of time necessary for the planet to make a single observed orbit. Thus, the moon was the closest, then Mercury, Venus, the Sun, Mars, Jupiter and Saturn. (Uranus, Neptune and Pluto were unknown at the time as they can only be observed by telescopes, which were not invented until the Scientific Revolution.)

Because the universe was expected to be an orderly place, the spheres were understood to turn in a perfectly uniform motion. However, even without a telescope one can observe that the planets do not move in straight lines across the sky at a constant speed. They appear to slow down, speed up, and periodically move backward.

Planet in orbit around earth with a single epicycle
Epicycle orbits and observed motion

Epicycles

The orbit an planet took around the earth is known as a deferent. An epicycle is an orbit revolving around a point on the deferent. As a planet rotates around the earth, it also rotates around a point on that orbit. This could roughly, but not completely, explain the predictable but not uniform motion of the planets. It was also acceptable in the common world view of an orderly universe, because the planets were still moving in an orderly fashion within their epicycles. To be more exact in explaining the motions of the planets, the epicycles eventually got their own epicycles.

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