Researchers have long wondered and argued about how the Vikings were able to successfully navigate their way around the Northern Hemisphere in the late eighth to 11th centuries, hundreds of years before the magnetic compass reached Europe around 1300. Besides the direction of the wind, waves, and swell, the only way to navigate during the day away from shore is by knowing the sun’s direction. But that’s not so easy on a foggy or stormy day, or during the long twilight of Northern summers.
Historians have speculated that, due to their optical properties, crystals of calcite (a common form of calcium carbonate) could have been used to tell direction, but until now the theory hadn’t been tested.
Icelandic spars are crystals of calcium carbonate which have a special property called birefringence: light hitting the mineral is split and follows two parallel paths through it, which explains how calcite makes objects look doubled. The relative brightness of the two images--the amount of light following the two different paths--depends on the light’s orientation to the crystal. The researchers showed that this can be used to locate a hard-to-find light source, like the sun on a cloudy day.
The basic idea is that at a certain orientation to a light source, the crystal produces two light beams of equal brightness, a contrast the eye can measure surprisingly well. If you figure out what orientation of the crystal produces this effect when the sun is visible, you can repeat the procedure in the fog to find the sun.
Testing the method with various Icelandic spars, the researchers were able to establish the direction of the sun to within 5 degrees.
So far no crystals have been found in known Viking settlements or artifacts. But one such crystal was recently found in the 1592 shipwreck of an Elizabethan vessel in the English channel. The researchers think its likely to have been used to aid navigation, due to its shape and their calculation that the presence of even the one large cannon found aboard would interfere with compass readings. This suggests these types of sunstones were in use more than 200 years before polarized light was first discovered, and possibly used even earlier by the Vikings to navigate the open seas.