The Great Sunstone
The sunstone is a type of mineral attested in several 13th—14th century written sources in Iceland, one of which describes its use to locate the sun in a completely overcast sky. Sunstones are also mentioned in the inventories of several churches and one monastery in 14th—15th century Iceland. A theory exists that the sunstone had polarizing attributes and was used as a navigation instrument by seafarers in the Viking Age. 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. Their sunstone came within 1% of the true location of the sun even after it had dipped below the horizon.
Danish archaeologist Thorkild Ramskou posited that the sunstone could have been one of the minerals (cordierite or Iceland spar) that polarize light and by which the azimuth of the sun can be determined amid a partly overcast sky or when the sun is just below the horizon. The principle is used by many animals and was applied during polar flights before more advanced techniques became available. Ramskou further conjectured that the sunstone could have aided navigation in the open sea in the Viking period.
A polarizing mineral only gives the horizontal angle of the sun (azimuth), which is only of marginal value when navigating the open sea. A polarizing crystal would have been useful as a sundial, especially at high latitudes with extended hours of twilight, in mountainous areas or in partly overcast conditions.
Viking navigation from Norway to America in the northern latitudes remains a mystery for physicists, historians and archaeologists. Polarimetric methods using absorbing crystals as polarizers to detect a hidden Sun direction using the polarized skylight has often led to controversies.
Indeed, these techniques may lack in sensitivity, especially when the degree of polarization is low. In the new study it is demonstrated theoretically and experimentally that using the transparent common Iceland spar as a depolarizer, the Vikings could have performed a precise navigation under different conditions. When simply rotated, such a crystal can completely depolarize, at the so-called isotropy point, any partially polarized state of light, allowing to guess the direction of the Sun.
A precision of a few degrees could be reached even under dark 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.