Did Vikings navigate the seas using crystals?

Vikings might have navigated foggy seas using crystals to analyze light from the sky, a trick similar to what honeybees do to stay on course on cloudy days, researchers suggest.

Scientists are now planning experiments to see if they can replicate these practices.

The Vikings dominated the North Atlantic from 900 to 1200 by skillfully navigating across the open sea. For instance, when the sun was out, archaeologists found the Vikings could have navigated with the aid of sundials — by tracking the sun's trajectory in the sky from east to west, they would've been able to locate north.

But how did the Vikings navigate when the sun was obscured by fog or clouds that could last for days along key parts of their sailing routes? A controversial answer proposed by Danish archaeologist Thorkild Ramskou in 1967 was the use of mysterious crystals known in Viking sagas as sunstones.

To understand how sunstones might have worked, one first can think of all light waves as either rippling up and down, left and right, or at any angle in between, a property known as polarization. Scientists conjectured that sunstones were naturally occurring crystals that served as polarizing filters — they blocked out all light except for that polarized in specific directions. Although it's unclear what sunstones might have been made of, researchers suggest they could have been composed of cordierite, tourmaline or calcite, all common stones in Scandinavia.

The idea is that ancient mariners looked up through these sunstones on overcast days, when the entire sky looked equally bright. Light making its way through a cloudy sky is often polarized — if the way the crystal was angled matched the polarization of this light, the sky would look brighter, but if not, it would look darker. By rotating the sunstones to and fro, the sky would thus appear to periodically brighten and fade. Then, by looking for the patch of sky that was brightest regardless of the clouds, Vikings could have identified where the sun was and then have used the sundial to figure out which direction was north.

For instance, in the Viking saga, "The Legend of Sigurd," it reads, "The weather was very cloudy, it was snowing. Holy Olaf, the king ... 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 might have been right.

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