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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.


gl




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 
>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@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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