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Geoffrey Kolbe wrote:
>
> 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.
>   
For those interested in the subject an article was published by 
R.Hegedus (Could Vikings have Navigated under Foggy and Cloudy 
Conditions by Skylight Polarization?) in the Proceedings of the Royal 
Society London (2007), vol. 463, nr. 2080, pp. 1081 - 1095

Nicol�s


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