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    Re: Impressed with the Raft Book
    From: George Huxtable
    Date: 2010 Feb 1, 15:51 -0000

    Mike Boersma wrote-
    "There is a quote in Duttons that Captain Weems made 10 horizon sights on 
    six occasions with an average error of 2 miles and a maximum error of 4 
    miles. Duttons does not explain how he did this. He must have had extremely 
    good conditions."
    Mike doesn't provide the paragraph number in Dutton's, which could be useful 
    in order to understand the context of that report. Weems' errors are just 
    what are expected for altitude observations, up from a sharp horizon, by an 
    experiened navigator with a decent instruments from a biggish ship avoiding 
    adverse conditions. But only so long as the observed objects are well above 
    the horizon (by 5º or more) so that refraction is predictable and can be 
    properly compensated. It shows the sort of precision that traditional 
    celestial obsevation can provide, under real-world conditions.
    On the other hand, attempts to make deductions from the timing of rising or 
    setting of any body will always be very imprecise, as a result of the 
    extreme and unpredictable atmospheric refraction that occurs around zero 
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 

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