Page 17 - GALIET PHYSICS BLOSSOMS I+

“C” to “Q”
Point “Q”
is placed symmetrically on the opposite side of C on the deferent’s apogee-perigee line.
“Q” becomes the equant point or punctum aequans.
[more details of this are seen on page 11-12]
In other words, the epicycle’s center circles around a circle offset from the earth.
Remarkably, predicted motion agreed well with observations.
In Ptolemy’s final model, Jupiter as illustrated in Figure 2 travels uniformly around its epicycle while the center of the epicycle on its deferent travels uniformly around the equant Q. In this way, regular motion is maintained. One can partially discern the many apparent retrograde and prograde motions of Jupiter as it circles the deferent. Venus is also partially shown here for simplicity’s sake. Figure 3, on the other hand, shows the intricacy of Ptolemy’s system if the five planets, the sun and moon are included.
Ptolemy chose the Grecian epicycle model as his basic mathematical tool, because it shows not only open curves, but it also generates every conceivable closed curve:
(i.e. circles, squares, ellipses, etc.)8
8 This figure and eplanation is drawn from Dr. Robert A. Hatch’s http://www.clas.ufl.edu/users/ufhatch/HIS-SCI-STUDY-
GUIDE/0034_summaryPtolemiacAstron.html
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