Viking Sun Stones?

Top row ~ John Stetson finds the direction of the sun using a calcite crystal.

Calcite crystals, Iceland Spar, produce double images. The two black shapes are the doubled shadow of a paper sticker on the crystal upper face. When the two shadows are equally dark the long side of the crystal points to the sun. A sunstone or sun seeker. The sun itself was only 1° high and hidden by trees.

Bottom row ~ Ever the sceptic, Les Cowley also tests a crystal to see if it really seeks the sun. It was remarkably accurate and sensitive, locating the sun direction to within a degree or so. The late afternoon sun was 4° high.

Images ©John Stetson & Les Cowley
Blue sky is a product of sunlight Rayleigh scattered by air molecules. Its light is preferentially polarised with the electric vector perpendicular to the sun's rays and the polarisation is almost complete at 90� from the sun. Analysis of the polarisation gives the sun direction and so any device that splits blue skylight light into two polarised components and compares their intensities can act as a sunstone or sun seeker.

An Iceland Spar, calcite, crystal is just such a device. The crystal, built of arrays of negatively charged carbonate and positive calcium ions, is highly anisotropic. Light entering the crystal splits into two polarised components which travel in different directions. It is 'doubly refracting' and objects viewed through it appear doubled.

Place a paper sticker, or blob of Viking Stockholm tar, on the crystal top face and as seen through the bottom face it is doubled. Hold the crystal to the sky with its upper face level and rotate it to and fro. The two images change in intensity. When they are equally dark - a condition that can be judged rather precisely - the crystal long edges point in the sun's direction.

To be effective the sun must be low - as needed anyway for the Viking horizon board - with moderately blue sky overhead. The sun seeker does not work on completely cloudy days because any light from blue sky beyond them is depolarised by multiple scattering from the cloud water droplets.

Test the operation of a sunstone on cloudy days by looking through it at a white LCD monitor screen (strongly polarised). As the crystal is rotated the two images change in intensity.

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed
The Vikings started to settle in Iceland in the 860-70s. Inhabitable land was eventually taken and the Icelandic Sagas have it that a new land, Greenland, was reconnoitred in the 980s by Eric the Red. Two colonies of 3-4000 people were later established on its western coast. North America was probably first sighted by Bjarni Herjolfsson who, driven off course in 985/6 en route to Greenland from Iceland, reached but did not set foot on land to the west. Erik the Red's son, Leif the Fortunate, made a deliberate voyage westwards in ~1001, crossed the Davis Straight and landed on Baffin Island. He then sailed down the coast of Labrador to Newfoundland where a Viking settlement has been discovered. It is uncertain whether he sailed further south although vines and grapes were said to have been found and the Vikings called the land Vinland. From then on Viking longships regularly sailed back and forth across the Atlantic.

The Vikings were remarkable seafarers. There is no evidence that they had a magnetic compass. With our almost total reliance on advanced electronics and satellites for navigation it is seen as near impossible that Vikings in their small open ships could make such voyages without some navigational device. Their voyages were made in summer and at high latitudes the days were long and the sky did not get completely dark. Celestially, they had to rely on the sun rather than stars.

There is some evidence that they used an horizon board to help find latitude. This had the bearing of the sunrise and sunset marked with peg holes for their home port and a number of dates. To sail due west and reach Iceland and then Greenland they could sight the sun at sunrise and sunset each day and compare it with pegs on the board to set their course west.

However, the North Atlantic especially in Arctic or near Arctic waters is frequently cloud covered or there is a sea fog layer that hides a low sun but irritatingly shows blue sky overhead - thus the need for a sun seeker. On a typical Arctic morning with thin fog hiding sunrise an Iceland Spar sunstone could easily give an accurate bearing. Similarly towards sunset when distant clouds often hide the sun but leave some blue sky overhead the sunstone also works. And Iceland Spar crystals could be found on the ground in Iceland.

This is speculation, there is no direct evidence that they used sunstones. Less exciting perhaps, they were simply consummate sailors. The smells of the air and of the ocean, the character of the waves and winds, birds and moods of the sky were all cues that they were likely very sensitive to. They perchance had little need of magical devices.

Many thanks to John Stetson for alerting OPOD to sunstones.