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    Re: determination of longitude and the prime vertical
    From: Bill B
    Date: 2007 Apr 12, 15:42 -0400

    Good eye George,
    
    First, there is a typo in LHA.  It should read 71d 09!9.  My apologies.  The
    rest of information is as the site presented it.
    
    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.
    
    From the almanac, I calculated the the following values (using a velocity of
    15d per hour, actual velocity approx. 15d 00' 04").
    
    GHA 131d 09!9 (131d 09' 51")
    LHA  71d 09!9
    dec 15d 09.5
    
    Using velocity of 15d 00' 04"
    GHA  131d 09!9 (131d 09' 53")
    
    In this case the site information looks good.  I again apologize for the
    confusion caused by my poor keyboarding skills.
    
    I do caution that in my experience if an exact value--especially for stars
    and planets--is required (star-to-star sextant checks etc.) stick with the
    N.A. or Frank's online almanac.
    
    Bill
    
    
    > From: "George Huxtable" 
    > But a minor matter cropped up in that mailing that worries me a bit.
    >
    > Bill quoted some predicted Sun positions, that had come from Andres Ruiz'
    > website.
    >
    > I will refer just to that first set of numbers from his mailing, as follows-
    >
    > | Date:     1 May, 2007
    > | UT:       20:41:45
    > | Position: N40 W60
    > | Hc:       24d 00!2
    > | Az:       270.00
    > | LHA:      71d 09!5
    > | dec:      15d 09!5
    >
    > I don't have a 2007 Nautical Almanac. But I have compared those numbers with
    > my own predictor, held on a pocket calculator, which differs slightly.
    >
    > That gives a Sun position, at 20:41:45 GMT on 1 May 2007 of 15 deg 09.5' N,
    > exactly as Bill gave it, but a GHA of 131deg 09.8', and so with a long of
    > 60deg W, a LHA of 71deg 09.8', just 0.3' more than the figure Bill stated.
    >
    > That difference isn't a large one, but to me it's a bit of a worry,
    > depending on which of us is in error. I expect the Sun predictions of that
    > program to be generally well within a millidegree. One might expect
    > divergences of 0.1', as a result of rounding errors, but not more. If my
    > program is out by that amount I need to investigate why.
    >
    > So I wonder if Bill will kindly recheck that figure, and ask anyone with a
    > current Nautical Amanac or computed equivalent, to provide a value for Sun
    > GHA at that moment, please.
    >
    > George.
    
    
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