Long before the magnetic compass reached Europe, the Vikings were sailing across oceans to both the east and west, discovering new lands in the west such as Iceland and Greenland and even discovering America.
Many have wondered how they managed to navigate across the ocean with no land in sight. Of course they used the sun and the stars for navigation, but in the summer time you see no stars this far north, and on a cloudy day, you see no sun either.
In the sagas you can read about something called a Sunstone, a magical stone that could help navigators find the sun even on a cloudy day. In the Hrafns Saga it says: "the weather was thick and stormy . . . The king looked about and saw no blue sky . . . then the king took the sunstone and held it up, and then he saw where [the Sun] beamed from the stone." So when the traditional methods failed, the sunstone was the magic tool good navigators used.
In 1967 Danish archeologist Thorkild Ramskou put forward a theory about the sunstone as being a natural crystal used for the polarization of the skylight. Crystals that could be used for navigation this way with 5° accuracy are found in southern Norway and in Iceland you can find a common calcite crystal called the Icelandic spar, that can be used for navigation with 1° accuracy. Ramskou's theories were widely accepted in the beginning but no evidence has though been found to support it. Crytical voices have therefore become ever more louder.
Yesterday, researchers led by Guy Ropars from the Rennes University in Bretagne, France, published a study in the journal "Proceedings of the Royal Society A: Mathematical Physical & Engineering Sciences" where they claim to have discovered what crystal they used and how.
"The Vikings could have discovered this, simply by choosing a transparent crystal and looking through it through a small hole in a screen," study researcher Guy Ropars wrote in an email to LiveScience. "The understanding of the complete mechanism and the knowledge of the polarization of light is not necessary."
I used to play with these crystals as a kid, seeing how the light polarized going through them, reading double when I laid them over text and so forth. But I never realized they could be used as a compass as well. When held up against the center of the sky, the sunlight would get polarized into an ordinary beam and extraordinary beam. By seeing how these two beams line up according to the angle of the sun, it can be used to locate the sun even though it is not visible. While there is sunlight this works, even though there are clouds blocking the sun or though it has gone a few degrees below the horizon. That would have been particularly useful as the sun goes just below the horizon during the bright summer nights, leaving no stars visible but yet no direct sun.
No evidence have yet been found in Viking settlements of the use of this crystal, but a suncompass with a an Icelandic crystal was discovered in a ship wreck from 1592 in the English channel. One thing that supports that this crystal was indeed a sun compass is that on board that ship was a large metal cannon that would have distorted a magnetic compass.
The study group created a prototype of a Viking sun compass using this crystal, that turned out to work particularly well as a navigation tool when the sun is beyond the horizon, and even after the stars come out (see image above).
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