The sunstone is known from a few Icelandic texts, of which the twelfth or thirteenth century Rauðúlfs þáttr describes how King Olav Haraldsson II (St. Olav) used a crystal to locate the sun behind a thick layer of clouds to verify a prediction made by a man named Sigurður about the sun’s location: “Then the king made them fetch the solar stone and held it up and saw where light radiated from the stone and thus directly verified Sigurður's prediction” (trans. Thorsteinn Vilhjalmsson).
Because the medieval texts mentioning the stone are allegorical in nature, many have dismissed the stones as fictional. But in 2013, a crystal was found in a sixteenth century English shipwreck near Alderney, which some have interpreted as evidence that sunstones were both real and in used six centuries earlier by a completely different culture. According to the Alderney museum official Gates interviewed, the only connection between the Elizabethan shipwreck and the Vikings is speculation that the crystal found in the 1592 wreck was a sunstone and not, say, intended for jewelry, magical rituals, etc. The stone was found close to a navigational instrument, but this doesn’t imply that the stone was used this way. Some suggest that it may have been used as a portable sundial. It’s a bit challenging to believe that sunstones were used for navigation for six centuries and produced a grand total of two or three mentions in all of medieval literature, and not a lick of notice from anyone in England where the only existing stone was found. Stranger things have happened, but it is a bit unusual.
Interestingly, the medieval texts do not specify that the sunstones, should they have existed, were used in navigation. That conclusion came from the Danish archaeologist Thorkild Ramskou, who in the 1960s postulated that polarized stones could have been used to keep track of the sun while sailing through cloudy weather. It’s a reasonable conclusion, but not one explicitly supported by the texts, which do not imply that the sunstone was necessary for the coast-hugging sailing described in the voyage of St. Olav.
Gates visits the shipwreck and dives into the waters, but of course he doesn’t find any more sunstones. Gates then travels to Norway in search of more information about St. Olav. At a medieval cathedral, Gates is shown the crypts where the medieval king may be buried. His exact place of entombment within the catacombs is unknown. Gates moves on to Oslo, where he eats reindeer. I wonder if it tastes like venison, which I’ve had. Anyway, he moves on to a museum where the largest intact Viking ship ever found is on display. There, he meets with Dr. Jan Brill, who tells Gates that the crystal found in Alderney is a polarized form of calcite. He sends Gates to a closed Norwegian mine to excavate a piece of calcite, though this is more an excuse to take a trip into the mine than it is a necessity to get calcite. But as they say, getting there is half the fun. This leads into a long segment of descending into tunnels that date back to the seventeenth century.
After chipping a piece of clear calcite out of rock a thousand feet below ground, Gates meets with an expert in light polarization, Balázs Bernáth, who explains how one might use a polarized stone to find the location of the sun. Bernáth then explains how one might use a “twilight board,” or sun compass, which uses the angle of the sun to deduce direction with the help of a shadow stick. By combining the two, the sun stone could therefore be used to determine the location of the sun and aim the sun compass when the sun wasn’t visible. Bernáth published a paper about this last year.
Gates next travels to a reconstruction of a Viking village to learn about medieval Norse life, and then in the last minutes of the show, he and Bernáth are on board a reconstruction of a Viking ship to test whether the presumed sunstone and the twilight board actually work. Gates determines the presumed direction based on the medieval technology, and then Bernáth compares the direction to a modern compass reading. They are close, though not exact; but it does not prove as Gates thinks that the sunstone is “real.” There are two factors not considered: A magnetic compass finds magnetic north, while the sunstone should, in theory, find geographic north. At Oslo, I believe the magnetic declination is about 2.5 degrees east, which is likely not enough to be significant and probably accounts for the variation seen on screen. Second, just because this reconstructed and back-formed system works does not imply that the Vikings used it. There remains no textual evidence that the sunstone was ever used for navigation, nor that it was ever combined with the sun compass, which had its own independent uses.
So, overall, this was an interesting experiment and a possibility in terms of Viking navigation, but one that requires more archaeological evidence to fully prove.