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On 2019-03-13 7:01, I wrote:
> Note that RA proper motion is great
> circle units, not coordinate units. I.e., the unit is the same size as
> the declination proper motion.

That's a pitfall in the computation of proper motion. Several years ago,
in a discussion about a possible occultation in the 3rd century BC, I
noticed a bug in the Swiss Ephemeris correction for proper motion. It
incorrectly assumed right ascension proper motion in the Hipparcos
catalog was the instantaneous coordinate rate. Due to the convergence of
the hour circles, this unit becomes smaller (relative to declination
proper motion) as you approach the poles. Since the star was near the
equator (I think it was Spica) the bug might have escaped detection if
not for the remote epoch.

Nevertheless, the simple assumption that both coordinates change at a
constant rate is satisfactory over a short time span if utmost accuracy
isn't important. But in the long term it implies each coordinate makes a
full circle! To realize the full accuracy of the Hipparcos catalog, see
Section 1.2 in the catalog, or Section B of the Astronomical Almanac.
Although they are not the same, both methods express proper motion as
space velocity vector which is assumed constant.

   

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