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    Re: Refraction
    From: George Huxtable
    Date: 2007 Dec 15, 22:35 -0000

    This is interesting stuff, from Nicolas, about Barents' observations in Nova
    Zemblya, and the attached pictures helped me to understand it better.
    Presumably, what's being argued is the following.  If the Sun could be seen
    a few days later, as appearing to be above the horizon when by rights it
    ought to be a few degrees below it, then similar atmospherics could be
    affecting Jupiter during that conjunction, raising it to appear visible in
    the same way. And presumably the argument goes that the Moon is not being
    affected in the same way, or not nearly as much, so the relative positions
    of Jupiter and Moon are very significantly shifted, vertically, by that
    abnormal refraction, ducting Jupiter's image around the curve of the Earth.
    And that could greatly change the timing of an occultation, and therefore
    any deduced longitude.
    So a moral of the story could be that one shouldn't rely on measurements
    made near the horizon, something most navigators are only too aware of,
    today. One problem of navigation in high latitudes, however, is that Sun,
    Moon, and planets tend to be low in the sky at the best of times.
    But that particular conjunction would have been very difficult to time,
    anyway, even without that extreme refraction effect. The Moon and Jupiter
    were apart by all of three degrees, or about 6 Moon diameters. Nicolas is
    correct, that when the Moon is near first or last quarter, its bisecting
    shadow-line indicates a contour of equal ecliptic-longitude, but such a long
    lever-arm of 6 diameters is asking for a long extrapolation of that
    diameter, to time the moment that Jupiter crosses it. Halley made very 
    similar measurements, but using stars rather than a planet, and he looked 
    for much closer passage than that, between body and Moon. Of course, there 
    are so many stars in the sky that there are a lot more close-passages (or 
    "appulses"). With planets, such events come so rarely that you have to take 
    what you can get.
    > 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.
    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
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