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Magic Oddball [soylentnews.org] writes:

Scientists & historians have long been unable to explain how the Vikings navigated successfully to and from the Americas at such northern latitudes under cloudy skies. A professor of Earth & Planetary Science studying the properties of calcium carbonite (calcite) who attended a presentation on Vikings suspects his studies might hold the answer [berkeley.edu]:

The calcite's birefringence causes one ray to go through normally and another to pass at an angle. The light's direction changes as it goes through calcite, and this change tells where the light is coming from — in this case, the sun. There is a nerve in the back of your eye that can recognize this change and causes an optical illusion, "Haidinger's Brush," which looks like a fuzzy yellow bowtie. The sun is perpendicular to the yellow shape.

The professor is working with a research assistant to re-create a device the Vikings likely used, based on a 2011 Royal Society study. Hopefully the project overall will give him some interesting ideas for his investigation of the properties & potential uses of calcium carbonite.

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