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    Re: Northing correction to Noon longitudes.
    From: George Huxtable
    Date: 2005 Jun 8, 17:58 +0100

    Perhaps there's a bit more to say about the proposed method for obtaining
    long. from observations of the Sun made around noon. If I refer to it as
    the "Frank Reed" method, I think we will all know what we are discussing.
    
    The question is: how well do we know the speed of our boat?
    
    The speed over the ground is what I am referring to. We can check the speed
    through the water by using the log. How well do we trust the log? Perhaps,
    at a guess, to half a knot.
    
    But then, we need to know the speed of the current. In my English Channel
    home-waters, there are tidal currents of up to 7 knots in the Alderney
    Race, which can make for an exciting wild ride. But there, and as a general
    rule for tidal waters, these are pilotage areas in which celestial
    navigation isn't called for.
    
    What about ocean currents, then? The Gulf Stream comes to mind, a sinuous
    stream of water with a sharp boundary, which wanders with time, and reaches
    at a guess 2.5 knots, perhaps more locally. American navigators will know
    more about that than I do.
    
    Of course, GPS would provide speed-over-ground very precisely, but here we
    are doing celestial nav by the Frank Reed method, in or (perhaps) out of
    the Gulf Stream. How well can we estimate velocity over the ground, in that
    situation?
    
    It's the Northerly component of the speed-over-ground that concerns us, so
    assume that our heading is North, and the current too is generally
    Northerly.
    
    The correction that Northing makes to the Frank Reed time-of-noon is 15.3
    (tan lat - tan dec) seconds. In Florida, in Summer, that amounts to 3.5
    arc-minutes of long., for each knot of boat-speed. If that speed is known
    precisely, then it can be corrected precisely, by Frank's proposed method
    or by my alternative. But any knot of error in the estimated ground-speed
    will give rise to an error in long of 3.5 arc minutes. It gets somewhat
    worse at higher latitudes.
    
    It's not an overwhelming defect of the Frank Reed method, but one that's
    worth bearing in mind, to add to its other problems.
    
    George.
    
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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