Greek Astronomy and Beyond

The Lyceum after Aristotle

Aristotle met Theophrastus on the island of Lesbos (ca. 371-ca. 286), and Theophrastus returned to Athens with him in 335. Theophrastus participated in the Lyceum for 13 years until the Aristotle's death in 322, at which point he took the reins for the next 36 years. Theophrastus wasn't afraid to disagree with some of the teachings of his mentor; in fact, he disagreed with Aristotle's:

1. teoleology- Theophrastus believed that not everthing has purpose

2. 4 elements theory- Theophrastus questioned Aristotle's status of fire as an element

3. theory of light and vision- questioned Aristotle's opinion that light is the actualization of the transparency of the medium

When Theophrastus died, he left the library of his and Aristotle's books to Neleus, whom he may have intended to succeed him. However, Strato was elected to be the new leader of the Lyceum, at which point Neleus took the books home with him to Skepsis in Asia Minor. The books were eventually purchased from Neleus's ancestors and remained in Athens until the 1st century B.C. when the Romans swept through, taking the books with them. The Romans edited and circulated them. Strato led the Lyceum from 286-268; his material ideas included calling into question the theory of 4 elements. Strato also demonstrated acceleration and discredited Aristotle's theory of motion.

Summary

     Early Greek astronomy was considered mathematical rather than physical. It consisted of the observation and mapping of the stars but was not concerned with cause as was the physicist (or naturalist) within Aristotle's paradigm of distinction between the sciences. It was concerned only with prediction of celestial events, not explaining them. 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 apparent irregularities.

Going by Plato's idea of an orderly universe that merely appears disorderly, the main goal for most early Greek astronomers was to decipher what combination of uniform and orderly motions could 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 described 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 could produce figure-8 motion (hippopede) in a planet, which is in agreement with observation.  For better understanding of Eudoxus' hippopede model a diagram follows:

Two more astronomers who tried to justify variant celestial velocities were Aristarchus and Appollonius. Aristarchus 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.  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 by observing their position and brightness 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

epicycle-deferent model: Apollonius' other model of planetary motion that used opposing circular motions to describe retrograde motion.  It is equivalent to the eccentric-circle model in its explaination of planetary motion.

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

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

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

physical astronomy: concerned mainly with figuring out what the matter of the universe is and why the planets behave and move as they do.

equinox: the two points on the earth where the sun's path crosses the equator

procession of the equinoxes: theory proposed by Hipparchus that said the equinoxes of earth have shifted and will continue to shift over the years. One circuit through the heavens back to the point of origin is    23,000 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)