Medieval Mondays: Viking Sunstones, magical myth or primative high-tech?

Welcome to Medieval Mondays! As a medievalist I get a lot of questions about the Middle Ages, so every Monday I'll be giving you some interesting facts about my favorite historic period.

Despite a century of research showing the Vikings were an advanced civilization, they still have a reputation as booze-swilling brutes who could barely grunt out a coherent sentence, let alone make any significant inventions. Hopefully a recent study will help dispel this myth.

We all know the Vikings were expert seafarers, generally the only ability granted to them besides being able to kick some serious ass. They were helped in their navigation by a "sundisc", a type of sundial. But when the skies were cloudy as they often are in the North Sea and northern Atlantic, how did they know which way to go?

A tantalizing clue comes in one of the Norse Sagas. In Rauðúlfs þáttr, King Olaf asks the hero Sigurður to point out the Sun while it's snowing. He points it out, even though it's invisible. Then the king had his men "fetch the solar stone and held it up and saw where light radiated from the stone and thus directly verified Sigurður’s prediction.”

What's going on here? Is it only a tall tale, or something more? Some scientists believe the "solar stone" was actually a type of crystal called a double-refracting crystal. Such crystals like cordierite, tourmaline, or calcite are common in Scandinavia. These crystals only allow light through them that's polarized in certain directions and thus appear darker or lighter depending on the polarization of the light behind it. While the Sun may be blocked by clouds, it's still sending out a concentration of polarized light that can be detected by the crystal as it's moved around. For a more technical description of how it works, check out this article.

So we can imagine a Viking standing on the deck of a ship in bad weather, trying to find the way to the nearest Irish monastery for an afternoon of relaxing looting. He pulls out his handy sun stone and moves it around the sky until a flash of light shows him where the Sun is. From that he can determine direction, and his seaman's skills do the rest.

Viking ships are fascinating. The one I'm showing here is called the Gokstad ship and it's at the Kulturhistorisk museum, Oslo. It was found in 1880, and dates to around 890 AD, when it was used as a grave ship for a Viking chieftain. Thanks to James Cridland for this cool photo!