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    Re: Navigation in Fog
    From: Gary LaPook
    Date: 2007 Feb 19, 12:09 -0800

    just a test
    
    On Feb 18, 3:22 am, Nicolàs de Hilster  wrote:
    > Dear Gary,
    >
    > Great pictures!
    >
    > Please correct me if I am wrong. From what I understand is that you get
    > great repeatability, but not a good absolute value. I assume you will
    > need to calibrate the whole setup so that when the polarising filter
    > 'disappears' against the background the orientation is truly towards the
    > sun. Is there an easy way to do this calibration? Would you be able to
    > do this experiment while still observing the sun directly, or should the
    > sun already be below the horizon? Have you any idea how accurate such a
    > calibration would be?
    >
    > Nicolàs
    >
    > glap...{at}PACBELL.NET wrote:
    > >To follow up on my previous post about the "sky compass."
    >
    > >As an experiment I cut out a small rectangle of celophane and laid it
    > >on top of a polaroid camera filter. I then balanced this on top of my
    > >astro-compass and placed a small mirror below it so that I could look
    > >through the celophane-polaroid filter combination up at the zenith. I
    > >used the astro-compass because it has a base that can be leveled and
    > >it rotates against an azimuth scale so that I could make  accurate
    > >readings. I waited for the sun to be setting and then rotated the
    > >astro-compass to cause the light to match on the filter and celophane.
    > >This match point is very distinct. Rotate it slightly to the right and
    > >the celophane portion is darker than the uncovered part of the filter,
    > >too far left and the celophane was lighter than the other part. By
    > >rotating the compass both right and left and reading the azimuth at
    > >the match point I determined that the repeatabliity of this reading
    > >was plus or minus only two degrees, close enough for practical
    > >navigation. If anyone is intersted I took some pictures of this
    > >operation and I could scan the relevant pages from H.O. 216. and could
    > >send them to you.
    >
    > >Gary LaPook
    >
    > >On Feb 17, 10:50 am, "Marcel Tschudin" 
    > >wrote:
    >
    > >>I could imagine that this information would be useful for the authors
    > >>of the paper. Of interest to me would be how such a "sky compass"
    > >>would look like manufactured with the skills of the Vikings.
    >
    > >>The article which I read was written by one of the co-authors and
    > >>appeared in a Swiss newspaper. For those of you who are interested
    > >>(Wolfgang?), you can find it herehttp://www.nzz.ch/2007/02/14/ft/articleEV4VN.html
    >
    > >>Marcel
    >
    > >>On 2/17/07, glap...---.net  wrote:
    >
    > >>>This technique was used in the "sky compass" (invented in 1948) which
    > >>>mounted in the sextant mount of aircraft for polar navigation. It
    > >>>works by determining the polarization of light directly overhead, at
    > >>>the zeneth, which then shows the azimuth to the sun even though the
    > >>>sun is not visible and possibly several degrees below the horizon.
    > >>>These "twilight periods" can last for weeks in high latitudes. The
    > >>>sky compass is completely discribed in "Air Navigation" H.O. 216
    > >>>(1962) and is also mentioned in "Bowditch" (1962.)
    >
    > >>>You can experiment and see how it works. Get a polarizing filter,
    > >>>either for a camera or a polarized sun glass lens. Hold it overhead,
    > >>>looking through it at the zenith and rotate it in azimuth and you will
    > >>>see the sky alternately lighten and darken as the polarizer
    > >>>alternately lines up with the polarization axis of the sun light, it's
    > >>>azimuth, and then crosses it. The light and dark points are 90º apart.
    > >>>Although this can give you a general azimuth to the sun the change in
    > >>>intensity is hard to judge accurately. So the next step is to get a
    > >>>piece of cellophane from a craft shop (or they may still use it on
    > >>>cigarette packs). Cut a piece to cover half of the poarizer and place
    > >>>it on top of the lens, between the lens and the sky, then try the
    > >>>experiment again. The two halves of the view through the polarizer
    > >>>will lighten and darken out of phase with each other. (You may have to
    > >>>try different orientations of the cellophane.) You will find four
    > >>>points where both sides will be the same intensity and this is very
    > >>>sharp and can be judged accurately but this causes a 90º ambiguity. By
    > >>>adjusting the orientation of the cellophane you will end up with four
    > >>>match points but two will be darker than the other two. This leaves
    > >>>you with an easy to manage 180º ambiguity.
    >
    > >>>It works.
    >
    > >>>Gary LaPook
    > >>>On Feb 14, 7:40 pm, "Peter Fogg"  wrote:
    >
    > >>>> Marcel Tschudin wrote:
    >
    > >>>>>Just read in German a summary on a published paper. The author(s?)
    > >>>>>investigate(s) whether it was possible for the Vikings to use the
    > >>>>>polarised light of the sky for navigation, this with the help of the
    > >>>>>"sun stone" (Sonnenstein). Those of you who are interested in this may
    > >>>>>read the original paper in English:
    >
    > >>>> Polarising filters can indicate the direction of the sun, even under
    > >>>>overcast skies, and have proved useful for this, especially in Arctic
    > >>>>regions.
    >
    > >>>>Here is that abstract, 
    from:http://www.journals.royalsoc.ac.uk/(byfonk55wemothqi02m1ir45)/app/hom...
    >
    > >>>>*Abstract:*
    >
    > >>>>In sunshine, the Vikings navigated on the open sea using sundials. According
    > >>>>to a widespread hypothesis, when the Sun was occluded by fog or clouds the
    > >>>>Vikings might have navigated by skylight polarization detected with an
    > >>>>enigmatic birefringent crystal (sunstone). There are two atmospheric optical
    > >>>>prerequisites for this alleged polarimetric Viking navigation under
    > >>>>foggy/cloudy skies: (1) the degree of linear polarization *p* of skylight
    > >>>>should be high enough and (2) at a given Sun position, the pattern of the
    > >>>>angle of polarization *α* of the foggy/cloudy sky should be similar to that
    > >>>>of the clear sky. Until now, these prerequisites have not been investigated.
    > >>>>Using full-sky imaging polarimetry, we measured the *p*- and *α*-patterns of
    > >>>>Arctic foggy and cloudy skies when the Sun was invisible. These patterns
    > >>>>were compared with the polarization patterns of clear Arctic skies. We show
    > >>>>here that although prerequisite (2) is always fulfilled under both foggy and
    > >>>>cloudy conditions, if the fog layer is illuminated by direct sunlight,
    > >>>>prerequisite (1) is usually satisfied only for cloudy skies. In sunlit fog,
    > >>>>the Vikings could have navigated by polarization only, if *p* of light from
    > >>>>the foggy sky was sufficiently high.
    
    
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