Summary

     Early Greek astronomy was basically concerned with the observation and mapping of the stars.  It was not until the 4th century B.C. that major changes began to occur under the prodding of Plato and Eudoxus, who produced the two-sphere universe (Lindberg p. 86).  The main astronomical question the ancient Greeks came across was how to compound the circular motion of the planetary system with the irregularities that also occur, such as retrograde motion. For example, Mars, Jupiter, and Saturn all stop and go backwards for about a month and then resume their normal path.  They produced the two-sphere universe. Ancient Greek astronomers wanted to explain how it was possible for the universe to be a regular orderly place, as Plato concluded, and still have these irregularities appearing; they wanted to demonstrate how the irregularites were regular

Going by Plato's idea of an orderly universe that merely appears disorderly, the main goal for most early Greek astronomers was to find a way to compound circular motions to account for irregular apparent motions of the celestial bodies. One of the first who attempted to answer this question was Eudoxus. According to Eudoxus, each planet has four nested spheres of motion. The outermost sphere was the celestial sphere, which accounts for the daily west to east motion of the celestial body. The next sphere was the ecliptic sphere, which describe the annual west to east motion of the body at a 23.5 degree tilt (like the sign's ecliptic). The innermost rings were called planetary rings, and they moved in the opposite direction, counterbalancing the motion of the outermost spheres. These four spheres combined can produce a figure 8 motion (hippopede) in a planet, which is in agreement with observation.

Apollonius was another astronomer who invented two separate mechanisms to explain the motion of the heavenly bodies. The first was called the eccentric model. It supposed that the planets and other celestial bodies do indeed move in circular orbits around the Earth, but the Earth is not at the center of the circle. Because Earth is slightly off-center, we observe varied speeds in other celestial bodies (the sun, for example). The second model was the epicycle on deferent model. This model placed the Earth at the center, and gave celestial bodies circular orbits around the Earth. This circle is called the deferent. In addition, each body orbiting the Earth also moved in an epicycle (its own circular orbit around the main circle) in the opposite direction.

Primary Sources

  "... the astronomer, when he proves facts from external conditions, is not qualified to judge of the cause, as when, for instance, he declares the earth of the stars to be spherical; sometimes he does not even desire to ascertain the cause, as when he discourses about an eclipse; at other times he invents by way of hypothesis, and states certain expedients by the assumption of which the phenomena will be saved. For example, why do the sun, the moon, and the planets appear to move irregularly?"

- Simplicius 'Commentary on Aristotle's Physics'

This segment of Simplicius really ties in with the whole attempt of ancient Greek philosophers to keep their belief of an orderly universe intact while explaining the irregularities in it.  They thought that the universe was an orderly place, and they knew there were irregularities, therefore they believed there was some way to explain that the irregularities were regular.

Key Terms and Definitions

eccentric-circle theory: a theory Apollonius came up that concluded the earth was slightly off the center of the universe, which explains why irregularities are observed

retrograde: means moving backward in space or time, used to explain the shared behavior of Mars, Jupiter & Saturn

physical astronomy: interested in the why's, 'is this really the way things are constructed?'

mathematical astronomy: interested only in predictions, makes no difference how the end resuslt came about 

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