OK - let me get this straight.
Scientist think the Vikings used these stones to navigate on foggy and cloudy days.
And the stones contain crystals that REQUIRE SUNLIGHT to work.
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“Vikings to navigate on foggy and cloudy days could provide accurate results.
Vikings living between 900 and 1200AD did not have magnetic compasses, and their ability to navigate was attributed in part to the use of calcite, cordierite or tourmaline crystals which functioned as linear polarizers to help them determine geographic north. The crystals can split sunlight into two beams, and when the crystal is turned, splitting the two beams at the same brightness, a navigator could see the polarized rings around the sun, effectively showing its placement in the sky.”
Foggy days include daylight. The sun is up somewhere over your horizon, therefore the sunstone can form the two beams and the halo when turned- when the halo is seen at its brightest, even if dull(er) compared to the effect in full sunlight, that is the direction of the sun.
The crystals collect the diffuse light coming thru the clouds or fog and re-concentrate it into a single fuzzy point...............