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    Re: Star proper motion
    From: Bill Noyce
    Date: 2001 Nov 20, 7:42 AM

    > The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version)
    > (Hoffleit+, 1991)
    > Position (J2000) RA 6h 45min 8.9sec Dec -16 42' 58"
    > Proper motion   RA -.553 arcsec/a  Dec -1.205 arcsec/a
    >
    > data from SKY2000 - Master Star Catalog (Myers+ 1997)
    > Position (J2000) RA 6h 45 min 8.821sec  Dec -16 42' 56.85"
    > Proper motion -0.0385 arc/a  Dec -1.205 arcsec/a
    
    Since the difference is close to a factor of 15, it looks
    to me as if -.553 is measured in arcseconds, but -0.0385
    is measured in seconds of time.
    
    There's a further difference to be aware of: I believe the
    -.553 measures the speed of the star in the RA (or -SHA)
    direction, but that's not quite the same as saying it's the
    amount to add to RA each year.  Consider it a 'departure'
    rather than a 'delta-longitude', and divide it by cos(decl)
    first.
    
    I think if you make both these corrections, the data will
    start to agree better.
    
    
    PS- If you're calculating star positions, be sure to consider
    precession (about 1'/year) and annual aberration (about 0.3')
    which both have a greater effect than proper motion.
    
            -- Bill
    

       
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