Maybe the 'Sunstone' wasn't a legend after all!

I'm very interested to read that the legendary Viking 'sunstone', an aid to navigation mentioned in one of the Icelandic sagas, may not be so legendary after all, but very real.

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 B1.

The Vikings, seafarers from Scandinavia who travelled widely and settled in swathes of Northern Europe, the British Isles and the northern Atlantic from around 750 to 1050 AD, were skilled navigators, able to cross thousands of kilometres of open sea between Norway, Iceland and Greenland. Perpetual daylight during the summer sailing season in the far north would have prevented them from using the stars as a guide to their positions, and the magnetic compass had yet to be introduced in Europe - in any case, it would have been of limited use so close to the North Pole.

But 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.

A large crystal of calcite, or Icelandic Spar (image courtesy of Wikipedia)

Light consists of electromagnetic waves that oscillate perpendicular to the direction of the light's travel. When the oscillations all point in the same direction, the light is polarized. A polarizing crystal such as calcite allows only light polarized in certain directions to pass through it, and can appear bright or dark depending on how it is oriented with respect to the light.

Scattering by air molecules in the atmosphere causes sunlight to become polarized, with the line of polarization tangential to circles centred on the Sun. So Ramskou argued that by holding a crystal such as calcite up to the sky and rotating it to check the direction of polarization of the light passing through it, the Vikings could have deduced the position of the Sun, even when it was hidden behind clouds or fog, or was just beneath the horizon.

. . .

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."

She and Horváth are now planning further experiments to determine whether volunteers can accurately work out the Sun's position using crystals in various weather conditions.

There's more at the link.

Here's a video clip demonstrating double refraction (another optical effect) through a calcite (Icelandic spar) crystal.

As one trained in nautical navigation, I find this fascinating. To think that a simple rock crystal could guide Viking navigators through fog and low cloud, when later Western European navigators were essentially helpless in such conditions, and had to wait for the sun to come out once more . . . that's mind-boggling! Shows that our ancient ancestors weren't as 'simple' as we might think.

Peter