A paper published on Tuesday in Proceedings of the Royal Society A: Mathematical Physical & Engineering Sciences may offer the explanation of how Viking mariners a thousand years ago were able to navigate the North Atlantic between Europe and the Old World, traveling great distances by sea in high latitudes in which cloudy weather frequently denied visibility of the sun.
Yahoo News explains:
Before the invention of the magnetic compass, navigating with a sundial would have been difficult, particularly on overcast days. Ancient Viking lore suggests that they had a magical tool to find the sun, even when the star was hidden.
Researchers have now discovered the crystal that would have made such a magical apparatus possible. The Vikings could have used a common calcite crystal, called an Icelandic spar, to find the sun in the high latitudes where they would have had to battle long twilights and cloudy skies to navigate. This special “sunstone” could find the direction of the sun even when it was out of view because it plays a trick with the light.
“The Vikings could have discovered this, simply by choosing a transparent crystal and looking through it through a small hole in a screen,” study researcher Guy Ropars wrote in an email to LiveScience. “The understanding of the complete mechanism and the knowledge of the polarization of light is not necessary.”
To use the crystal, the Vikings would have held the stone up to the center of the sky (from their perspective). When sunlight hits the crystal, that light gets polarized and broken into an “ordinary” and an “extraordinary” beam.
On a clear day, the Vikings would have rotated the crystal until the two beams lined up. Since these two beams line up and have the same brightness at only one angle, by noting where the sun is when this happens the Vikings could establish a reference point that could be used even when the sun wasn’t visible.
There are several other types of crystal that have this same property, but they wouldnâ€™t have been too useful because they aren’t as clear or as common, the researchers said. The Icelandic spar that the researchers analyzed is very common along the coasts of Iceland, and is also common today in Brazil and Mexico.
While none of these Icelandic spar crystals has been found in a Viking settlement, one was recently discovered in an Elizabethan shipwreck from 1592 in the English Channel.
Guy Ropars, Gabriel Gorre, Albert Le Floch, Jay Enoch and Vasudevan Lakshminarayanan, “A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight:”
Viking navigation from Norway to America in the northern latitudes remains a mystery for physicists, historians and archaeologists. Polarimetric methods using absorbing dichroic crystals as polarizers to detect a hidden Sun direction using the polarized skylight have led to controversies. Indeed, these techniques may lack in sensitivity, especially when the degree of polarization is low. 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.