Mythical crystals and a "sixth sense?" Really? But a group of physicists and optometrists say they have cracked the optical properties of the Viking sunstone, which legend has it aided the northerly, often storm-beset navigators long before the invention of the compass. But to do it requires using a squid-like sense of direction.

A report released today in the Proceedings of the Royal Society A lays out the possible inner workings of the Vikings' legendary sólarsteinn, which was said to reveal the true bearing of a hidden sun, even on overcast days and during long summer twilights in the northerly latitudes. Researchers long speculated that the sunstone might have been a transparent type of calcite, common in Iceland, that has optical properties akin to linearly polarizing filters for a camera (see photograph above).

Light passing through such a crystal, including the common Iceland spar, changes in brightness and color as the crystal is rotated. Vikings presumably could have used such crystals to observe polarization patterns and thereby pinpoint the direction of the sun. But exactly how this was done was an enigma, until now. Guy Ropars and Albert Le Floch of the University of Rennes' Laser Physics Laboratory in France, led the latest study, which has solved the mystery of the myth they say by attacking the problem backwards.

"Rather than thinking in term of polarizer, we have deliberately chosen to 'destroy' the polarization of the light," Ropars told Discovery News. "Iceland spar behaves theoretically and experimentally like a perfect depolarizer." In other words, with the crystal held up to the sky, there is one specific angle of rotation, called the isotropy point, at which the crystal eliminates all polarization of the light passing through it.

Here's where the "sixth sense" comes in: The investigators say that if you look through the crystal in its depolarizing position and then pull it away suddenly from your line of sight, you can catch a glimpse of a faint, elongate yellowish pattern known as a Haidinger's Brush. The key here is that the ends of that yellow shape point directly toward the sun.

The Haidinger's Brush phenomenon amounts to a greatly scaled-down version of the specialized ability of many insects, cephalopods, amphibians and other animals to "see" polarization patterns in the sky or water. That's how those animals navigate. Turns out the Vikings may have too. When Ropars' group asked test subjects to use their method to identify sun direction, their answers were accurate within 5 degrees.

Coupled with a second technique observing the changing polarization patterns passing through the crystal, also tested and described for the first time in this study, the Vikings could have established a reference point that could be used even when the sun was fully hidden, upping the sunstone's accuracy to within 1 degree.