The Viking Sunstone

Boards creak beneath the feet of a Viking warrior as the ship bobs in the water. Waves crash against the bow sending ocean spray into their face as they look out across the water. It is their first raid, and they can feel their heart race with excitement. There is only one problem that stands in their way. With the sky heavy with clouds, the sun is impossible to see. How are they supposed to navigate with no sun or stars to point the way? 

Viking legends may hold the answer. Tales like 'The Saga of King Olaf' tell of sólarsteinn or sunstone that could identify the sun's location even with cloudy skies. No evidence of the sunstone has been uncovered in the Viking archaeological records, but a whitish crystal identified as calcite (Iceland spar) was found next to navigation tools of a 16th-century Elizabethan shipwreck. No one is entirely sure where the stone came from or what purpose it served, but some researchers have suggested that the stone was used for navigation. 

To determine if calcite could be the fabled Viking sunstone, researchers began testing the calcite crystal along with cordierite and tourmaline, each of which shares similar characteristics. All three stones have a property called birefringence, meaning they split a beam of sunlight, creating two images. Researchers believe the Vikings could have taken advantage of this characteristic to locate the sun, even on cloudy days or when the sun was below the horizon. While it is not known exactly how this was done, one theory suggests a mark was placed on the top of the crystal using tar or charcoal. Navigators would point the stone at the brightest point in the sky while looking up through the bottom of the stone. The light would split allowing them to see two marks, one brighter than the other. They would move the stone across the sky until both marks appeared equally bright. The navigator would then know the stone was pointed at the sun. With practice, navigators may have been able to detect the sun’s location within a matter of one or two degrees.   

Could the use of a crystal help Vikings navigate? Experiments suggest the answer is yes. Biophysicist Gábor Horváth and his colleague Dénes Száz decided to test the theory. Along with a group of researchers, they created a computer simulation of a voyage between the Viking village of Hernam, Norway, and Hvarf, a Viking village in Greenland, a three-week voyage almost directly to the west. They ran 3,600 simulations covering a time spanning from the spring equinox to the summer solstice with only three variations: cloud coverage, type of crystal, and how often the sunstones were consulted.     

The experiment showed that it was not the crystal, but the frequency navigational readings were taken, that determined navigational accuracy. If a navigator took a reading every four hours, they reached Greenland 32% to 59% of the time. A reading every five or six hours meant the ship had a decidedly poor chance of finding Greenland, while readings taken every three hours gave the ship a 92% to 100% chance of reaching land. They also determined that readings should be taken equally in the morning and evening. Morning readings would shift a ship too far north while evening readings would send a ship too far south, with the possibility of missing Greenland completely.  

With no physical evidence, it is impossible to prove that Vikings used the sunstone to navigate. There are multiple other forms of navigation they could have used including locating familiar landmarks, tracking wave patterns, or observing migrating animals. No matter what method of navigation they used, the Vikings were able to harness their knowledge to cover the vast northern seas.