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    Re: Navigation in Fog
    From: Peter Fogg
    Date: 2007 Feb 15, 14:40 +1100


     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/home/contribution.asp?referrer=parent&backto=issue,9,22;journal,1,137;linkingpublicationresults,1:102023,1

    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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