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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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