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    Re: Testing pocket sextant
    From: George Huxtable
    Date: 2006 Jun 14, 00:21 +0100

    Alex wrote, about his observed errors with a box sextant-
    
    | So where could these -3.5 possibly come from?
    | This cannot be any anomalous dip (the whole dip correction
    | was between -2.2 (when I sat on a stone just next to the water edge)
    | and -3.5). It is hard to believe to such large arc irregularity at
    | such moderate angles.
    
    "Cannot" is rather overstating things, It's perfectly possible though
    not very likely, for temperature gradients near the water surface to
    give rise to such anomalous dip.  If, for example, warm air from over
    the adjacent Sun-heated land blows over the surface of a cooler body
    of water, that's a recipe for anomalous values for dip; significantly
    worse than in open-sea conditions. Just because the dip itself is
    small (because near the horizon) does not necessarily imply that such
    refraction errors in the dip must also be small.
    
    If Alex is certain that the weather was calm enough to dismiss Bill's
    suggestion (overlapping waves on the distant horizon toward the head
    of the fjord) then he is left with anomalous dip or a serious error in
    marking (or reading) of the scale near the zero-degree point.  The
    interesting thing would be to test again under very different local
    weather conditions or in the open sea.
    
    George
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    

       
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