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    Re: Refraction
    From: George Huxtable
    Date: 2007 Dec 12, 22:42 -0000

    Nicolas wrote-
    
    "Barents observations were taken at 76�15�.4 N 68�18�.6 O.:
    January 24th, 1597 - It was indicated that they saw a glimpse of the sun
    January 27th, 1597 - They saw the sun "in zijn volle rondicheyt" (in its
    full roundness)"
    
    Comment from George-
    
    That, in itself, is a bit contradictory. Seeing "a glimpse of the Sun" is a 
    very different matter from seeing the Sun "in its full roundness".
    
    I am presuming that the date given is a new-style Gregorian date, not a 
    Julian date (the switchover date was a rather complicated matter, in 
    Holland). In which case, at local noon on the civil day of 27 Jan 1597, the 
    Sun's altitude was -4d 41' 38", according to Skymap, so it agrees with the 
    figure quoted..
    
    Not unreasonably, the Nautical Almanac predictions for refraction go down 
    only to an apparent alitude of zero, for which the refraction is given as 
    33.8'. To see the Sun when it's actually so far below the horizon must be an 
    extreme case of "ducting" of light, by a strong temperature gradient close 
    to the surface. I have heard that such effects are not uncommon in Arctic 
    regions.
    =================
    
    I tried to access the English-language article referred to by Nicolas, but 
    all I could get, without privileges, was to the abstract.
    
    Presumably, any argument about Barents' Jupiter-Moon conjunction relates to 
    Jupiter being theoretically at least 2 degrees below the horizon at the time 
    the conjunction occurred, on 24 Jan. If the Moon and Jupiter could be seen 
    together at the time of conjunction, again, there must have been a lot of 
    abnormal refraction. But I strongly doubt whether any worthwhile longitude 
    could ever be derived by observing that conjunction. It wasn't a very close 
    event, the Moon never coming closer to Jupiter than about 3 degrees, so it 
    would be hard to give a time for that conjunction to within an hour or two, 
    even if a precise Moon position prediction had been available. It would 
    depend a lot on the definition of the moment of a conjunction. The 
    astronomers, compiling an ephemeris, would probably predict it as the moment 
    when the Moon and Jupiter had the same Right-Ascension (or perhaps the same 
    ecliptic longitude), but all the observer could do, at a guess, is to time 
    his best estimate of when they had the same azimuth. Halley used a similar 
    technique, but he required the Moon and (in his case) star to come much 
    closer than that.
    
    I don't see refraction errors, which will usually change the altitude only, 
    having a big impact on the determination of the moment of conjunction.
    
    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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