According to ancient Icelandic texts, a mysterious object known as a sunstone could locate the sun in a clouded-over sky. Such an object might explain how ancient mariners like the Vikings traveled across the oceans with otherwise rudimentary technology. But did sunstones actually exist? Or were they merely allegorical references?

Recently, researchers discovered a slab of mineral in a 16th century shipwreck. This mineral, known as Iceland Spar, might just be the mythical sunstone. Here’s more from Raphael Satter at the Associated Press (posted at R&D Mag):

A rough, whitish block recovered from an Elizabethan shipwreck may be a sunstone, the fabled crystal believed by some to have helped Vikings and other medieval seafarers navigate the high seas, researchers say.

In a paper published earlier this week, a Franco-British group argued that the Alderney Crystal—a chunk of Icelandic calcite found amid a 16th century wreck at the bottom of the English Channel—worked as a kind of solar compass, allowing sailors to determine the position of the sun even when it was hidden by heavy cloud, masked by fog, or below the horizon.

That’s because of a property known as birefringence, which splits light beams in a way that can reveal the direction of their source with a high degree of accuracy. Vikings may not have grasped the physics behind the phenomenon, but that wouldn’t present a problem.

“You don’t have to understand how it works,” said Albert Le Floch, of the University in Rennes in western France. “Using it is basically easy.”

(See the rest at R&D Mag)