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Sunstone
Continuing my exploration of ancient navigation techniques, I picked up a copy of Secrets of the Viking Navigators the other day. It's not the greatest book in the world (hint: applying photoshop filters to real pictures does not make them look quaint, it just reduces their illustrative value), but it's okay.
One thing that really intrigued me was the talk about viking sunstones. They're mentioned as treasures in certain sagas, but never explained. In 1967 the Danish archaeologist Thorkild Ramskou theorized that they could have been using natural crystals available to them to detect the polarization of the sky. The atmosphere polarizes light passing through it, the more so the greater the angle from the sun. Even if the sun itself is hidden (behind a fog bank, for instance) you can determine its azimuth if you can see any blue sky above you.
Conveniently, the vikings did have a crystal available to them that has polarizing properties: Iceland spar, aka calcite. Thus, claimed this book, that must have been what the sunstones were. Given the slightly dubious vibes I was getting from the book, I figured I would have to try it out myself. One quick ebay transaction later...
A nice little rhombohedral crystal of calcite. The dot on the top will be explained in a bit.
What makes calcite so interesting is that it refracts incoming light into two beams of opposite polarization. High quality calcite has been in demand for use in optics for hundreds of years -- the same veins that might have supplied the vikings were used during WWII for bomber sights.
This double refraction is where the dot on the top comes in. The idea is, you hold the crystal up to blue sky and look through it. You'll see two dots. As you twist the crystal back and forth, the polarized light from the sky will get filtered differently. If you twist it so the darkness of the two dots are equal, you've aligned the crystal with the polarization of the sky, which means you've aligned the crystal along a line pointing at the sun. Cute theory, but would it be practical?
Yes, I think so. The effect is clear, could be done from the pitching deck of a knarr, and it's not hard to imagine this property being discovered. It isn't a particularly powerful navigation tool -- it only works if you can see blue sky, it's only accurate to a few degrees, it works best when the sun is near the horizon, which wouldn't happen very much in the summer at high latitudes. But it works well enough I could certainly see some grizzled navigator keeping one in a pouch, just in case. Knowing where the sun sets or rises is enough to tell you which way north is.
That said, this is where we run up against the limits of experimental archeology. Just because something could have been done, doesn't mean it was. There is no direct archaeological evidence supporting the idea. For now it will have to remain simply a fun idea.
One thing that really intrigued me was the talk about viking sunstones. They're mentioned as treasures in certain sagas, but never explained. In 1967 the Danish archaeologist Thorkild Ramskou theorized that they could have been using natural crystals available to them to detect the polarization of the sky. The atmosphere polarizes light passing through it, the more so the greater the angle from the sun. Even if the sun itself is hidden (behind a fog bank, for instance) you can determine its azimuth if you can see any blue sky above you.
Conveniently, the vikings did have a crystal available to them that has polarizing properties: Iceland spar, aka calcite. Thus, claimed this book, that must have been what the sunstones were. Given the slightly dubious vibes I was getting from the book, I figured I would have to try it out myself. One quick ebay transaction later...
A nice little rhombohedral crystal of calcite. The dot on the top will be explained in a bit.
What makes calcite so interesting is that it refracts incoming light into two beams of opposite polarization. High quality calcite has been in demand for use in optics for hundreds of years -- the same veins that might have supplied the vikings were used during WWII for bomber sights.
This double refraction is where the dot on the top comes in. The idea is, you hold the crystal up to blue sky and look through it. You'll see two dots. As you twist the crystal back and forth, the polarized light from the sky will get filtered differently. If you twist it so the darkness of the two dots are equal, you've aligned the crystal with the polarization of the sky, which means you've aligned the crystal along a line pointing at the sun. Cute theory, but would it be practical?
Yes, I think so. The effect is clear, could be done from the pitching deck of a knarr, and it's not hard to imagine this property being discovered. It isn't a particularly powerful navigation tool -- it only works if you can see blue sky, it's only accurate to a few degrees, it works best when the sun is near the horizon, which wouldn't happen very much in the summer at high latitudes. But it works well enough I could certainly see some grizzled navigator keeping one in a pouch, just in case. Knowing where the sun sets or rises is enough to tell you which way north is.
That said, this is where we run up against the limits of experimental archeology. Just because something could have been done, doesn't mean it was. There is no direct archaeological evidence supporting the idea. For now it will have to remain simply a fun idea.
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