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    Re: Additional error found in H.O. 249
    From: Gary LaPook
    Date: 2009 Oct 07, 19:04 -0700

    I don't know how negligible it is. For example, Dubhe in epoch 1985 was 
    SHA 194� 18', DEC 61� 50' N; epoch 2010, SHA 193� 55', DEC 61� 42' N. 
    Similar changes for Deneb, SHA 49� 46' to 49� 33', DEC 45� 14' N to 45� 
    So you can't use the 1985 positions today. You can use the epoch 2010 
    positions from 2006 to 2014 by applying the precession and nutation 
    correction. Also remember that if you use the "Q" correction table for 
    Polaris epoch 2010 that you must also apply the precession and nutation 
    correction to the Polaris LOP.
    douglas.denny@btopenworld.com wrote:
    > Quote:-
    > "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
    > Brad
    > >
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