Re: Sir Hoyle on Copernicus and Ptolemy

["John S. Denker" <jsd@MONMOUTH.COM>]

Savinainen Antti wrote:

> I understand that a reference frame can be attached to the Earth as
> well as to the Sun.

Good.

> But this does *not* imply, I believe,  that there is no way of
> telling if the Earth revolves around the Sun or vice versa. As far as
> I know, the Earth (and the Sun) revolves around the center of mass
> (CM) of the Earth-Sun system. This CM lies near the center of the
> Sun.

The CM lies the sun, but not exactly centered on the sun.

So the earth is a non-inertial reference frame,
and the sun is a non-inertial reference frame.
If you're going to allow one, you have to
allow the other.  It's both or neither.
Maybe one is "more nearly" inertial, but that's
a difference in degree, not a difference in kind.

For many purposes, e.g. the construction of
sundials, the fact that the earth is a non-inertial
frame is negligible, and for such purposes it
is perfectly OK to say that the sun orbits the
earth, without applying any corrections.

There is a range of situations where using the
earth frame would require complex corrections
but the sun frame would not.  But this is a
bit dodgy, requiring you to look closely
enough to see one set of corrections but not
closely enough to see other corrections.

If you look closely enough, you either need
to use the CM frame (in which case the sun
or the earth are equally unacceptable) or
to apply appropriate corrections to your
non-inertial frame (in which case the sun
and the earth are equally acceptable).