Viking sunstone

The Vikings navigated by the position of the sun. But what did they do when it was cloudy?

Icelandic spar - maybe the sunstone
ArniEin, CC BY-SA 3.0, via Wikimedia Commons

In 1948, an archaeologist in Greenland dug up a nondescript fragment of wood: the Uunartoq Disc. It was a sun compass, a sundial-like tool that Scandinavian sailors could use to find True North and thus navigate effectively. Vikings did not have magnetic compasses, but they managed to sail far across the world; they travelled distances rivalled in Medieval times only by Polynesian sailors in the Pacific.

(It’s well-known that the Vikings got as far as Greenland and Newfoundland in North America, but they also got as far as the Caspian Sea, Sicily, and North Africa.)

A sun compass is a very useful tool, but it has one fatal flaw: you need the sun. Indeed, any sun-based navigation system requires a fairly precise measurement of the sun’s current position. A ship stuck in fog or heavy cloud is a ship lost.

But Medieval Viking seafarers had a trick up their sleeves. Consider the following passage, from the Medieval Icelandic story Rauðúlfs þáttr:

The weather was thick and snowy as Sigurður had predicted. Then the king summoned Sigurður and Dagur (Rauðúlfur’s sons) to him. The king made people look out and they could nowhere see a clear sky. Then he asked Sigurður to tell where the sun was at that time. He gave a clear assertion. Then the king 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.


[I know the site looks a bit janky, but it seems to be the only version of Thorsteinn Vilhjalmsson’s translation online.]

An Iceland spar crystal is a rather nondescript transparent piece of calcite with a surprising property. When held up to a cloudy or dusky sky, it can supposedly identify the exact position of the sun. Is this the mysterious “sunstone”?

Iceland spar does this by refracting the sun’s light into two beams. Those beams differ in brightness according to the sun’s polarisation, so the relative brightness will change according to where it’s pointed. Hold the crystal up to a random place in the sky, and those two rays will have different brightnesses. But, if you hold it up directly to the sun, both beams will have the same polarisation – the brightnesses will match. Now you know exactly where the sun is, even if you cannot see it directly.

There’s a decent demonstration of this effect in this video:

Or, for a more technical description, see the third link below. No archaeologists have found an actual Viking sunstone yet – but given the paucity of Viking artefacts that’s not too surprising. The Iceland spar sunstone remains a popular but unverified theory about Viking navigation.

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