Summary

     Early Greek astronomy was considered mathematical and consisted of the observation and mapping of the stars. Developed from the ideas of Babylonian astronomy during the Hellenistic period, it would dominate Western and Arabic forms of astronomy until the seventeenth century.  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, especially retrograde motion, that also occur.  For example, Mars, Jupiter, and Saturn all stop and go backwards for about a month and then resume their normal path.  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.

Going by Plato's idea of an orderly universe that merely appears disorderly, the main goal for most early Greek astronomers was to answer the question of what combination of uniform and orderly motions can account for the apparent motions of planets.  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 sun'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.

Two more astronomers who tried to justify celestial varient velocities were Aristarchus and Appollonius. Aristachus tried to answer this question by looking at the size and distances of the Sun and Moon. He, along with his contemporary Heraclides, proposed that Earth itself rotates on an axis.  Moreover, Aristachus suggested the possibility of a heliocentric solar system, though he acknowledged that for the Greek observations, the point was moot.  A heliocentric solar system and a geocentric solar system were mathematically equivalent. Another important astronomer, Apollonius, 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 exact center of the circle. Due to this fact, 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.

Another major influence of the time was Hipparchus (2nd century B.C). All we have are just pieces of his works and an impressive and extensive cataloging of the stars according to their brightness and positions in the sky. During his work, Hipparchus cataloged thousands of stars just be observing them in the sky. He also proposed the Procession of the Equinoxes after observing that the equinoxes changed spots over hundreds of years. Hipparchus also used the idea of the Eccentric Circle from Apollonius to help explain retrograde motion of the planets, which was mathematically equivalent to the epicycle theory but easier to work with and understand.

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 

equinox: the two points on the earth where the sun crosses the equator

procession of the equinoxes: theory proposed by Hipparchus that said the equinoxes of earth have shift and will continue to shift over the span of the years

star catalog: the extensive catalog of stars recorded by Hipparchus that catagorized stars by their brightness and their positions in the sky

Relevant Links

  http://faculty.fullerton.edu/cmcconnell/Planets.html

(This is a link to the diagrams Dr. Ramberg used in class. Just click on the corresponding links to get more information)