At sea with a Viking satnav

The Vikings' seafaring exploits relied on some ingenious navigation techniques.

For reasons that there really isn't space to go into here, I have spent a lot of this year dealing with 9th Century Vikings. On the whole they've proved a rather admirable bunch and so I think it's about time we stopped thinking of them purely in terms of their robust attitude towards foreign policy and started appreciating them for their technological achievements.

In particular, I've been thinking about navigation and the sticky problem of finding places like Iceland, which sits in an awful lot of ocean. We know that the Vikings had reached Iceland by 874 when Ingólfr Arnarson is recorded as building his homestead in Reykjavík, and we know that the descendants of those early Icelanders went on to travel further west to Greenland and even Newfoundland. The question is, how? The answer, it would appear, is with a bird, a board and a lump of stone - the Viking equivalent of a satnav.

Of course, the Vikings were very good with ships and could easily navigate in sight of land by just using landmarks and their knowledge of the coast, shoals, rocks and harbours. Setting out across the open sea was another matter.

Flóki Vilgeròarson, who the 'Icelandic Book of Settlements', or 'Landnámabók', credits as being the first person to deliberately set out for Iceland, navigated using ravens. When he reached the Faroe Islands he took three ravens on board and headed off in the rough direction of Iceland. After a while he let a raven go, which flew up until it could see much further over the horizon than Flóki could, at which point it spotted the Faroes and promptly flew back home. Some time later he set another raven free which flew up, had a look round and promptly came back to the deck, indicating that they were really now quite a long way from land. Finally, he let the third raven go which flew up, spotted Iceland in the far distance, and headed off there. Fl'ki gratefully followed.

There is a more technical means of navigation mentioned in the history of the Faroes. This is the 'sun-shadow board', or solskuggerfjol, used for determining latitude by measuring the height of the sun over the horizon at noon. It was a circular wooden board, about 25-30cm in diameter, with a gnomon at its centre, the height of which could be set to the time of the year (the sun is lower in the sky at noon in the winter than the summer).

To keep it level, the board was placed in a bucket of water. Next, the shadow of the noon Sun was observed. A concentric circle on the board gave the line the shadow should reach if the ship was on the desired latitude. If the shadow was beyond the line, the ship was north of this latitude; if inside, the ship was south of it. So by checking the noon-day shadow reached the same circle, you could ensure you were going due east or west.

You'd need to know when noon was, so you should start making measurements 'around noon' and marking the position of the shadow on the board. The point where the shadow is shortest, and hence the Sun is highest in the sky, would be noon.

The final method of navigation is a shade more contentious. One of the sagas mentions an incident that must have been typical of northerly navigations. Navigation requires knowing where the Sun is, and this can be tricky under leaden, snowy skies. When King Olaf asked the hero Sigurd to manage this feat in the saga, Sigurd grabbed a 'sunstone' and correctly estimated where the Sun must be by looking through it.

Just what this sunstone is has long baffled historians and it has often been dismissed as a 'magical' device. However, it has been noted by archaeologists for some time that some forms of calcite, notably Icelandic Spar, have polarising properties, so perhaps these were the basis for real sunstones. They noted that calcite can variably polarise light depending on the orientation of the crystal. At one particular point, however, known as the isotropy point, the crystal eliminates all polarisation.

This property was exploited last year by physicists at the University of Rennes 1 who noted that if the calcite is then suddenly removed from the line of sight at this point, a faint, yellow streak, known as Haidinger's Brush, is briefly visible. This polarisation artefact in the eye rather conveniently points directly towards where the hidden Sun is.

Proof of concept is not, of course, proof of use, and to-date no Icelandic Spar has been found in Viking nautical contexts, although, to be fair, no-one has looked for it. Calcite has been found on later vessels, however, hinting perhaps at the antiquity of the technique. Perhaps Leif Ericson owed his discovery of North America not to luck or bravery, but to birds, boards and lumps of stone. *

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