by DavidCortner » Fri Jul 03, 2015 10:03 pm
Ann, as Cousin Ricky pointed out, epicycles (and epi- epicycles within epicycles) were the key. They are requrired to make the Ptolemaic system reflect what we see in the unmagnified sky. In the simpified version you posted, Venus could appear at any distance from the Sun, even opposite the Sun in the sky. That, of course, never happened, and so Ptolemy et al needed to add complexity: they needed something to insure that Venus was always within 50-ish degrees of the Sun. An epicycle worked just fine until Galileo saw that Venus was not always only partially illuminated: sometimes it was fully illuminated. Maybe a sufficiently motivated Ptolemaniac could find a way to save the appearances with ever more complex epicycles, but too much is just too much and Copernicanism offered a vastly simpler (if somewhat humbling) alternative.
Ann, as Cousin Ricky pointed out, epicycles (and epi- epicycles within epicycles) were the key. They are requrired to make the Ptolemaic system reflect what we see in the unmagnified sky. In the simpified version you posted, Venus could appear at any distance from the Sun, even opposite the Sun in the sky. That, of course, never happened, and so Ptolemy et al needed to add complexity: they needed something to insure that Venus was always within 50-ish degrees of the Sun. An epicycle worked just fine until Galileo saw that Venus was not always only partially illuminated: sometimes it was fully illuminated. Maybe a sufficiently motivated Ptolemaniac could find a way to save the appearances with ever more complex epicycles, but too much is just too much and Copernicanism offered a vastly simpler (if somewhat humbling) alternative.