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    Re: Dip uncertainty
    From: George Huxtable
    Date: 2004 Dec 6, 20:33 +0000

    Bruce wrote-
    >I hope people will continue to look for more information. The crux of my
    >argument, which I don't think has been addressed yet, was in the last
    >paragraph:
    >
    >"Set a prism between you and a line on the wall. Put the prism near the wall
    >and look at the line from across the room. You'll see the line not far from
    >its true position. Bring the prism close to your eye and you'll see the line a
    >considerable distance out of place."
    
    ================
    
    The error in Buce's argument comes from this:
    
    What he should be looking for is a deviation in the DIRECTION of the
    incoming ray from the horizon. Yhe sextant measures the angular difference
    between that ray and the direction of incoming light from a celestial body.
    
    If he looks through a prism, the prism deviates light through a constant
    angle. It doesn't matter a damn where he puts it in the light path, close
    up or far away. That constant angular deviation can indeed alter the
    apparent position of the pencil line, depending on how far away it is, and
    how far away the prism is. But the only thing that matters to a sextant  is
    the ANGULAR DIRECTION of the incoming light. Bruce's example doesn't quite
    correspond to the situation we are considering, because his pencil line is
    on the wall, not at "infinity", as is the horizon.
    
    In bending the light to give rise to anomalous dip. the only thing that
    matters is the amount of the angular bend, not where it comes from along
    the path.
    
    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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