On Aristotelian motion:

• The natural motion of all terrestrial object is rectilinear. The natural motion of the celestial is circular.
• Motion that is contrary to natural motion down is called 'forced' or 'violent' '(forced' is probably the better translation and that probably what Ptoley is referring to when he rejects the notion that the earth moves / turns), because it is contrary to the natural disposition of weighty objects to fall in straight lines.
• Nature tends to act in the same way unless impeded in some way [e.g., by a by another 'violent' force. So drawing a bow applies to the arrow a potential for 'forced' motion, so that when it is released it moves forward, but also will eventually be overtaken by natural movement and fall to the ground.

What does this all mean?

1. First, for the earth to rotate in a circular matter [the property of a celestial object] is not 'natural', but rather considered 'violent' or forced motion. And is ipso facto impossible.
2. Because the celestial objects move in circles and move at incomprehensible velocities, and because do not fall to the ground, they must in fact be of a substance different from the matter we know on earth. Aristotle called this substance 'quintessence' or 'ether', or the 'fifth element',
3. this was not a problem for the 'fixed stars', fixed as they are in reference to another, and moving very fast (but apparently not falling out of the cosmos). They appeared to be the most perfect and to move most perfectly.
4. but it was a problem for the 'wanderers' [aka planets]. with their retrograde motion and changing 'brightness'. But the anomalies were not so overwhelming that the basic model could be overthrown. and the models of Eudoxus and Ptolemy indicated how this might be done.
5. As Geminuus writes: "For the hypothesis underlying the whole of astronomy is that the sun, the moon and the planets move at uniform speeds in circles...For they [Greeks scientists] could not brook the idea of such disorder in things divine and eternal as that they should move at one time more swiftly, and at another time more slowly, and at another time stand still...but when the heavenly bodies are in question no reason can be assigned for their swifter and slower motion.




6. It was Galileo, the telescope and the the alternate center of rotation that really doomed the Aristotelian model.