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    Re: Multi-Moon line exercise in 2 parts
    From: George Huxtable
    Date: 2009 Aug 12, 00:54 +0100

    Christian and I are in accord.
    
    He wrote-
    
    "You say that determining the azimuth is impractical is impractical. ... I
    agree, it is impractical."
    
    Good.
    
    he followed this by-
    
    | Are you of the opinion that Charles Cotter's method to determine the
    passage
    | of time between the sun's meridian passage and its maximum height is a
    | simplification, however acceptable in view of its purpose? I understand
    that
    | you are an expert on Cotter.
    
    No not an expert. What I've done, years ago, in collaboration with other
    listmembers, is to point out an immense number of errors in Cotter's
    "History of Nautical Astronomy" (1968), list attached. This is a later work
    than his "Elements of Navigation", which Christian has mentioned, and which
    I haven't read.
    
    That list of errata was produced to be useful for anyone that owned tor used
    the book, to print it out and slip it within the pages.
    
    In his "History", Cotter refers to the use of equal-altitude observations,
    either side of noon, to determine the time of the apparent peak; then
    correcting that time by an amount that takes account of the rate of change
    of ship's latitude and of the object's declination. I wouldn't describe it
    as "Cotter's method", Cotter was reporting, rather badly, on developments
    made by others.
    
    It's a perfectly viable way of doing the job, particularly if a wide
    time-span is chosen, and to my mind is a more efficient use of navigator's
    time than by trying to measure and plot many altitudes close to noon. The
    vessel's speed and course will usually be known to sufficient accuracy to
    allow the correction to be estimated. The problem is that in that text
    Cotter gets much of the technical detail hopelessly wrong, and provides an
    incorrect formula. In contrast, Jim Wilson gets it right.
    
    The correction is simple to calculate, and gets complicated only if there's
    a course-change between the observations, such as a tack of a sailing
    vessel.
    
    It is, indeed, only a special case of the standard St Hilaire method of
    crossing two position lines of a single body, ideally separated in time
    enough to provide a decent angle of cut. To my mind, the standard St Hilaire
    plot does the job as well as any, and doesn't require the two observations
    to be made symmetrically about the peak. Would-be navigators can get seduced
    by the simple-sounding notion of "longitude at noon", even though it has to
    be "longitude around noon"; indeed, the further from noon, the better it
    gets.
    
    George,
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
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    File: 109476.cotters-errors.doc
       
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