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    Re: Additional error found in H.O. 249
    From: Douglas Denny
    Date: 2009 Oct 7, 14:13 -0700

    "I do understand that stars have proper motion." 
    They do, but even the fastest moving have completely negligable proper motion 
    and are difficult to measure with the most sophisticated photographic methods 
    over long periods of time.
    Precession and nutation are the main reasons for apparent star motions, simply 
    speaking measured by and referenced to the changes of the first point of 
    Aries around the equinoctial and the obliquity.
    The changes are only slight, with a period of 25800 thousand years for full 
    precession around the equinoctial;  with  nutation having  much shorter 
    period linked to the motion of the Moon of 18.6 years but the effects are 
    miniscule: in the order of longitude by 17" and obliquity by 9".
    Douglas Denny.
    Chichester. England.
    Original Message:-
    Hi Gary
    What is the magnitude of these errors?  Several arc-seconds? An arc-minute?  
    Several arc-minutes? A degree?!!?!?!?
    My 1853 Bowditch gives a long term almanac, in which the motion or rate for 
    the stars is given for a year, and you just multiply by years and add to the 
    base value.  Surprise, it agrees quite
    readily with current values (not perfectly, but dang close).  It didn't seem 
    like a huge delta for a 156 year run (1853 -> 2009).  I can't imagine this 
    to be a huge injected error for 1982 -> 2009.
     Wouldn't one obtain the SHA 
    and declination of a star from the Nautical Almanac for the day in question?  
    Why would it be preferable to obtain
    this from HO 249 other than convenience?
    Best Regards
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