You might think that seafaring Vikings who from 900 to 1200 AD traveled thousands of kilometers in the rough waters would have floated adrift on cloudy days. Without a magnetic compass, they often traveled in the long days of summer at high latitudes that would hinder them from viewing any stars for navigation. But they didn’t lose their way, thanks to what was once considered a mythical navigational tool: the sólarsteinn, or sunstone – and scientists are finding new evidence to support its existence. In 1967, Danish archaeologist Thorkild Ramskou suggested that the sólarsteinn could have been a piece of Icelandic spar, which is calcite in the form of a transparent polarizing crystal, commonly found in Iceland. Other possible candidates for the sunstone include cordierite and tourmaline, both of which are common in Scandinavia. Polarizing crystals allow light polarized in only one direction to pass through and, depending upon the orientation to the polarized light, they will appear dark or light. Ramskou predicted that the Vikings rotated the sunstone to check the polarization direction, deducing the position of the sun when it was hidden by fog or when it was just beneath the horizon. Knowing that our atmosphere can scatter sunlight and polarize it, scientists from Eötvös University in Budapest and from Lund University in Sweden decided to put the calcite to the test. Using photographs of the cloudy sky, they tested how well volunteers could estimate the sun’s position, and their conclusion was: not very. The volunteers were highly inaccurate, making errors of up to 99°. In 2005, the researchers traveled the Arctic Ocean measuring the polarization patterns of the sky, finding that they were very similar on clear and cloudy days. While the polarization appeared weaker on overcast days, their findings suggested that the Vikings could have used this information if they were in possession of polarized crystals. The scientists next plan to see whether the volunteers can use polarization crystals to determine the sun’s location under a variety of weather conditions. The research was published in Philosophical Transactions of the Royal Society B, March 2011 (doi: 10.1098/rstb.2010.0194).