Ptolemy’s
explanation: Epicycles
•Uses multiple levels of “circular” motion.
•Planet moves on a small circle called en epicycle.
•Center of epicycle moves on a larger circle called a deferent.
•Earth is fixed near (not exactly at) the center of the deferent.
•Motion around deferent is only constant as seen from point called equant.
•Add epicycles on epicycles to refine motion.
Advantage:
Could predict precise positions of planets
Disadvantage: No physical explanation of why motion is like this
From our text: Horizons, by Seeds