How did the Vikings navigate under cloudy skies?

In a recent paper published in Philosophical Transactions of the Royal Society B, CAnMove coordinator in collaboration with a team lead by an optical physicist Dr Gabor Horváth at Eötvös University in Budapest report on recent findings on how Viking navigators possibly could have used so-called sunstones to determine the position of the sun under foggy and cloudy conditions. The team for part of their work measured the skylight polarization under different optical conditions during an expedition (Beringia 2005) to the North Pole in 2005 organized by the Swedish Polar Research Secretariat. Based on these and additional measurements and psychophysical laboratory experiments we suggest that the optical physical conditions required for sun navigation by using sunstones are met under cloudy and foggy conditions, such that the pattern of skylight polarization are transmitted through the clouds and correspond to the pattern seen under clear sky conditions. Overall there is a reduction in degree of polarization of the incoming light under cloudy and foggy skies, but the pattern of skylight polarization is very similar to what is seen in a clear sky. It has been suggested that Vikings have used Icelandic spar (calcite) or possibly crystals of tourmaline, which are birefringent crystals (linearly polarizing crystals), to determine the Sun’s position when it could not be seen in the sky. It is a theory which has been discussed for a long time, however, still such sunstones have not been found in association with Shipwrecks from the Viking ages. Now we show that the optical prerequisites for sunstone navigation are met.

A more popular account of the recent findings is presented at e.g.
Nature News.

In the issue of Phil Trans also other work are presented from CAnMove scientist Rachel Muheim on polarization navigation in birds and by Marie Dacke and Eric Warrent in the Vision group at the Department of Biology at Lund University on the use of polarized light day and night-active insects.

Susanne