Ancient Viking mariners didn’t have GPS or even a magnetic compass, but they still managed to beat Christopher Columbus to America by nearly 500 years. While it’s possible to navigate by observing the position of the sun, just how did seafarers in the northern latitudes manage to find their way when the sky was cloudy?

Icelandic sagas make mention of a "sunstone" that could be used to find the sun even when it was hidden, thereby guiding sailors safely even in dark and stormy hours. Now, new scientific research implies that the sunstone isn’t just a fable.

Even when the sun is hidden behind clouds or just below the horizon, it's possible to pinpoint its location by detecting the polarization of light. Scientists have thought that Vikings’ sunstones may have been made of a kind of calcite crystal called Iceland spar, which is “birefringent,” meaning that it splits the light that enters it and produces a double image. As an example, see this picture below of a calcite crystal resting on top of graph paper:

Depending on the polarization of the light, one of the two "double" images may be brighter than the other. Theoretically, the calcite would be used to navigate by moving the crystal so that the two images are equal in brightness, thus pinpointing the rings of polarization that point toward the sun like a bull's-eye.

In 2011, a research team led by a French physicist named Guy Ropars built a sunstone using a bit of Iceland spar set into a wooden case that directed light from the sky onto the crystal. They then used it to measure the position of a sun on an overcast day, and compared it to measurements with modern technology. Their "sunstone" turned out to be quite accurate, coming within a few degrees of the exact position of the sun.

No intact sunstones have been found at known Viking sites, so it’s still unclear if Icelandic spar really was the sunstone featured in their sagas. But a slightly more recent find may lend some credence to the theory.

In a paper published last week in the Proceedings of the Royal Society A, Ropars and his colleagues examined a chunk of crystal recovered from a sixteenth-century ship that sank off the coast of Alderney, one of the islands in the English Channel. The crystal was found near some navigational equipment, which led researchers to suspect that it may have been more than just decorative.

Nowadays, the Alderney crystal is a bit worse for wear thanks to being immersed in sea water and scraped by sand for a few centuries -- making it too cloudy to be used now for navigation. But tests confirmed that the crystal is Icelandic spar, the same material used in the previous experiment.

“Alderney-like crystals could really have been used as an accurate optical sun compass as an aid to ancient navigation, when the Sun was hidden by clouds or below the horizon,” the team wrote.

SOURCE: Le Floch et al. “The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass?” Proceedings of the Royal Society A published online 6 March 2013.

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