Aristotle's heavy and light
This is going to be a very short post. I was skimming the part in the De Caelo where Aristotle considers the meaning of heavy and light, and also the meanings of upward and downward movement. He says that upward movement is movement which is toward the extremity of the universe, downward movement is that which seeks a center.
Now the center he refers to above is most probably the center of the earth as he conceives this to be the center of the universe, yet it's interesting that the definitions remain a bit open ended. It seems that upward motion could correspond to the so-called inertial motion by which a body maintains its speed and direction even unto the edge of the universe. It is even more obvious that account downward motion as centripetal motion corresponds to gravitational motion by which every body tends to every other body as a center.
I intend to read fully books 3 and 4 of De Caelo which discuss the natural motion of terrestrial bodies. Hopefully this will help me to determine if planetary motion is natural, given Newton's proof the they are moved according to the same principle by which lower bodies are moved.
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