According to Viking legend, the marauding seafarers found their way in bad weather using glowing sunstones that revealed the position of the sun even when it was obscured by cloud or had sunk beneath the horizon.
Scientists have long argued over whether such a trick is feasible, but new research on a crystal recovered from a 16th century shipwreck reveals that such stones could indeed have helped the Vikings navigate from Norway to North America.
Vikings are known to have sailed vast stretches of open water to reach North America more than a thousand years ago, finding their way from the sun and stars, and the direction of the wind, waves and swell.
As skilful as the Vikings were as navigators, their ambitious voyages would have been beset by thick fog, cloudy skies and the prolonged twilights of the polar summer, which would have made direct observations of the sun and stars all but impossible.
The enigmatic sunstone appears as an extra navigational aid in an Icelandic saga featuring a sailor called Sigurd who, frustrated by the weather, holds a sunstone aloft to locate the sun and so set his ship's course.
In 1967, Thorkild Ramskou, a Danish archaeologist, speculated that Viking sunstones might have been Icelandic spar, a clear calcite that is common in the region. Calcite splits incoming rays of light in two, known as birefringence. The same property makes the crystal appear light or dark when held up to light of different polarisations.
Light is not polarised as it leaves the sun – in other words the electromagnetic waves vibrate in all directions perpendicular to the direction in which they are travelling. But as sunlight passes through the Earth's atmosphere, it is scattered and becomes polarised in a particular direction.
Vikings might have calibrated calcite crystal sunstones by scanning them across a clear sky and noting the sun's position when the crystal brightened. They could then repeat the trick to locate the sun when it was no longer visible.
Writing in the journal Proceedings of the Royal Society A, a team led by Guy Ropars at the University of Rennes in Brittany describe tests on a a piece of Icelandic spar found aboard a sunken Elizabethan military vessel. The ship was discovered in the 1970s by a fisherman off Alderney in the Channel Islands.
Through a series of experiments, they found a different way to use the crystal to pinpoint the position of the sun. They covered the crystal with an opaque sheet that had a hole in the centre. When viewed through the hole, they noticed the crystal cast two distinct shadows. Rotating the crystal made one shadow get lighter as the other darkened and vice versa.
Further tests showed that they could pinpoint the sun's position with an accuracy of one degree in either direction by rotating the crystal until the darkness of the shadows matched. As before, the crystal had to be calibrated on a day when the sun was visible.
"Such sunstones could have helped the Vikings in their navigation from Norway to America, as the magnetic compass had yet to be introduced in Europe," Guy said. A crystal measuring 3cm on each side would have been large enough to work, he added.
The calcite crystal may also have proved invaluable on the Elizabethan vessel. Just one of the cannons aboard the ship would have disturbed a magnetic compass by as much as 90 degrees, Guy claims. "To avoid navigational errors when the sun is hidden, the use of a [calcite crystal] could be crucial even at this epoch, more than four centuries after the Viking time," he said.