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    Re: Short Term Proper Motion
    From: Paul Hirose
    Date: 2019 Mar 12, 20:42 -0700

    On 2019-03-11 20:01, Bill Bertrand wrote:
    >    -- The j1991.25 Dec value is listed (it differs ~3x10^-5) degrees for the 9 year difference
    Here are Bellatrix data from the original (1997) Hipparcos catalog and
    the 2007 re-reduction. Values are RA, dec, parallax (mas), RA PM
    (mas/yr), dec PM at J1991.25. The RA and dec I have converted to
    sexagesimal at sufficient precision. 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.
    081.28278416 +06.34973451 13.42 -8.75 -13.28 (1997)
    (05 25 07.868 +06 20 59.05)
    081.28278339 +06.34973457 12.92 -8.11 -12.88 (2007)
    (05 25 07.868 +06 20 59.05)
    I don't see those coordinates in the supposed Hipparcos data you quoted.
    Where are they?
    In addition, the "RA/Dec (on date)" aren't meaningful without knowledge
    of the basis of the coordinates — the stuff I mentioned in the previous
    message, such as astrometric place vs. apparent place and ICRS vs. true
    equator and equinox. Until you resolve that question, it's futile to
    compare those values to yours.
    Roger thinks they may be apparent place, but if so, they aren't very
    accurate — about 7 arc seconds from my computation with respect to the
    true equator and equinox.

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