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    Re: determination of longitude and the prime vertical
    From: George Huxtable
    Date: 2007 Apr 13, 00:08 +0100

    Bill wrote-
    |
    | It is quite possible this site's program
    | (http://www.tecepe.com.br/cgi-win/cgiasvis.exe by Omar Reis) may be off on
    | GHA, LHA etc, by more than a rounding error in it's Navigator Star Finder
    | section.  It depends on the body.  Other areas of concern are stated RA's
    of
    | bodies converted to SHA and compared to the N.A.  Also note the
    declinations
    | given for Polaris for May: Site 89d 16!7, N.A. 89d 17!8.
    
    Comment from George-
    
    Well, if so, that's a bit of a worry. It's an unacceptable discrepancy, of
    1.1 arc-minute, in the prediction of Polaris' declination! My own pocket
    calculator gives for 1 May 07, 89 deg 17.7', which is close enough to the
    Nautical almanac figure that Bill gives of 89 deg 17.8'.
    
    So what's the origin of that erroneous declination of 89 deg 16.7' in the
    Omar Reis program, I wonder? It seems too big to have come from neglecting
    aberration, which can only shift Polaris dec from its mid-value by about
    0.3' or so, either way; not even when nutation is added. Is there something
    wrong with its allowance for pression, perhaps? I ask Bill to double-check,
    please, to make sure that he is comparing predictions for the same year. If
    Polaris is indeed that far out, can any predictions from that site be
    trusted, for precise work? I wonder if others have comments about that.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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