Modeling Velocities

[Steve Allen]

On Sat 2018-12-22T13:40:40-0500 Arnold Rots hath writ:
>   if so, is it always EQUATORIAL?
> Usually, but I don't think it needs to be, necessarily; could be converted
> to Galactic, for instance
> Caveat: it can be either specified by separate RA and Dec components, or as
> total PM and a direction (CCW from North)

Double caveat.

Existing PM catalogs sometimes measure as motion per year, sometimes
as motion per century.  In both cases the distinction between Julian
or Besselian is not relevant for *most* catalogs.

Existing PM catalogs for objects not near the pole sometimes express
as change of coordinate per time rather than change of arc length on
sky per time.  Therefore some catalogs have a cosine declination
factor in the proper motion for right ascension.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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