Between AD 750 and AD 1200 the Vikings, were the dominant sailors of the North Atlantic. When the Sun was shining, geographical north could be determined with a special sundial or ‘sundisc’. However, how the Vikings could have navigated in cloudy or foggy situations, when the Sun’s disc was unusable, is still not fully understood.
A hypothesis was formulated in 1967, suggested that under foggy or cloudy conditions, Vikings might have been able to determine the azimuth direction of the Sun with the help of skylight polarization, just like some insects. According to this theory, the Vikings could have determined the direction of the skylight polarization with the help of an enigmatic birefringent crystal (double-refracting crystal), like cordierite, tourmaline, or calcite, which are common in the Scandinavian region and even mentioned in a Viking saga, functioning as a linearly polarizing filter.
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