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    Short Term Proper Motion
    From: Bill Bertrand
    Date: 2019 Mar 9, 08:19 -0800

    I am unfortunately having a problem calculating simple short term proper motion.  Since I am off by orders of magnitude, it's certain I missed a point obvious to others.

    To use the simplest case, I am confining my example on proper motion in declination, to avoid RA's need to be a function of the cos(declination).

    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'
    • 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!
    • I calculate 16.99 Julian years between J2000 and the date above at 365.25 JDays/JYear
    • Stellarium pmDec =-13.6 (mas/yr), Hipparcos pmDec = -12.88 (mas/yr) milliarcseconds/year
    • @-12.88 mas/yr  I get -218.86 mas in 16.99 years or -0.218 arcseconds
    Where did I go wrong?
    Thank you in advance,
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