‘Sunstone’ crystals may have helped Vikings navigate on cloudy days.
Between AD 750 and AD 1200 the Vikings, were the dominant sailors of the North Atlantic. When the Sun was shining, geographical north could be determined with a special sundial or ‘sundisc’. However, how the Vikings could have navigated in cloudy or foggy situations, when the Sun’s disc was unusable, is still not fully understood.
A hypothesis was formulated in 1967, suggested that under foggy or cloudy conditions, Vikings might have been able to determine the azimuth direction of the Sun with the help of skylight polarization, just like some insects. According to this theory, the Vikings could have determined the direction of the skylight polarization with the help of an enigmatic birefringent crystal (double-refracting crystal), like cordierite, tourmaline, or calcite, which are common in the Scandinavian region and even mentioned in a Viking saga, functioning as a linearly polarizing filter.
The Viking saga 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 in Philosophical Transactions of the Royal Society B . (Feb 2011)
Viking legends, including an Icelandic saga ‘Rauðúlfs þáttr’ with the hero Sigurður , 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 Sigurður on the location of the Sun. To check Sigurður‘s answer, Olaf “made them fetch the solar stone and held it up and saw where light radiated from the stone and thus directly verified Sigurður’ s prediction”. 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.
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.
Air molecules scatter the light in the atmosphere causing 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.
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.
“Using full-sky imaging polarimetry, we have shown that one of the two atmospheric optical prerequisites of the polarimetric Viking navigation is always fulfilled under both foggy and cloudy conditions,” saidHorváth. “The distribution (pattern) of the direction of polarization of skylight on the foggy or cloudy celestial hemisphere is similar to that of the clear sky, which was a great surprise for us. However, we would like to emphasize that the Dutch meteorologist Guenther P. Koennen has already hypothesized this phenomenon in his famous book Polarized Light in Nature (Cambridge University Press, 1985).”
Through the clouds
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.
Sean McGrail, who studied ancient seafaring at the University of Oxford, UK, before retiring, says that the studies are interesting but there is no real evidence to indicate that the Vikings actually used such crystals. “You can show how they could be used, but that isn’t proof,” he says. “People were navigating long before this without any instruments.”
Surviving written records indicate that Viking and early medieval sailors crossed the north Atlantic using the Sun’s position on clear days as a guide, in combination with the positions of coastlines, flight patterns of birds, migration paths of whales and distant clouds over islands, says Christian Keller, a specialist in North Atlantic archaeology at the University of Oslo. “You don’t need to be a wizard,” he says. “But you do need to combine a lot of different sorts of observations.”
Keller says he is “totally open” to the idea that the Vikings also used sunstones, but is waiting for archaeological evidence. “If we find a shipwreck with a crystal on board, then I would be happy,” he says.
Faulkes, Anthony. 1966. “Rauðúlfs þáttr: A study”. Studia Islandica 25. Heimspekideild Háskóla Íslands og Bókaútgáfa Menningarsjóðs. Reykjavík. ISSN 0258-3828. 92 pp.
On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers, Gabor Horvath et al., Phil. Trans. R. Soc. B 12 March 2011 vol. 366 no. 1565 772-782. doi:10.1098/rstb.2010.0194
Hegedus, Ramon, Akesson, Susanne, Wehner, Rudiger, and Horvath, Gabor. “Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies.” Proc. R. Soc. A. 463 : 1081-1095 (2007).
Barta, Andras, Horvath, Gabor, and Meyer-Rochow, Benno. “Psychophysical study of the visual sun location in pictures of cloudy and twilight skies inspired by Viking navigation.” Journal of the Optical Society of America A 22: 1023-1034 (2005).