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EARLY COSMOLOGY TUTORIAL TOPICS Dr. Brian Monson has created this group of pages as a supplement for the unit on early astronomy that is taught in most college astronomy courses and has graciously granted ScienceMaster permission to republish them. It is by no means a complete discussion of this topic. To explore more work by Dr. Monson please visit his Planetary Conjunctions page. |
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Second Iteration
Hipparchus & Ptolemy
Between 160 B.C. and 125 A.D. many improvements were made to the original model of Aristotle. An astronomer from Rhodes named Hipparchus came up with a simpler way to explain the retrograde motion of the planets. He placed the planet on another sphere called an epicycle. This sphere revolved around the earth attached to a larger sphere called the deferent. The revolution of the deferent about the earth led to the motion of the planet along the ecliptic. When the planet was on the outside portion of its epicycle, it would appear to be moving from west to east. When it crossed to the inside portion, it would appear to be moving from east to west or retrograde. The combined motion of these two spheres gave the planets complex, looping paths around the earth instead of simple circles as shown in this animation.
The planets do not move along the ecliptic at a constant speed as predicted by Aristotle's model. Instead they speed up and slow down in a regular pattern. Hipparchus introduced a new geometric device called the eccentric in an attempt to explain this behavior. He displaced the earth from the center of the cosmos by a small amount. The deferent spins about the center at a constant speed but since the distance to the planet varies it appears to speed up and slow down as it approaches and recedes from the earth.
This still did not completely solve the problem of non-uniform motion. About 250 years after Hipparchus, Claudius Ptolemy introduced another geometric device called the equant to help with this problem. It is an imaginary point from which the motion of the deferent appears uniform. If the motion appears uniform from the equant, however, it will not be uniform about the center.
The combination of all three of these devices by Ptolemy led to a model that explained the motions of the planets reasonably well. It was accurate to about ± 5°.
The Church at this time interpreted several passages in the Bible as meaning that the Earth stood still. Therefore they were pleased by this model and tried to suppress any competitors. The complexity of the model and its deviation from pure Aristotelian ideals led some thinkers to attempt a replacement model with the sun at the center of the universe.
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