NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Short Term Proper Motion
From: Paul Hirose
Date: 2019 Mar 13, 10:40 -0700
From: Paul Hirose
Date: 2019 Mar 13, 10:40 -0700
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.
