Viking Sunstones

The thirteenth-century Hrafns Saga contains the first mention of a mysterious bit of Viking lore called the sunstone:

The weather was thick and stormy... The king looked about and saw no blue sky. Then the king took the sunstone and held it up, and then he saw where [the sun] beamed from the stone.

It was first suggested back in the 1960s that a sunstone might be a piece of Icelandic spar, a transparent form of calcite that polarizes light. One problem with this theory was that nobody had ever found a piece of Icelandic spar on a Viking archaeological site, so there was no evidence that medieval people knew of the stone and its properties. Then in the 1970s a piece of Icelandic spar was found in the wreck of a 16th-century English ship, giving new credence to the idea.

Now a team of intrepid scholars from the University of Rennes have tried to navigate using a piece of Icelandic spar and found that, yes, it really works:

Here, we demonstrate theoretically and experimentally that using the transparent common Iceland spar as a depolarizer, the Vikings could have performed a precise navigation under different conditions. Indeed, when simply rotated, such a birefringent crystal can completely depolarize, at the so-called isotropy point, any partially polarized state of light, allowing us to guess the direction of the Sun. By equalizing the intensities of the ordinary and extraordinary beams at the isotropy point, we show that the Sun direction can be determined easily, thanks to a simple sensitive differential two-image observation. A precision of a few degrees could be reached even under dark crepuscular conditions. The exciting recent discovery of such an Iceland spar in the Alderney Elizabethan ship that sank two centuries before the introduction of the polarization of light in optics may support the use of the calcite crystal for navigation purposes.

It works like this:

Simply rotating the crystal gave a good chance of detecting the sun, the team found. But when the stone was covered with an opaque sheet with a hole at its center, two distinct shadows were created. Rotating the crystal now made one shadow lighter and the other darker, according to their angle to the sun.

They were consistently able to determine the location of the sun to within a few degrees, even when it had dipped below the horizon. I would put the sunstone theory in the "very likely to be true" category -- "confirmed," as the Mythbusters would say -- and the next time I teach about the Vikings I plan to mention it.