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Did Vikings navigate with 'sunstone'?

Reuters file

A Viking warship replica, Havhingsten af Glendalough (the Sea Stallion of Glendalough), makes its way into Dublin's port in 2007. New research suggests the Vikings used sunstones to navigate in cloudy and foggy conditions.

Vikings could have navigated the oceans in inclement weather with the aid of a crystal that pinpointed the sun's location behind banks of clouds and fog, a new study suggests.

Such a tool, known as a sunstone, is known from legend, but until now experimental evidence that it could actually work as hypothesized was lacking.

Researchers led by Gábor Horváth of Hungary's Eötvös University decided to see if the legend has real world legs. Their results were published online on Monday in Philosophical Transactions of the Royal Society B.


The theory
The Vikings were Scandinavian seafarers who traveled widely in the North Atlantic, roughly between the year 900 and 1200. Under clear and partly cloudy skies, archaeological evidence indicates that they used sundials to find their way around.

But a sundial is only useful when the sun is shining, raising the question of how the Vikings navigated in cloudy and foggy conditions, which can last for days along their known sailing routes.

In the 1960s, Danish archaeologist Thorkild Ramskou suggested that the Vikings used a sunstone to filter the sunlight so that it all had the same polarization, or direction. By rotating the crystal to and fro, the light would appear brighter or darker, depending on how the crystal was oriented. The brightest point would be toward the direction of the sun.

Two sunstone readings from different points in the sky would allow navigators to pinpoint the sun's location. They could then hold up a torch in that direction to mimic the sun's location, allowing its light to cast a shadow on the sundial.

A widely cited reference to this method of navigation appears in the Sigurd legend, a Viking saga. Horvath and his colleagues refer to the passage in their paper:

"The weather was very cloudy. It was snowing. Holy Olaf the king sent out somebody to look around, but there was no clear point in the sky. Then he asked Sigurd to tell him where the sun was. After Sigurd complied, he grabbed a sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible sun. It turned out that Sigurd was right."

The tests
Critics have questioned whether this technique was actually needed, since experienced navigators could likely estimate the position of the sun even in cloudy weather. They also questioned whether the method would actually work under cloudy and foggy skies.

To answer the critics, the researchers made photographs of cloudy skies with a fisheye lens and asked subjects in the lab to find the sun. The experimental subjects had errors as great as 99 percent, leading the researchers to report that "Viking navigators might have needed some aid to navigate on open seas during cloudy or foggy weather conditions."

So they tested out the sunstone idea under a range of weather conditions on expeditions to Tunisia, a sail across the Arctic Ocean, and at home in Hungary. For a "sunstone" they used a polarimeter, a device that measures polarization.

Their results showed that the method worked in cloudy and foggy weather, though the method wasn't as reliable under completely overcast skies. Further research will test whether the actual crystals from Scandinavia and Iceland work as well as the sensitive polarimeter.

Christian Keller, a specialist in North Atlantic archaeology at the University of Oslo, told Nature News that he is open to the idea that the Vikings used sunstones to help navigate. However, he said the available evidence suggests that they used the sun's position on clear days as a guide, combined with knowledge about the flight patterns of birds and the migration paths of whales, among other cues.

"You don't need to be a wizard," he said. "But you do need to combine a lot of different sorts of observations."

Navigation in nature
Using polarized light for navigation is common in nature, as highlighted in the other papers published this week in Philosophical Transactions B. In one study, for example, Australian researchers conclusively demonstrate that honeybees steer with the aid of a built-in polarization compass.

Another study shows that when dung beetles navigate using the polarization patterns of the moon, they're as accurate in celestial navigation under a crescent moon as they are under a full moon, and that their skill equals that measured for species that orient themselves using the sun, which is up to 100 million times brighter.

But researchers from the University of Queensland in Australia found that other navigating species such as loggerhead sea turtles, which are known to orient via a magnetic compass, appear unable to use polarized light for navigation, at least when they're juveniles.

All of these studies, and several more, are available for free from the journal. To navigate there, simply click here.

More about navigation:

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John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).