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    Re: Sextant Positions versus Map Datums?
    From: Bill Murdoch
    Date: 2002 Jan 20, 3:52 PM

    In a message dated 1/19/02 6:43:01 PM Eastern Standard Time,
    lunav{at}ABELHOME.NET writes:
    
    
    > Any insight into why my reasoning is completely opposite the truth is
    > welcome...
    >
    >
    
    Lou, I just thought of another way of looking at the problem.
    
    On a spinning earth, a plumb bob at a pole or a plumb bob on the equator
    would point at the center of the earth because neither would be deflected by
    the centrifugal force caused by the earth's spin.  There would be no
    centrifugal force acting on the plumb bob at the pole because the spin radius
    would be zero.  The centrifugal force at the equator would be parallel to the
    string and make the plumb bob there a little lighter but would not deflect it
    at all.
    
    A plumb bob near 45 degrees north would be deflected a little to the south by
    the centrifugal force of the spinning earth.  A pan of water there would be a
    little deeper at its south edge than it would be on a stationary earth.  The
    horizon at sea would not quite be "level".
    
    I wonder if the math (physics) would give the deflection at N45 due to
    centrifugal force to be 11'32", I wonder if the fact that we do not correct
    for that means that celestial is really done on an ellipsoidal earth, and I
    wonder if that gravity-centrifugal force ellipse is pretty close matching one
    of the map datums.
    
    Bill Murdoch
    

       
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