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    Re: Problem with a sextant
    From: Peter Fogg
    Date: 2006 Apr 27, 08:33 +1000

    > Bill wrote:
    > I guess the answer to my question would be in two parts:
    >
    > 1.  How close were the intercepts to true position for each observation?
    > 2.  How much error in COG and SOG between observations?
    >
    > Let's limit it to question #2.  How close do you think one can come in a
    > 30-40 ft sailing craft over a 4-5 hour period of time?  Given points of
    > sail, varying leeway, sea conditions, tides etc.?
    >
    > If I recall, Sumner's DR was off 1 mile in 75 for latitude, but 1 in 20
    > for longitude.
    
    If the observer is on a racing yacht that is often changing direction, plus 
    all those other factors, then an accurate DR could be difficult to achieve. 
    But Sumner, and methods he helped to evolve, were for running long distances 
    across oceans. Days (if not weeks) following the same course under similar 
    conditions. Apparently those navigators did manage to achieve remarkably good 
    DRs - and depended on them.
    
    An inaccurate DR will lead to longer intercepts, an inherent enemy of 
    accuracy. The remedy is to take the indicated Fix as an improved DR and 
    repeat the sight reduction exercise.
    
    Incidentally, methods of sight reduction that use a presumed DR expressed in 
    whole degrees suffer the same inherent defect. From reading this List, and 
    other sources, such methods seem the most popular today; presumably because 
    they remain the main methods taught. An example of the triumph of inertia 
    over an informed mind.
    
    
    

       
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