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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...@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...@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 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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