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    Re: Refraction
    From: Gary LaPook
    Date: 2007 Dec 12, 16:16 -0800

    Gary LaPook wtites:
    Attached is the refraction table from H.O 249 which has values for 
    negative altitudes since such values  are possible from an airplane.
    George Huxtable wrote:
    >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 
    >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.
    >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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