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    Re: Time of meridian passage accuracy
    From: George Huxtable
    Date: 2009 Sep 28, 11:46 +0100

    Gary wrote-
    
    "There is nothing magic or special about an LOP taken at noon. It is only
    due to its simple computation that made it useful in the past to navigators
    without the necessary math skills to do the trig for a Sumner line of for
    the St. Hilaire method that gave it any usefulness."
    
    Gary has omitted the most important reason why it was historically so
    useful. It didn't call for any knowledge of time.
    
    "Since the normal use of these tables allow you to choose an AP within 30 NM
    of the DR this allows a 4 minute period that you use the tabulated Hc, two
    minutes to the east and two minutes to the west. So certainly using the time
    to one one minute falls into the normal level of accuracy for celnav LOPs."
    
    Gary is referring to the normal use of an observation at LAN for determining
    latitude. There is, indeed, much leeway in timing that observation, just as
    he says.
    
    But Bowditch, rather unexpectedly, read "To calculate latitude AND LONGITUDE
    at LAN, the navigator seldom requires the time of meridian passage to
    accuracies greater than one minute." It's in calculating longitude from
    around local apparent noon that a much better time of the moment of noon
    than to the nearest minute is called for.
    
    George.
    
    
    contact George Huxtable, at  george{at}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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