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    Re: Navigation in Fog
    From: Marcel Tschudin
    Date: 2007 Feb 17, 20:50 +0200

    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 here
    http://www.nzz.ch/2007/02/14/ft/articleEV4VN.html
    
    Marcel
    
    On 2/17/07, glapook@pacbell.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 90�� 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 90�� 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 180�� 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 *��* 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 *��*-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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