The Vikings, explorers and conquerors of the northern seas, have long fascinated historians with their navigational skills. Between the 8th and 11th centuries, they covered vast distances, from Scandinavia to North America, without the aid of the compass, which didn't arrive in Europe until the 13th century. For these navigators, the sea was a high-risk terrain, where the slightest error in direction could be fatal. Their expeditions depended on their ability to interpret the signs of nature, such as waves and stars, but when the sky was overcast, their resources were limited.
Their ability to find their way without modern instruments still raises many questions. One hypothesis is the use of a "sun stone", described in medieval accounts as a possible guide on the high seas. Through scientific research and modern simulations, this mysterious mineral is gradually revealing its secrets, and could well shed new light on navigation methods of the time. Sunstone would thus have been a crucial innovation, making up for the lack of precise navigational instruments.
A mysterious stone mentioned in Icelandic sagas
The sun stone, or "sólarsteinn" in Old Norse, the origin of today's Scandinavian languages, is mentioned in Icelandic manuscripts as early as the 13th century. This stone, described as an aid for spotting the sun through clouds, has long raised questions among researchers. Although written sources cast doubt on its reality, archaeological discoveries and scientific simulations tend to show that a mineral, such as Icelandic spar, could have played this role. For Viking sailors, who often navigated the high seas without visual cues, such a tool would have been an invaluable asset.
Optical properties of calcite: the effect of birefringence
Formulated in 1967 by archaeologist Thorkild Ramskou, this hypothesis suggests that minerals such as cordierite, heliolite and calcite could have been used to locate the sun's position even on cloudy days, by capturing variations in light polarization.
Studies on sunstone have led researchers to focus on Icelandic spar, a variety of calcite, for its birefringent properties. This crystal, transparent and common in Scandinavia, divides light into two distinct rays of different intensities. When correctly oriented to face the sun, the rays superimpose to form a clear image of the star, even on cloudy days. The polarization of light, the basis of this technique, would enable navigators to pinpoint the exact position of the sun, essential for maintaining their course at sea.
A series of studies to test the effectiveness of sun stone
To test the feasibility of this technique, researchers at Loránd Eötvös University in Budapest carried out computer simulations, reproducing the conditions of Viking journeys between Norway and Greenland. By testing various minerals, including calcite, they obtained convincing results: by using sunstone every three hours, the virtual navigators reached their destination 92% to 100% of the time. Reducing the use of the stone to a check every four hours dropped this success rate to less than 60%. These results confirm that the sunstone could have been a reliable navigation tool under certain conditions, although it remains dependent on experience and frequency of use by sailors.
While the sunstone seems effective under ideal conditions, it does have its limitations. Light polarization works best when the sun is at a certain height in the sky, usually around thirty-five degrees elevation. So, at the beginning or end of the day, or under a completely overcast sky, the Vikings probably had to use other skills to compensate for the inaccuracy of this tool. These constraints are a reminder that at sea, the sunstone did not replace sailors' empirical knowledge, but complemented it, enabling them to navigate more efficiently when the sky offered bright openings.
