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    Re: Sextant accuracies - coastal piloting
    From: Arthur Pearson
    Date: 2003 Mar 17, 23:05 -0500

    CATCH UP ON AN EXCHANGE THAT DIDN'T GET TO THE FULL LIST THE FIRST TRY:
    
    On Mon, 17 Mar 2003 18:28:32 -0500, Arthur Pearson wrote:
    
    >Using a sextant for distance off an object of known height is a very
    >practical use of the sextant along shore. A compass bearing combined
    >with a distance off by sextant give a fix with only one object in
    sight.
    >For example, approaching the Maine coast, you pick up Mt. Desert
    >(1,500') before the other islands appear and a bearing and distance off
    >give you a position.
    
    Later Mon, 17 Mar 2003, Rodney replied:
    
    If the other islands, say Frenchboro Long Island and Gt Duck, are still
    below the horizon what are you going to measure the height from? I may
    be missing something, but I think you could do better calculating when
    MDI would show at your height of eye. If you are talking about making
    landfall.
    
    Closer in, you could use that method on Gt Duck when the bottom of it
    is also visible. But then you hardly need to. If I can see Gt Duck and
    Frenchboro, I just go between them.
    
    Arthur responds:
    
    My '58 Bowditch provides the following definition of the "sextant
    altitude" for use in Table 9, Distance by Vertical Angle: "The vertical
    angle between the top of the object and the visible (sea) horizon is
    measured and corrected for index error and dip only. If the visible
    horizon is not available as a reference, the angle should be measured to
    the bottom of the object, and dip short of the horizon used...".
    
    I take this to mean I can measure the "altitude" of Mt. Desert Island
    from the summit to the visible horizon while the bulk of the island is
    still below the horizon and get a distance off.  You don't need to see
    the bottom of the object to get a distance off by vertical angle.
    Bowditch provides the formula which I have plugged into my palm pilot
    for quick use.
    
    Arthur
    
    
    
    
    
    Rodney Myrvaagnes                                   J36 Gjo/a
    
    Chicken hawks unite! Your children won't go anyway.
    
    
    

       
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