The Viking Sun Stones: navigational marvels of ancient seafarers!
Everyone knows that the Vikings were skilled navigators and that their only compass was the sun: but how would they have managed, at those latitudes, to sail without losing their way on days when the sky was entirely covered by a thick blanket of clouds?
It is known that the Vikings traveled thousands of kilometers towards Iceland and Greenland, and most likely discovered North America around the year 1000, long before Christopher Columbus. Their ability to navigate without a compass over such long distances and in very unfavorable weather conditions (polar night, snow, clouds, etc.), still remains a mystery.
Some Icelandic sagas (epic tales based on true episodes of Viking history) tell of the so-called Sun Stones, stones with which the ancient Vikings were able to locate the position of the stars and orient themselves in any climatic condition. Ancient stories provide no explanations of how these enigmatic stones worked.
In 1969, a Danish archaeologist hypothesized that the Viking's stones could have been used to measure the polarization of the sunlight. Polarization is a phenomenon that occurs when light encounters an obstacle, such as a shiny surface or a fog bank, deviating to a particular orientation. Some animals, such as bees, are known to be able to detect the polarization of the sunlight.
A study published by Proceedings of The Royal Society A (https://royalsocietypublishing.org/doi/10.1098/rspa.2011.0369) in 2011 proposed an interesting explanation: the legendary sun stones contained a special mineral used by the Vikings during sailing.
It is possible that these forerunners of modern compasses really existed and that there was nothing magical about their functioning. To identify the position of the sun, when hidden by the fog or by the clouds, it was only needed to calculate the orientation of the light waves along the path. Even on a cloudy day, the sky appeared to the Vikings' eyes as a pattern of concentric rings of polarized light with the sun at the center: using a crystal capable of depolarizing the light it was possible to calculate the position of the rings around the hidden sun.
The special mineral was probably the Icelandic Spathus, a transparent calcite crystal, actually found in Scandinavia, which allows with a simple rotation to calculate the position of the large star in the sky, even under low visibility conditions, thus providing exact coordinates for navigators. The light passing through the calcite is split into two “beams,” which form a double image on the far side. The brightness of each image depends on the polarization of the light.
Therefore, by passing light through the calcite and changing the orientation of the crystal until the projections of the rays are equally bright, it is theoretically possible to detect the concentric rings of polarization and consequently the position of the sun.
Guy Ropars, a physicist at the University of Rennes, conducted an experiment with a crystal that may have been used as a “Sun Stone” by the ancient Vikings: a piece of Icelandic Spathus found aboard the Alderney, a British ship that sank in 1592. The stone is less than 1 meter large and features a pair of navigational dividers, suggesting it may have been stored with the ship's other navigational instruments.
In the laboratory, Mr. Ropars and his colleagues irradiated the piece of Icelandic Spathus with partly polarized laser light. Passing through the crystal, the light split into two rays, polarized and not polarized; by rotating it, they discover that there was only one possible angle where the two rays had the same intensity. The angle at which the light enters depends on the position of the beam. Twenty volunteers took turns looking through the crystal on cloudy days trying to locate the position of the sun. It turned out that, on average, it was possible to locate the sun with only one degree of error, out of the 360 into which the celestial vault is traditionally divided.
The results confirm that the Icelandic Spathus is a crystal that can be used with great precision to locate the sun.
In 2010 it was demonstrated that meteorological conditions influence the polarization of light at Arctic latitudes: a phenomenon that the Vikings should have taken into account.
The same team from the University of Rennes published a further study in the Proceedings of the Royal Society A (https://royalsocietypublishing.org/doi/10.1098/rspa.2012.0651) discovering that the crystal is composed of calcite, a form of calcium carbonate, and that it was originally transparent, unlike when it was found, given that centuries spent under water had made it opaque. It was probably used aboard the Elizabethan ship to help correct errors with a magnetic compass.
