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    Re: Refraction
    From: Nicolàs de Hilster
    Date: 2007 Dec 14, 16:45 +0100

    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".
    About this the authors say that what they might have observed back in 
    1597 on January 27th was a flattened but clearly 'roundy' (or 
    'round-like') sun (I do not know how to better translate the Dutch 
    'rondachtig'). In the article there is an series of calculated images 
    showing the sun as how the authors think it might have looked back then 
    (see attachment calculated-views-1597.jpg, I added the red number). 
    Images 1-4 is what they might have seen on the 24th, images 11-15 on the 
    > 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..
    That is correct. According to the authors this has been a dispute for 
    some 400 years. They claim to have proven the observations using the 
    same atmospheric conditions for all three days (the third day is January 
    25th on which day the conjunction of Jupiter was observed in the early 
    morning). It is mentioned in the article that the observations are in 
    Gregorian date.
    > 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.
    They had the advantage that it was almost half moon. The moon was near 
    to her first quarter, hence showing an almost straight line in the 
    direction perpendicular to the ecliptic.
    > 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.
    In the article an image shows Jupiter and moon for that morning. I added 
    it as Jupiter-moon-1597-bw.jpg to this mail. I also added a second 
    modified image (Jupiter-moon-1597-color.jpg) that has additional 
    verticals through Jupiter (green lines), lines through the moon, 
    perpendicular to the ecliptic (red lines) and the real position of 
    Jupiter in blue (which I estimated, but should be good enough to show 
    what happened). The next bit is a rough shortened translation of what 
    the authors say about the conjunction:
    The conjunction was predicted by Iosephus Scala for the 25th at 01:00 am 
    (00:24 UT). The real ecliptic conjunction would have been at 00:14 UT. 
    The moon would have had a height of 1°14', while Jupiter would have been 
    at -2°02'. In the image the moon is shown at twice her actual size. At 
    00:14 Jupiter and moon are in ecliptic conjunction, at 02:30 they are in 
    azimuthal conjunction. The 'true moon' is already above the horizon and 
    therefore hardly affected by the refraction, but Jupiter is lifted 
    vertically due to the refraction. Based on the ecliptic Jupiter and moon 
    were not in conjunction until 02:00 in the direction of 12° easterly of 
    north. The direction is within one degree of what was calculated.
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