NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: George Huxtable
Date: 2007 Dec 12, 22:42 -0000
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@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---