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    Re: Short Term Proper Motion
    From: Paul Hirose
    Date: 2019 Mar 10, 16:37 -0700

    On 2019-03-09 16:31, Bill Bertrand wrote:
    > The example:  Bellatrix on Date: 2016/12/28 T 23:14:15 UTC
    >    -- (Stellarium output for the date is listed below and the Bellatrix 
    fragment of the Hipparcos catalog below that)
    >    -- Dec J2000            (Stellarium &  Hipparcos): 6°20'58.7" or 6°20.98'
    Note that the epoch of the Hipparcos catalog is J1991.25, so any J2000.0
    coordinate must have been extrapolated. It can't have come directly from
    the catalog. That's true for both the original Hipparcos catalog and the
    re-reduction published about 10 years later.
    >    -- Declination on that date (USNO, Stellarium): 6°21'41.0" or 6°21.68'
    >    -- The Declination on the 2 dates differ by *0.7 minutes of arc!*
    Are you comparing like to like? To be unambiguous about a coordinate,
    its origin (solar system barycenter, heliocenter, geocenter,
    topocenter), "place" (geometric, astrometric, apparent), and reference
    system (ICRS, true equator and equinox, etc.) needs to be clear.
    For Bellatrix the parallax is only .013", so the coordinate origin makes
    no difference in this case. And there's no distinction between the
    geometric and astrometric place of a star. However, there's the matter
    of geometric place vs. apparent place and the ICRS vs. the true equator
    and equinox of date. I suspect the explanation for your discrepancy lies
    in that direction.
    At 2000-01-01 12:00 TT (= J2000.0) I get
    81.28276 6.34970 geocentric geometric RA & dec (ICRS)
    81.28827 6.34949 geocentric apparent (ICRS)
    81.28459 6.34767 geocentric apparent (true equator and equinox)
    At 2016-12-28 23:14:15 UTC I get
    81.28273 6.34964 geocentric geometric (ICRS)
    81.28833 6.34952 geocentric apparent (ICRS)
    81.51471 6.36103 geocentric apparent (true equator & equinox)
    The decrease in geocentric geometric declination is .00006°, which is
    consistent with -12.88 mas/yr in the catalog.
    Basis of computations: Hipparcos catalog 2nd reduction (van Leeuwin,
    2007), IAU 2006 precession and 2000B nutation.

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