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National Geographic Icelandic spar may have revealed sun's position on cloudy days, study says. 
Vikings may have navigated by looking through a type of crystal called Icelandic spar, a new study suggests.In some Icelandic sagas- embellished stories of Viking life- sailors relied on so-called sunstones to locate the sun's position and steer their ships on cloudy days. The stone would've worked by detecting a property of sunlight called polarization. Polarization is when light- which normally radiates randomly from its source- encounters something, such as a shiny surface or fog, that causes the rays to assume a particular orientation. Due to this property, as sunlight moves through the atmosphere, the resulting polarization gives away the direction of the original source of the light. Detecting light's polarization is a natural ability of some animals, such as bees. In 1969, a Danish archaeologist suggested real-life Vikings might have used sunstones to detect polarized light, using the stones to supplement sundials, stars, and other navigational aids. Since then, researchers have been probing how such a sunstone might have worked. On that point, though, the sagas were silent. Sun-Revealing Crystal Now, Guy Ropars, a physicist at the University of Rennes in France, has conducted an experiment with a potential Viking sunstone: a piece of Icelandic spar recently found aboard the Alderney, a British ship that sank in 1592. In the laboratory, Ropars and his team struck the piece of Icelandic spar with a beam of partly polarized laser light and measured how the crystal separates polarized from unpolarized light. By rotating the crystal, the team found that there's only one point on the stone where those two beams were equally strong- an angle that depends on the beam's location. That would enable a navigator to test a crystal on a sunny day and mark the sun's location on the crystal for reference on cloudy days. On cloudy days, a navigator would only be able to use the relative brightness of the two beams. Icelandic Spar "Ideal" for Navigating The team then recruited 20 volunteers to take turns looking at the crystal outside on a cloudy day and measure how accurately they could estimate the position of the hidden sun. Navigators subdivide the horizon by 360 degrees, and the team found that the volunteers could locate the sun's position to within 1 degree. The results confirm "that the Icelandic spar is an ideal crystal, and that it can be used with great precision" for locating the sun, said ecologist Susanne Akesson of Sweden's University of Lund, who was not part of Ropars's research team. In 2010 Akesson and colleagues showed how local weather conditions may have influenced how light polarizes in the sky at Arctic latitudes, something Vikings would've needed to account for in their navigation. "But the question remains," she said, "whether [Icelandic spar] was in common use" in Viking times. On that point, physics is also silent.
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