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    Re: Short Term Proper Motion
    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.

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