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    Viking sun compass artifact and Viking sun stones.
    From: Geoffrey Kolbe
    Date: 2009 Jun 07, 07:06 +0100

    There was an interesting program on the TV back in 1999 when a
    reconstruction of a Viking trading ship was sailed out of Roskilde (I
    think) to somewhere via an island in the Shetlands where it was
    deemed quicker to drag the ship over a narrow isthmus rather than go
    around the island. (It wasn't!) I can't remember the details, but it
    was a deep sea journey of some days and they had Robin Knox-Johnston
    along as navigator. He made up a little sun compass along the lines
    of the one that had been excavated in Greenland in  1947 (another was
    discovered in Poland in 2000 it seems). It seemed to work quite well.
    A day or so before the journey, Robin Knox-Johnston calibrated the
    device by scribing a line along which the tip of the short gnomon's
    shadow tracked across the base-plate during the day. The implication
    was that it was a use once and throw away sort of device, which would
    possibly explain why the Viking original was rather crude and why we
    have not found more of them. Of course, the device works only when
    direct sunlight is available and as I recall, Robin Knox-Johnston's
    other navigating skills were required to keep the boat on course. I
    see Robin Knox-Johnston has a piece on his website about this voyage.
    See http://www.robinknox-johnston.co.uk/da/20090
    
    I have used a sun compass of a different sort in the Sahara desert.
    See www.pisces-press.com/C-Nav/images/sun_compass.jpg For this, the
    time of day is required and the device is rotated for the sun's
    azimuth at that time of day. It was a small device, but good to a
    couple of degrees none-the-less and certainly good enough to keep a
    dead-reckoning track of our car.
    
    The Viking 'sun stone' uses the fact that the blue colour of the sky
    is due to Rayleigh scattering of sunlight from the air molecules.
    Blue is more efficiently scattered than red, which is why the sky is
    blue, but there is also some polarization of the scattered light
    which is strongest at a scattering angle of 90 degrees. It was fun
    when in the Sahara to look up at the sky with my back to the sun
    while wearing polarized sun glasses. There in the sky, at an altitude
    about 90 degrees away from that of the sun, a large black hole could
    be seen as the horizontally polarized blue light was blocked by the
    glasses. This phenomenon is not much use as a navigation aid in the
    Sahara, of course, as the sun is not usually blocked by clouds. I
    have tried looking for the 'black hole' on cloudy days up here in
    Scotland using my polarized sun glasses, but I have to say with very
    limited success - in fact, no success at all. I have not heard of
    anyone actually using a 'sunstone' effectively as a means of
    navigation. But for the fact that the 'sun stone' was described in
    the Sagas no less than three times (see
    http://www.nordskip.com/vsagas.html ), I would put very little
    credence at all in this as an instrument of navigation.
    
    Geoffrey Kolbe
    
    
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