Twenty-five years ago last Thursday, spring was in the air. Soft rains were awakening the land from its winter sleep, and the rich perfume of new life was everywhere evident. Almost everywhere, that is. All I could smell in the windowless cell that served as a geology lab was the dust of ancient rock. That arid lab was the room where my biweekly class in optical mineralogy met, and the prof’s lecture reflected the setting. It, too, was dry as dust. Then the prof held up a thin slab of a clear, colorless mineral. “This is Iceland spar,” he said. My eyelids drooped. I was dangerously close to falling asleep. But the prof had more to say: “Crystals just like it were used by the Vikings in navigating the northern ocean.” That got my attention. My drooping lids snapped open, and I had no trouble staying awake for rest of the hour. Here’s what I learned: Iceland spar is strange stuff. It polarizes light. Thanks to this quirk of nature, you can use a piece of it to find the sun’s position in the sky even on a cloudy day. And if you’re navigating by the sun in latitudes where clear skies are rare—as the Norsemen often did—that’s a very big deal, indeed. To geologists, Iceland spar is simply a transparent form of the mineral calcite, but to the Norse it was magic. They called it the “sunstone.” It’s easy to see why.

Not everyone agrees, however. My professor knew a good story when he came across one, but scholars are still arguing whether or not the sunstone was used as an aide to navigation by the Northmen. There’s evidence in the sagas to suggest that it was, but the sagas are…well…sagas, wonderfully embellished accounts written down long after the fact. Still, my prof got one thing exactly right. It’s a great story. And it might even be true. In 2005, studies aboard an arctic research vessel lent some weight to the sunstone theory. If you’re curious, you’ll find the details in the Proceedings of the Royal Society.

 

 
Luckily, backcountry photographers don’t need to take sides in this academic dispute. We don’t even have to understand the phenomenon of polarization. We just need to know how and when to use it. Read more…

 

 
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