A calcite crystal found in a 16th century shipwreck may hold the key to how the Vikings found their way across the Atlantic to North America. It may explain the nature of mysterious â€œsunstonesâ€ alluded to in Viking sagas that helped them find the location of the sun, even in cloudy or foggy weather. From the Guardian:
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 â€“ 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.
Thanks to Heike Kubash for this item.