Simulation Suggests Viking Sunstones of Legend Could Have Worked

If they existed, the crystals—used to locate the sun’s position on cloudy days—could have helped Vikings sail to far away places

Jason Daley / Smithsonian

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Viking marinersLeif Erikson pointing toward North America. Did he use a sunstone to navigate the open seas? (Wikimedia Commons)
Vikings have gone down in history as legendary navigators, sailing their longships to places like Britain, Ireland, Greenland and even Newfoundland. Without magnetic compasses or tools like astrolabes, the Vikings likely relied on primitive solar compasses to navigate, which uses the position of the sun to determine north.

The problem is, in the North Sea and North Atlantic, their primary stomping grounds, the weather is iffy at best, and the sun spends considerable time behind the clouds. That’s why some archaeologists have hypothesized that Vikings used sunstones, or sólarsteinns, crystals they held up to the sky to reveal the sun’s position, even through heavy cloud cover. Now, reports Ben Guarino at The Washington Post, computer simulations show that, if sunstones were used, they would have been a huge aid in navigation, at least in certain conditions.

Sid Perkins at Science reports that the sunstone theory isn’t something pulled out of thin air. The crystals are referenced in Viking stories, including “The Saga of King Olaf.” In 1967, Danish archaeologist Thorhild Ramskou first hypothesized that chunks of crystals found naturally in Scandinavia could have been used as navigational aids. In 2011, when experimenting with a common crystal in the region called Iceland spar, a transparent variety of calcite, researchers found that if they held it up to the sky and rotated it, the crystal polarized and depolarized light in a certain pattern which could reveal the position of the sun. With that reference point, navigators could calculate their position and correct their course as necessary.

While the idea works in theory, optical researchers at the Eotvos Lorand University in Budapest, Hungary, wanted to determine if the supposed navigational aid would actually work if put to the test in the North Atlantic. Using data from previous studies on how well the sunstones work, they set up a computer simulation of a voyage between the Viking village of Hernam, Norway, and Hvarf, a Viking colony in Greenland. They then analyzed 1,000 possible routes of the three-week voyage between the spring solstice and summer equinox, randomly changing the cloudiness of the sky and assessing the performance of three types of possible sunstones including the crystals of calcite, cordierite, and tourmaline. The study was published in the journal Royal Society Open Science.

The success of the voyage depended on several factors. During cloudy trips where the navigators consulted the sunstone every four hours, they reached the mountains of Greenland 32 to about 59 percent of the time. If they used the stone every one, two or three hours, however, the success rate jumps to 92 to 100 percent. Using the sunstone in equal amounts in the morning and later in the day also improved success rates. Comparing the sunstones themselves, cordierite proved to be the most accurate sunstone for navigation while the calcite had the worst performance.

In the paper, the authors caution that the success rate did not factor in the things that inevitably go wrong on the high seas like storms, heavy winds, ocean currents or ships drifting during the night. Co-author Gábor Horváth also makes it clear to Guarino that the work is not proof that the Vikings used sunstones, only evidence that the technology could have worked. “Nobody knows what the Vikings’ navigation practices were,” he says.

Finding an actual sunstone in the remains of a Viking ship would be much more convincing evidence of the practice, but those types of remains are few and far between. However, there is one piece of evidence suggesting that European navigators knew about sunstones. In 2013, French researchers found a crystal the size of a deck of cards among navigation equipment in a British shipwreck in the English Channel. While that wreck is from 1592, several centuries after the Vikings’ heyday, it does suggest that the use of sunstones was real at some point and not just a myth.