How did Vikings sail the high seas to within one degree of accuracy, long before the compass arrived in Europe?

According to a new study, Norsemen may have used polarizing crystals called sunstones to navigate the treacherous northern latitudes from Norway to Iceland, Greenland, and possibly even North America.

These transparent crystals are commonly found in Iceland and can be used to depolarize light for detecting the sun's position, even in very overcast conditions.

Such a calcite crystal was recently discovered in the Alderney Elizabethan ship that sank off the United Kingdom in 1592. The researchers speculate this may have been used as an alternative to the modern compass which could have been affected by cannons on the ship, for instance.

An international team of researchers showed that sunstones can be easily used for accurate navigation under different conditions by determining the direction of the sun.

They tested crystals using a simple sensitive differential two-image observation -- when a dot is place on top of the crystal and viewed from below, two dots can be seen.

"Then you rotate the crystal until the two points have exactly the same intensity or darkness. At that angle, the upward-facing surface indicates the direction of the sun," Guy Ropars at France's University of Rennes told AFP by telephone.

"A precision of a few degrees can be reached even under dark twilight conditions -- Vikings would have been able to determine with precision the direction of the hidden sun."

The study was published online in the Proceedings of the Royal Society A Mathematical, Physical, and Engineering Sciences on Nov. 2.