To avoid getting lost on their voyages across the North Atlantic 1000 years ago, Vikings relied on the sun to determine their heading. (This was long before magnetic compasses were available in Europe.) But cloudy days could have sent their ships dangerously off course, especially during the all-day summer sun at those far-north latitudes. The Norse sagas mention a mysterious "sunstone" used for navigation. Now a team of scientists claims that the sunstones could have been calcite crystals and that Vikings could have used them to get highly accurate compass readings even when the sun was hidden.

The trick for locating the position of the hidden sun is to detect polarization, the orientation of light waves along their path. Even on a cloudy day, the sky still forms a pattern of concentric rings of polarized light with the sun at its center. If you have a crystal that depolarizes light, you can determine the location of the rings around the hidden sun.

Calcite is such a crystal. It has a property called birefringence: Light passing through calcite is split along two paths, forming a double image on the far side. The brightness of the two images relative to each other depends on the polarization of the light. By passing light from the sky through calcite and changing the crystal's orientation until the projections of the split beams are equally bright, it is theoretically possible to detect the concentric rings of polarization and thus the location of the sun.

Theory is one thing, practice is another. To see if calcite is accurate enough for navigation, a team led by Guy Ropars, a physicist at the University of Rennes 1 in France, built a sunstone. They used a chunk of calcite from Iceland spar, a rock familiar to the Vikings, and locked it into a wooden device that beams light from the sky onto the crystal through a hole and projects the double image onto a surface for comparison. They then used it over the course of a completely overcast day. They took the measurements from a point on land where they knew the sun's exact trajectory.

If the Vikings were clever enough to use calcite as a sunstone, it would have enabled them to navigate on cloudy days, the researchers report today in the Proceedings of the Royal Society A. Their sunstone came within 1% of the true location of the sun even after it had dipped below the horizon. Ropars cautions that archaeologists have yet to find a sunstone among Viking shipwrecks or settlements.

The study reveals "an ingenious solution to the problem of open-sea navigation," says John Phillips, a biologist who studies animal navigation at Virginia Polytechnic Institute and State University in Blacksburg, adding that birds may also use polarization to navigate. But even if it is possible, using such a sunstone on a rolling Viking ship at sea would have been a challenge, he says. "Perhaps [they used it] when the Viking sailors encountered islands or ice packs during their travels."

Correction: A previous version of this story stated that the magnetic compass had not been invented by the 10th century, but it had already been invented in China by then. It has been corrected to reflect that.