Re: rotation of the earth

George Murphy (
Sat, 13 Dec 1997 20:32:41 -0500

Glenn Morton wrote:
> At 01:45 PM 12/13/97 -0500, George Murphy wrote:
> > First, I think we need something from the historians of science
> >about whether or not this aspect of Magellan's voyage _was_ seen as a
> >challenge to the Ptolemaic model in the early 16th century. ................
> Ask and you shall receive. Here is what William Manchester says, ..............

> I want to point out that Ted Davis, (if I recall correctly) and I discussed
> this thing at length a while ago. His concern was that Manchester was
> wrong. I was able to point him to contemporary literature about the
> puzzelment of the crew. Here it is, ....................

Thank you for these citations. I do not want to seem like the
monks who refused to look through Galileo's telescope, but I am still
quite puzzled by this. The primary problem is just that I have never
heard of this having had any influence on the debates about the
Ptolemaic & Copernican theories in the 16th & 17th century, as
Manchester's account suggests that they did.

> > It seems to me you've made things more complicated than
> >necessary by introducing 4 motions. The essence of the problem can be
> >seen just with the rotation of the "fixed stars" and the motion of the
> >sun around the earth in an approximately circular orbit. The epicycles
> >& deferent are unneeded refinements for this purpose.
> If one is going to see if the voyage of Magellan had any implication for
> Ptolemy, one must use the system that Ptolemy and the Medievals used. Their
> system was a whole bunch of complicated cycles and epicycles etc. Mercury
> rotated around an epicycle, which in turn rotated around a center which also
> rotated around its own circle. I would refer you to A.C. Crombie, "Medieval
> and Early Modern Science, Vo. 2, (new York: Doubleday, 1959) p. 171

I'm quite aware of the complexity of the Ptolemaic system - &
also of the fact that all the epicycles &c weren't removed by
Copernicus. But while the general idea of geocentricity was essential
to the Ptolemaic-Aristotelian world view of the Middle Ages, the precise
system of epicycles &c wasn't _de fide_, & as I understand it, there was
some freedom for astronomers to play with these details. It seems to me
at the very least that an intelligent geocentrist would have said,
"Well, I can't see how to get the details to work out, but the basic
"loss of a day" shouldn't be a problem" - using the type of argument I

George L. Murphy