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    Re: The Complete On-Board Celestial Navigator Second Edition/ Astronomical Refraction
    From: Peter Fogg
    Date: 2003 May 3, 21:17 +1000

    ----- Original Message -----
    From: "George Huxtable" ...I recently invested in a copy of George
    > Benntett's "The Complete On-Board Celestial Navigator Second Edition".
    This
    > contains five years of tables for all the navigational bodies, condensed
    by
    > working to a relaxed precision of the nearest 1 minute of arc, rather than
    > the 0.1 minutes of the normal almanac. Everything, interpolation,
    > correction, sight reduction, is done to that reduced precision, so these
    > approximations might sometimes combine to put the resulting position-line
    3
    > miles out, maybe  a bit more.
    
    Having used these tables, and comparing the results with those from a nav
    calculator, I can say that what mostly happens is that the small roundings
    tend to cancel each other out, and the final result is within one minute of
    arc. Occasionally it is within 2 minutes, and only rarely within 3 - when
    this happens I suspect operator error - mine!
    
    As an example, here are the intercepts from Silicon Sea No 69, which has
    fairly long intercepts:
                        On-Board        calculator
    Sirius                A28                A27.6
    Spica                T89                T89.1
    Avior                T51                T50.4
    Bellatrix            A71                A70.0
    Dubhe              A67                A66.8
    
    The resulting plotted fix agrees with the answer given.
    
    
    > However, I have reservations about Bennett's table for obtaining azimuths,
    > and warn users of the book to be aware that for a range of azimuths near
    to
    > due East and due west, the procedure introduces azimuth errors that can be
    > far greater than the "one or two degrees" that are claimed. Azimuths are
    > calculated in terms of sin az, and this makes for simple tabulation, but
    is
    > inaccurate and also ambiguous near 90 and 270 degrees. Bennett includes
    > instructions for resolving the ambiguity, but the inaccuracies remain.
    
    I find these Azimuth Solution tables quick and easy to use. The procedure to
    use when azimuth is within a degree or so of due east or west is given and
    explained. Then the alternative is given, Weir diagrams, beginning:
    'The accuracy of' (Weir Diagrams) 'is superior to' (Azimuth Solution
    tables).
    Remember that these are practical solutions for on-board sailors, somewhere
    else it is noted that an azimuth correct to within a degree or two is quite
    accurate enough for practical plotting purposes.
    
    
    

       
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