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From: jeff@aiai.ed.ac.uk (Jeff Dalton)
Subject: Re: Data Fitting & Ptolemaics
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References: <405u2c$n52@news.service.uci.edu> <807833022snz@longley.demon.co.uk> <1995Aug8.125421@cantva>
Date: Fri, 18 Aug 1995 17:34:35 GMT
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In article <1995Aug8.125421@cantva> misc183@csc.canterbury.ac.nz (Sean Broadley) writes:

>_Ptolemy's_ Ptolemaic system was very simple.  The idea that you should keep on
>explaining the data in a finer and finer manner by adding more and more
>epicycles ad infinitum was a later development.  Copernicus's epicycle system
>was horrifically convoluted (40 or 50 different circles, I believe).  It was a
>slight simplification on a geocentric approach of his time, but only slight.

I wonder about this "adding more and more epicycles ad infinitum".
That seems to be a common view of the later elaborations of the
Ptolemaic system, but it is correct?  Like many people, I wouldn't
have questioned it.  But I hadn't ever read a description of the
actual system(s).  A week ago, or so, I finally did read one,
and it was very different from the epicycle-piled-on-epicycle
impression we're often given.  There was just one epicycle.
Instead of adding more, two things were done.  The earth wasn't
at the center of the main circle -- it was inside but not quite
at the center.  And the epicycle's center did not move at a
constant speed around the large circle: there was a 3rd circle
that regulated the speed.  (This isn't as clear as it could be,
and I may have it somewhat wrong, but I don't have the book
handy.  A proper reference, and corrected details, can be posted
if desired, once I grab the book at home.)

-- jeff
