Re: [asa] geocentricity

You can use any coordinates you wish but also have to recognize that usually
no single coordinate system is good for the entire space (or space-time)
you're dealing with. E.g., the ordinary system of latitude & longitude
breaks down at the poles, which have no unique longitude. The so-called
"Schwarzschild singularity" - the event horizon of a black hole - is
actually a singularity of the coordinate system. In order to describe what
happens inside the event horizon you have to use other coordinates.

& you can use a frame rotating with the earth, so that the earth is
stationary. In fact, we do that in practice all the time. & such a frame
is fine for calculating the motion of projectiles, earth satellites &c
(including centrifugal & Coriolis forces). But the coordinates become
unphysical at a distance of about 30 astronomical units because objects that
would be used to mark the coordinates would have to move faster than c.

Shalom
George
http://web.raex.com/~gmurphy/
----- Original Message -----
From: <kbmill@ksu.edu>
To: <asa@calvin.edu>
Sent: Saturday, July 07, 2007 9:07 AM
Subject: Re: [asa] geocentricity

Quoting George Murphy <gmurphy@raex.com>:

 Of course it's true that the earth exerts a gravitational influence on
the sun as well as vice versa, so that what we usually call a
"heliocentric" frame is (when we're doing precise celestial mechanics)
one in which the CM of the solar system is at rest. But there is no
"must" about using this frame or any other. General relativity really
is general - you can use any space-time coordinates you wish (subject
to some modest conditions about continuity & differentiability).

_______________

I have a question concerning the limitation of reference frames. If the
Earth is taken as not rotating, then stars and ultimately galaxies end
up moving at velocities far exceeding the speed of light. However, we
can also calculate the speed of stars and galaxies relative to a
stationary Earth. Thus there is a way to exclude at least one possible
reference frame. It seems then that all conceivable reference frames
are not possible. Am I missing something?

Keith

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Received on Sat Jul 7 12:29:28 2007