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I'd mentioned sunstones in a TV & Media thread on the new History Channel drama about Vikings, but it's an interesting enough topic for Science & Technology.
The Vikings made news this week with partial confirmation of a mythical Viking sunstone (wiki, which was said to be a magic stone that allowed them to see where the sun was at even through clouds, making it extremely useful for navigation in Northern latitudes.
More from National Geographic on how the stone works, which involves dual refraction and polarization in a crystal of transparent calcite known as Icelandic spar. Researchers found that 20 test subjects could locate the sun to within one degree on an overcast day, even in twilight.
Here's an earlier video showing the dual refraction in the crystal, which is pretty interesting.
Apparently what's going on when using the crystals to find the sun might be pretty sophisticated, and I've seen references to either rotating the crystal until the two refraction bands are equally bright (mentioned in the National Geographic story) or to a polarization effect within the eye called Haidinger's Brush.
So does anyone have a transparent calcite crystal laying around? I think this might be a great science experiment for kids, and I'd like to play with it myself.