STC Questions

[Anita Richards]

>
> Note that this is only relevant (at least I think I have this list
> complete) for objects for which RA and Dec are time-variable due to:
> - proper motion
> - parallax
> - orbital motion: binary stars, solar system objects,
>  extra-solar planets, galactic rotation
>
> Epoch should not be confused with equinox which is a property of the
> coordinate system (as correctly pointed out in the previous messages),
> if applicable.

Very true.

I would point out that for observations madwe in B1950 the epoch is always 
relevant I believe, and a frequent cause of grief and wailing and gnashing 
of teeth is that for many years VLA positons were in B1950, epoch 1979.9 
(or something - not 1950) which was often overlooked by people retrieving 
archive data for combination with MERLIN data in B1950 epoch 1950; to make 
matters worse in some cases phase reference source positions would be 
taken from one epoch but the target position precessed to another epoch 
without taking that into account..

which is not a problem for STC but those of us implimenting it 
might want to find ways of encouraging anyone publishing sub-arcsec data 
in B1950 to record the epoch...

cheers
a

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Dr. Anita M. S. Richards, AstroGrid Astronomer
MERLIN/VLBI National Facility, University of Manchester, 
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K. 
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).