Hi Bill,

--- In guide-user@yahoogroups.com, Bill J Gray <pluto@...> wrote:

> I've been using the versions at
>
> ftp://cdsarc.u-strasbg.fr/pub/cats/VI/49/
>
Yes I had seen your note on the CDS, this is what gave me
the idea to check my calculations with Guide.

>...I doubt it would make a huge difference -- certainly not an 18"
>one but again, it shouldn't hurt to do the math accurately.


Hmmm.. I use the " symbol for arcsec, I am not sure in English ?
I supposed that by 18" you mean the 18 time-minutes difference
between JPL and Guide. Indeed the two preceding possible causes
of error cannot account for more than one or two minutes of time.
For me they could be the cause of the small differences
(+- 1 minute of time) I got between Horizons and my calculations.


>
> > And after all that there is the aberration question.
>
> True. And the difference you're seeing is an aberration-sized
>one. But again, I am reasonably confident that the boundaries ought
>to be defined to be "fixed" in either B1875 or J2000 coordinates;
>i.e., a star with near-zero proper motion and parallax shouldn't
>change constellations twice each year, simply because its apparent
>coordinates cross a particular RA or dec boundary. (Nor twice every
>eighteen years as the earth nutates.)

This was the point that most bothered me. Now I am sure you are right
and I shall change my calculations to work only with the mean
positions. They will agree no more with Horizons but they will do
with Guide. Thanks a lot for all.
Luc