WaPo – Vikings may have used crystals to navigate across the Atlantic

https://www.washingtonpost.com/news/speaking-of-science/wp/2018/04/04/vikings-may-have-used-crystals-to-navigate-across-the-atlantic/?utm_term=.cc8f3047aa8a

  •  A thousand years ago, the trip between Norway and Greenland by longboat took about three weeks. Viking navigators used shadows cast by a sun compass, a ring with a central fin like a sun dial’s, to identify geographic north
  •  such compasses work only when the sun shines, not on foggy or cloudy days
  • In 1967, Danish archaeologist Thorkild Ramskou proposed Vikings had a backup tool for navigation – they tracked the sun through the clouds using chunks of crystal called sunstones.
  • sunstone hinges on a property of sunlight called polarization
    • means that the light has a non-random orientation
    •  When sunlight travels through the atmosphere, it forms polarized rings, with the sun at the center like a bull’s eye.
  • Crystals like calcite, also called Iceland spar, can reveal the direction of polarization
    • “Rotating such a crystal in front of our eyes to and fro, the intensity of skylight transmitted through the crystal changes periodically,” 
    • even on cloudy days
    • When the sunstone is brightest, the crystal points at the sun
  • In 2013, a crystal of Iceland spar was found amid the wreckage of a British ship that sank in 1592
  • legends, like the “The Saga of King Olaf,” refer to navigation by sólarsteinn — a sunstone
  • uncertainty surrounds this model. “Nobody knows what the Vikings’ navigation practices were,”

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