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    Re: Refraction
    From: George Huxtable
    Date: 2007 Dec 11, 22:54 -0000

    Nicolas de Hilster wrote-
    
    "In the Dutch magazine Cornelis Douwes no. 165, Siebren van der Werf has
    written an article on sun's observations on Nova Zembla. There is a very
    nice series of pictures in it showing the sun setting on 30 March 2004
    behind Sheringham Point (as seen from Resolute Bay). The sun only
    disappeared when its altitude was already -51.6 arc minutes! This
    remarkable effect was caused by what they called the Toboggan effect (a
    Toboggan is a small sledge) as the light follows a cold layer along the
    slopes of Sheringham Point. Why was this researched? Simply because in
    1597 Willem Barentsz and his crew established their longitude by
    observing the Jupiter/Moon conjunction at the same spot on January 24th
    and 25th of that year. Due to the Toboggan effect they saw this
    conjunction no less than two hours after the real conjunction, resulting
    in a 30 degree (!) error in longitude."
    
    ====================
    
    I'm a bit puzzled as to why a Sun altitude of -51.6 arc minutes, at its
    moment of disappearance, should be thought so unusual.
    
    When the last glimpse of the Sun's upper limb can be seen at the horizon,
    the apparent altitude of that limb is zero. If we look at the table for
    standard refraction in a modern Nautical Almanac, it tells us that in that
    case the refraction is -33.8 arc minutes. And if the semidiameter of the Sun
    was, say, 16 arc minutes, a rather typical value, that would put the Sun's
    expected position (of its centre) at -49.8. Not much of a discrepancy with
    the value Nicolas quotes of 51.6.
    
    George.
    
    contact George Huxtable at george---.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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