We all know that ancient seafarers navigated using the position of the sun by day, and the position of the stars by night. But what did they do when skies were cloudy? Without a magnetic compass, surely they’d be stuck?
The methods of Viking sailors, who reached Iceland, Greenland and eventually Newfoundland, have been the subject of intense speculation for decades. Famed for their skills at using landmarks and subtle natural cues to navigate, there’s still one question that remains: could part of their seafaring success have been down to their discovery of the strange properties of sunstones?
Several Nordic sagas and other literary sources refer to these mysterious ‘sólarsteinn’, but don’t specify how they were used. In the epic TV series The Vikings, Ragnar Lothbrok convinces his men to sail West, across open seas they’d never crossed before, having discovered ‘a new way of traveling’ that he claims worked even when the skies were cloudy. But could sunstones really have been used for navigation, even in overcast conditions?
In theory, says Stephen Harding in The Conversation, they could. The coastline of Norway and Iceland is indeed dotted with crystalline chunks of calcium carbonate known as calcite or Iceland spar. When polarised sunlight enters one of these crystals, it is split in two, making things appear in double. More importantly, each ‘image’ has a different intensity in part dependent on the position and orientation of the crystal compared to the light source. By rotating the crystal, these differences can be observed, measured and used to estimate the position of the sun, and determine geographical north.
The questions is whether it actually works in practice. If Vikings really did have these sunstone crystals aboard their ships, and had worked out how to calibrate them, could the optical signals be strong enough in overcast conditions to be detected with the naked eye, and with enough accuracy to navigate?
A paper by Gabor Horvath and colleagues, which recently appeared in Royal Society Open Science, set out to address that very question. To do this, they examined the optical signals from three different types of crystal (calcite, tourmaline and cordierite) of differing purities under simulations of 10 different sky conditions (ranging from clear to thick cloud and fog) at 61° latitude – one of the main Viking sailing routes.
They found that in clear skies, Viking sailors could have navigated with net orientation errors of less than 3∘, but in overcast conditions navigational error increased dramatically depending on the type and purity of crystal used (calcite up to ≤6∘, cordierite up to ≤10∘ and tourmaline up to ≤17∘).
Still, something’s got to be better than nothing and being able to hold a course that’s even roughly right until the skies had cleared must have been a boon. But as Horvath’s team made clear, this is just the first step in determining whether sunstones could be used to navigate in cloudy conditions – there are other stages of calculation for which their experiments have not yet factored in any room for error. Add those on and you might end up well… who knows where.
As Stephen Harding said, ‘if the method does not work under cloudy conditions using the kind of imperfect crystals the Vikings would likely have had, then the theory is probably wrong. And on clear days it would have been easier just to use calibrated sundials’.
And yet… if Horvath’s team does establish that sunstones could have accurately been used to determine the direction of geographic north, then all that will remain to finally prove this theory will be to find one, calibrated and in place, onboard a real Viking ship. And that’s something only a team of very skilled, and very lucky, archaeologists can do.
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