The Vikings Had Stones...

...to navigate across the ocean in their little wooden boats.

A Viking legend tells of a glowing 'sunstone' that, when held up to the sky, revealed the position of the Sun even on a cloudy day. It sounds like magic, but scientists measuring the properties of light in the sky say that polarizing crystals — which function in the same way as the mythical sunstone — could have helped ancient sailors to cross the northern Atlantic. A review of their evidence is published today in Philosophical Transactions of the Royal Society...

Viking legends, including an Icelandic saga centring on the hero Sigurd, hint that these sailors had another navigational aid at their disposal: a sólarsteinn, or sunstone.   The saga describes how, during cloudy, snowy weather, King Olaf consulted Sigurd on the location of the Sun. To check Sigurd's answer, Olaf "grabbed a sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible Sun". In 1967, Thorkild Ramskou, a Danish archaeologist, suggested that this stone could have been a polarizing crystal such as Icelandic spar, a transparent form of calcite, which is common in Scandinavia...

Historians have debated the possibility ever since, with some arguing that the technique would have been pointless, because it would only work if the crystal was pointed at patches of clear sky, and in such conditions it would be possible to estimate the position of the Sun with the naked eye, for example from the bright lining of cloud tops.

Gábor Horváth, an optics researcher at Eötvös University in Budapest, and Susanne Åkesson, a migration ecologist from Lund University, Sweden, have been testing these assumptions since 2005. The special issue of Philosophical Transactions of the Royal Society B in which their review appears is dedicated to biological research on polarized light.

In one study, the researchers took photographs of partly cloudy or twilight skies in northern Finland through a 180° fisheye lens, and asked test subjects to estimate the position of the Sun. Errors of up to 99° led the researchers to conclude that the Vikings could not have relied on naked-eye guesses of the Sun's position.

To check whether sunstones would work better, in 2005 they measured the polarization pattern of the entire sky under a range of weather conditions during a crossing of the Arctic Ocean on the Swedish icebreaker Oden...  The researchers were surprised to find that in foggy or totally overcast conditions the pattern of light polarization was similar to that of clear skies. The polarization was not as strong, but Åkesson believes that it could still have provided Viking navigators with useful information.  "I tried such a crystal on a rainy overcast day in Sweden," she says. "The light pattern varied depending on the orientation of the stone."

I have got to try this.  I wonder where that old rock collection is?  And, just because this post is worthless without a picture..

Note: Viking picture highly inaccurate...