EXPLAINING THE MOTIONS OF THE WANDERING STARS

A nice illustration of the various models for the motions of the wandering stars is available on this YouTube video:

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Skip the first 16 seconds! The illustration of the models is from a planetarium in Italy, but someone tacked on a lousy attempt at illustrating retrograde motion of Saturn, and they tacked on an overly general statement. From 16 seconds forward the video is good.

The first model seen in the video shows Aristotle’s geocentric explanation for how a wandering star like Saturn moves:

This shows more detail than is discussed in the book, but notice that in this model the distance between the Earth and the wanderer does not change, so there is no explanation of why a wanderer increases in brightness when it goes through retrograde motion.

The second model seen in the video shows Ptolemy’s geocentric explanation for how a wandering star like Saturn moves:

This shows the epicycle and deferent in action. The epicycle causes the retrograde motion, and carries the wanderer closer and farther from Earth, explaining the why a wanderer increases in brightness when it goes through retrograde motion.

The last model seen in the video shows Copernicus’s heliocentric explanation for how a wandering star like Saturn moves:

The action of the Earth passing between the sun and the wanderer causes the illusion of retrograde motion (the wander itself never moves backwards), and the approach of the Earth toward the wanderer as it passes explains why a wanderer increases in brightness when it goes through retrograde motion. That the sun is the center of motion for both Earth and wanderer explains why retrograde motion occurs only when the sun, Earth, and wanderer fall along a line.