Re: rotation of the earth

George Murphy (
Sun, 14 Dec 1997 12:25:25 -0500

Glenn Morton wrote:

> At 08:32 PM 12/13/97 -0500, George Murphy wrote: .......................
>> 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.
> Now here I agree with you. Ever since my discussion with Ted I stop by the
> history section of my favorite book stores and look in the indexes for
> Magellan.
> I am looking for someone else to support or refute Manchester. So far I
> haven't come up with anything.

This definitely has piqued my interest. The problem, it seems
to me, can be posed in analogy to a similar paradigm shift, Newtonian
gravitation to general relativity. In a sense the deflection of
starlight was the new observation which convinced most physicists of the
superiority of Einstein's theory. This sort of corresponds to Galileo's
observation of the phases of Venus, which Ptolemaic theory can't
reproduce. But Einstein's theory was also strongly supported by the
excess perihelion precession of Mercury, which had been known for ~50
years & which Newtonian theory couldn't explain. The anomaly (if indeed
there is one) resulting from the Magellan voyage would correspond to
this. But as far as I know it wasn't cited by Copernicus, Galileo &c in
support of a heliocentric model. & that, even taking intoi account much
poorer scientific communication in the 16th century, seem very strange
to me.

> >> > 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 think we can appeal to continuity in support of my simplified
argument. Any anomaly seems to vanish if we just take a circular orbit
of the sun around the earth. Now the eccentricity of the apparent solar
orbit is small so the necessary solar epicycle is small (unlike those
for Mars &c) & should not produce a qualitatively different effect from
the simpler case. But I don't want to be dogmatic, never having done
celestial mechanics in detail with Ptolemy.

George L. Murphy