- 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,”