The vikings navigated by using sundials calibrated to show the direction of the north pole. How did they navigate on cloudy days? In references, there is mention of "sunstones" which are translucent pieces of calcite mined in Scandinavia and Iceland. The sky and the crystals polarize light. The sky polarizes it in concentric rings around the sun. The crystal polarizes in one direction. If held up to the sky and turned, the sky would get darker in one direction and brighter in the other. If two readings are taken at different points in the sky then the position of the sun may be made. Then a torch is held at the sun's position above the sundial and then their measurements are made.
The article is a good one. It makes a description of what is supposed to happen and how it can be accomplished. It's understandable and correct. It also points out objections. That being that the sky is only polarized in some directions and if it's not polarized enough then the technique won't work. It also outlines what the researchers plan to do in the future, acquire some crystals and start using them. Oh, I forgot to mention that it describes how they tested the idea with a polarimeter, already.
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