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    Re: I need some help please
    From: Michael Bradley
    Date: 2006 Dec 11, 00:23 +0000

    Hello Guy
    I don't know the book you are taking your exercises
    from, and so can only guess about the repeated
    systematic error...
    If these were real sights, there could indeed be a
    sextant index error, and/or an operator sextant drum
    reading error caused by using the wrong end of the
    vernier scale as the zero mark. However most current
    sextants have a vernier scale length of 4 minutes not
    5 minutes.
    As exercise sights, it may be that the exercise was
    constructed to give better results than you achieved
    at your first attempt. One way a repeated 5 minute
    error could occur in the sight reductions was if the
    exercise gave an index error of 2.5 minutes, but you
    applied the index error correction in the wrong
    direction - an oft repeated slip.
    Sound navigation is more than just position fixing,
    and uses self checking methods. The whole point of
    using pairs of sights in opposition to each other is
    to show up possible slips and/or systematic errors.
    Similarly, the point of using three bearings rather
    than two for a hand bearing compass 'fix' is to allow
    the navigator to assess the size of some ( not all)
    the possible systematic errors in the bearings.
    We do not need fabulous accuracy, just a self checking
    basis for our navigational decisions. That's exactly
    what Geoffrey's analysis shows so well. I strongly
    support the suggestion elsewhere that you get a copy
    of Mary Blewitt's Celestial Navigation book - it's
    small, cheap, timeless, succinct. Mary explicitly uses
    the concept of the most probable position. Some books
    do, many others don't. You seldom see any advice as to
    what symbol to use for the most probable position. In
    the UK tradition we use a triangle to surrond the dot
    for a plotted estimated position: I personally use an
    inverted triangle for 'most probable' astro positions
    when plotting. It helps to remind me and others on the
    yacht that nothing is certain out on the water.
    Michael Bradley 55 01.2N 001 27W
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