In the 1960s, Danish archaeologist Thorkild Ramskou suggested that the Vikings navigated by sunstone, using its polarization to find the sun through clouds and fog:
The idea of navigating by polarized skylight originated with a Viking saga, in which the Norse hero Sigurd “grabbed a sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible Sun.” The stone would like have been made of a so-called birefringent material, like calcite or certain plastics, that can split light into separate rays.
The atmosphere similarly splits sunlight into a pattern of concentric rings. Looking through the crystal and rotating it would make the sky appear to brighten and fade, as certain directions of light were transmitted or blocked. When the light coming through the crystal was polarized the same way as through the atmosphere, the crystal would appear brightest and points toward the sun. By checking the polarization at two different points in the sky, the navigators could determine the invisible sun’s location, and hold a torch in that position to cast a shadow on the sundial.
Optics expert Gábor Horváth of Hungary’s Eötvös University ran five experiments to see if the proposed method worked — which it did, even in clouds and fog, but not when the sky was completely overcast.