On Thu, May 26, 2011 at 12:06 PM, Daniel C. <[email protected]> wrote:
> So if I lived in Greece in the 4th century BC, then the Sun I saw
> rising each day was stationary and the Earth revolved around it;
> unless I was a Pythagorean, in which case the Earth revolved around
> the Sun; but if I left Greece and moved to Egypt, it goes back to the
> Earth being stationary (in Egypt, at least), until I can convince
> everyone that they're wrong and I'm right and once that happens the
> Sun stops moving and the Earth starts up again.
>
> All of this is of course perfectly sensible and could not under any
> circumstances be challenged except by the most intellectually obtuse,
> but I'm still confused about something. Hopefully you can enlighten
> me. We know that in Egypt in the 4th century BC the Sun revolved
> around the Earth, since that's what everyone believed. But when half
> of the people in Egypt believe that the Earth is stationary, and half
> of the people in Egypt believe that the Sun is stationary, which one
> is moving and which one is holding still? The answer is probably
> obvious, but I'm just too dumb to see it. Can you help me out, Alan?
It is indeed true that the sun moves around the Earth, even today.
It's just that when you consider solar and planetary motion from an
Earth-centric frame of reference, it's much more difficult to model
and predict the paths of bodies, and it doesn't reflect the actual
forces that constrain the paths. It would also be easy to choose a
frame of reference in which neither the Earth nor Sun were stationary,
such as using the moon as the static point. This frame of reference
is less useful from the perspective of trying to model the motion, but
it would be a useful frame of reference to translate the paths into in
order to aim solar collectors or antennas on a future moon base.
Motion is all relative, and choosing the frame of reference from which
to analyze things is one of the first steps in any physics problem.
The frame of reference might change depending on what kind of analysis
you're trying to do!
--Levi
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