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    Re: Polaris' PM in RA?
    From: Paul Hirose
    Date: 2020 Sep 24, 14:02 -0700

    A diagnosis is difficult since Sean has not given his results. However,
    one frequent point of confusion is the exact meaning of the right
    ascension proper motion: seconds of arc vs. seconds of time, seconds per
    Julian century vs. milliseconds per Julian year, etc.
    Another issue is coordinate rate vs. great circle rate. In other words,
    does the unit of RA proper motion get smaller as you approach the poles,
    or is it always the same size as the declination unit?
    The latter is the modern tendency. Typically, both proper motions are
    given in mas (milli arc seconds) per Julian year (exactly 365.25 days).
    In other words, they are both in great circle units.
    For navigational stars the most reliable catalog is probably the
    re-reduction of the Hipparcos satellite observations (van Leeuwen,
    2007). That seems to be the default catalog used by SIMBAD.
    SIMBAD right ascension proper motions are in great circle units: mas per
    Julian year. A note to that effect appears if you hover the mouse
    pointer over the place.
    If you demand a high degree of accuracy it's not easy to validate your
    own software. You can't be sure how far to trust other applications
    until they have been tested. One good test is the stellar reduction
    example in every edition of the Astronomical Almanac. It's challenging
    since the fictitious star is close and has high proper motion. If your
    software succeeds here (within its intended accuracy, not necessarily
    matching the Almanac to the last digit), it's almost certainly OK.
    The Almanac example stops at the geocentric coordinates. For a reduction
    all the way to azimuth and altitude, see the Patrick Wallace example.
    It's rather intimidating, but if there's anything weak in your code this
    will reveal it.
    For a detailed discussion on these matters use the "contact me" link at
    my site.
    Paul Hirose

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