On Sat, 3 Feb 2007 17:05:23 -0800, The Ghost In The Machine
wrote:

In sci.physics.relativity, HW@....(Henri Wilson)
HW@
wrote
on Sat, 03 Feb 2007 22:38:56 GMT
:
Sure, they're a long way from us...but there are a great many out there in our
galaxy and every object must be in orbit around a mass centre of some kind.

Most do not appear to have moved much in thousands of years.
Should we not expect to see more movement than we do?

...my question may be naive and the answer trivial... so please enlighten me.

If one assumes that a star 50 light-years away is moving
at a speed of 2 * 10^-3 c[*], that means it will move
2 light years per millennium. Assuming that it is moving
sideways to us that resolves to about 1/25th of a radian
per millennium, or 2.30 degrees per millennium, or 8.05
arcseconds per year. Even were the star 10x closer (or
5 l-y) one only gets 80.5 arcseconds per year -- or 1'20.5".

Yes, thanks for that Ghost. More or less what I thought.

I was wondering why more stars were not seen changing places as they orbit
each other reasonably closely.

I suppose the answer is that all objects in optically resolvable orbits are
always moving very slowly around that orbit.
No large object in our galaxy appears to be moving at anything like c wrt
anything else.....something I find interesting.

Contrast this to Pluto's distance of 39.481 AU or 5.9 * 10^12 m
(semimajor axis), an average orbital speed of 4.666 * 10^3 m/s,
and an orbital period of 248.09 years; this translates into
1.45 degrees per year, despite the fact that Pluto is moving more
slowly (about 1.5 * 10^-4 c) in an absolute sense.

Hence the term applied to the 8 planetei -- Greek for "wanderer".
(Pluto got demoted. :-) )

[*] this is double the estimated speed at which the Sun
is swinging around the Galactic core. It is theoretically
possible for a star to be retrograde.