Nowadays, we can rely on GPS receivers or magnetic compasses to tell us how to reach our destination. Some 1000 years ago, Vikings had none of these advanced navigation tools. Yet, they successfully sailed from Scandinavia to America in near-polar regions where it can be hard to use the Sun and the stars as a compass. Clouds or fog and the long twilights characteristic of polar summers complicate direct observations of these celestial bodies. So how did they find their bearings? A new study published in Proceeding of Royal Society A shows that they probably used Iceland spar, a “sunstone”.
Centuries-old Viking legends tell of glowing sunstones that navigators used to find the position of the Sun and set the ship’s course even on cloudy days. In 1967, a Danish archaeologist named Thorkild Ramskou speculated that the Viking sunstone could have been Iceland spar, a clear variety of calcite common in Iceland and parts of Scandinavia.
This crystal has an interesting property called birefringence: a light ray falling on calcite will be divided in two, forming a double image on its far side. (This double image is easily seen by placing transparent calcite on printed text.) Further, the Iceland spar is a polarising crystal, meaning the two images will have different brightnesses depending on the polarisation of light.
Light is made up of electromagnetic waves with component electric and magnetic fields. If these components have a specific orientation, the light is said to be polarised, while in unpolarised light the orientation of these fields has no preferred direction. Calcite can appear dark or light depending on the polarisation of light that falls upon it.
Sunlight becomes polarised as it crosses the Earth’s atmosphere, and the sky forms a pattern of rings of polarised light centred on the Sun. Changing the orientation of calcite as light passes through it will change the relative brightness of the projections of the split beams, even when the Sun is hiding behind clouds or just below the horizon. The beams are equally bright when the crystal is aligned to the Sun.
It can be hard to determine when exactly these split beams have equal brightness. But the new study, led by Guy Ropars at the University of Rennes 1 in France, suggests Vikings could have built a simple device to better use the sunstone.
The technique consists in covering the Iceland spar with an opaque screen with a small hole in its centre and a pointer. As light passes through the hole onto the crystal, a dark surface below it receives the projection of the double image for comparison.
By rotating the apparatus and determining the direction at which the two images were equal in brightness, the team managed to pinpoint the Sun’s position on a cloudy day with an accuracy of one degree on either side. Researchers also conducted tests when the Sun was largely below the horizon. “We have verified that the human eye can reliably guess clearly the Sun direction in dark twilights, even until the stars become observable,” Ropars’ team writes in the paper.
Although archaeologists have not yet found Iceland spar among Viking shipwrecks, the new study adds credence to the idea that Viking seafarers used the crystal in their travels.
Further, the recent finding of a calcite crystal on a sixteenth century Elizabethan ship shows that navigators could have used Iceland spar even after the appearance of the magnetic compass. Cannons on ships could perturb a magnetic compass orientation by 90 degrees, so a crystal serving as an optical compass could be crucial in avoiding navigational errors and get sailors to a safe port.
By Bárbara Ferreira, EGU’s Media and Communications Officer