In 2014, the group suggested a medieval artifact called the Uunartoq disc was part of a sun compass that the Vikings used in conjunction with sunstones.
Their latest work uses a computer model that simulated 3,600 three-week trips from Norway to Greenland under varying cloud cover at two dates, spring equinox and summer solstice, which are marked on the Uunartoq disc.
If the Vikings oriented their ship with calcite, cordierite or tourmaline at least every three hours, the model showed, they had a 92 to 100 percent chance of getting within sight of Greenland. These are “surprisingly large success rates” for navigating in overcast conditions, the authors noted.
The key to sunstone navigation is polarization, a process that filters light rays so they can only move in one plane. Sunlight starts out oscillating in multiple planes, but atmospheric particles create concentric rings of polarized light around the sun, even on cloudy days. Though some animals, like ants and crickets, can detect these patterns, polarization is practically indiscernible to the naked human eye.
More Reporting on Archaeology
Crystals can help. When polarized light passes through calcite, it splits into two beams. By rotating a calcite crystal against the sky and noting changes in brightness between these beams, one can find the atmosphere’s polarization rings and figure out where the sun is. (Cordierite and tourmaline work a bit differently, but the principle is similar.)
The Vikings may have calibrated crystals in sunny weather, then used them to navigate on gray days, Dr. Horváth said in an email.
Amit Lerner, an animal vision researcher based in Israel, wonders if navigation by sunstone would have been too cumbersome for the Vikings to pull off.
“Small mistakes in navigation — even by a fraction of an angle — can lead to enormous diversions” in long-distance trips, he said.
Dr. Harding said we shouldn’t be quick to dismiss the Vikings, who “were brilliant” despite popular perception of them as brutes.
The study’s authors hope to settle the score with the ultimate test: a round-trip voyage between Norway and Greenland, navigated by sun compass and sunstones.