For 2026: Please note that units are considered "under revision" until two weeks before the class date, and page content, web assignments, and exercises may change before that deadline.

Unit 10: Aristotle, Ptolemy, and the Planets

Preparation

• History Web Lecture: We look now at the early models for planetary motion. The Greeks could not conceive of a vacuum, which they thought violated the laws of a perfect universe. Since everything touches, the motions of the things above influence the things below. There has to be some way to explain the interconnectedness of these motions. It is particular important now to keep the different philosophers in the right order, since the later theories are building directly on their predecessors.

• Science Web Lecture: The main topic here is the combination of motions of the moon around the earth and the earth around the sun, which produce the patterns of lunar observations: phases and eclipses. We spend some time on what was eventually realized as a false trail: the Ptolemaic theory of the planetary motion, so that the students realize that not all modern theories were obvious. Ptolemy's theory, for what it is worth, was not only accurate in predicting planetary motions (given the rough instruments available), it remained accurate and consistent well into and past the introduction of Copernicus' theory. It is important that the student realize that this model of planetary motion wasn't accepted simply because it was proposed by someone in authority. The name "Ptolemy" was probably given to Claudius because of his residence in a town of that name, not because of any relationship to the royal family that ruled Egypt. Ptolemy's model was accepted because for all intents and proper purposes, it worked the way a theory is supposed to work, accounting for all the phenomena and allowing the user of the theory to make predictions.

• Homework: The mastery emphasizes the categories of motions and causes, and details about the Ptolemaic system. Be sure that the student understands these; we are laying the foundation for understanding the Scientific Revolution later on.

• Discussion: We will spend time going over the points outlined above (matter, motion, expression in Ptolemy's system), and why many generations found this theory compelling.

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