The BBC article mentioned refraction, but Iceland Spar makes it possible to see the direction of polarization of skylight, which means it could be used on small, clear patches of sky on an otherwise overcast day to find the direction to the sun. This is another way it could act as a "sunstone" for navigation. Whether it actually was used this way is unfortunately not known.
I do not think that this paper proves anything about Viking navigation.
While the researchers seem to have been able to prove that the found crystal is Iceland spar, it was recovered from a Elizabethan shipwreck in the English channel from 1592, centuries after the magnetic compass started being used in Europe. While other articles on the topic claim that they suggest that sunstones would have been destroyed in Viking burials, I think that the lack of similar crystals from known Viking ruins is troublesome.
Fair concerns. An interesting and not entirely unrelated tangent is that of the nearby Basques. In Salt: A World History, Mark Kurlansky presents a theory that the Basques preserved their ancient and linguistically disconnected culture by bartering salt-preserved fish gained from the vast stocks on the Atlantic coast as protein stocks to neighbouring peoples during the European winters. Evidence of such tools may lend some further support to this theory.
(Edit: I just noticed he has also written The Basque History of the World: The Story of a Nation... that's definitely on my list now!)
Even if you concede that the basques did have an early start on exploiting the Newfoundland fisheries, that would have given them an edge for a few centuries after the year 1000 (more like 1400 if you wait for the necessary navigation technology). By the 3rd century AD, all of Iberia except for the Basque was speaking a romance language.
So, this theory -- very poorly documented in itself -- fails to explain at the very least these mere 7 to 10 centuries of preservation of Basque identity.
EDIT: by the way, I totally buy that Europeans were fishing for cod in Newfoundland before Columbus, and that at least some of those were basques. 15th century Portuguese maps calling those waters "Mare Baccalearum" are proof enough for me.
you can get Archimedes mirror working, only that (if I remember the math) it just has to be very big - so you'd just have to wonder if the ancients would've really gone through the effort of building it without knowing if it would really be that useful in real ship warfare. I'm sure someone tested a prototype with toy papyrus boats or something, but I doubt the large scale equivalent would have been build if it ended up more expensive than an actual warship. We have all sorts of "wonder weapons" prototypes in the military labs, but 90% of them will never end up being used in real warfare. I guess the same was with Archimedes, but it had the added coolness of using Sun's power to buuuuurn you enemies ;)
In the book 'Emergency Navigation' by David Burch ( http://www.amazon.com/Emergency-Navigation-Improvised-No-Ins... ), the method is explained more clearly. With a clear Iceland spar crystal 1 to 2 inches on a side, the direction of the sun can be pinpointed to within a few degrees. You can also get the same effect using a piece of cellophane and the lens of a pair op polarized sunglasses. You do need a patch of clear sky at a 90 degrees angle from the sun direction to observe the change in intensity.
According to the Wikipedia article on the Iceland spar (http://en.wikipedia.org/wiki/Iceland_spar): "Further research in 2011 by Ropers et al., confirms that identifying the direction of the sun to within a few degrees in both cloudy and twilight conditions was possible using the sunstone and the naked eye." So it could probably work closely after sunset or before sunrise, but not at night.
Well, they probably sailed mostly on summer (because of weather) when, at northern latitudes, twilight conditions are persistent for most of the "night".
The sun below the horizon is still a source of diffuse light, the crystal is used to find the angle to a source of light and in northern latitudes with long twilights the sun produces a glow long after it has set.
Nice. First 3D-printed replica wins my vote as a teaching aid and generally cool retro hack. We need more of this offline tech stuff.
Combine a compass and other navigational tools, and I see an interesting evolution as a sort of retro, creative-commons, git-forkable hacker-army object.
Elegant device that enables you to get an accurate Sun altitude at specific times of day, and hence a position fix. No moving parts.
I think the 'hack' here is to think through the navigation requirements of someone sailing across an ocean. Do they need to be able to find a position at an arbitrary time (the proper sextant) or would the ability to find a position at some time during daylight be enough? The result is a device that is robust and that can be kept in a 35mm film cassette.
When you figure out how to 3d print a birefringent material, let me know. In fact, when you get a hobby-grade 3d printer to print something you can actually see through, let me know.
Sure. Obviously not everything comes from a machine. One can probably rely on people finding some crystal separately and/or if feasible order bulk-printed 3D transparent material (no idea of light properties of those, though I know they exist... just saw some this morning!).
My understanding from the image is that the operation is based upon the combination of two directional light beams through holes driven by an opaque piece with a rotating disc. Whether this was used at the time or not, it still seems a clear way to demonstrate the concepts.
At the very least the operation could perhaps be usefully modelled three dimensionally using raytracing, perhaps also with the capacity to vary basic optical properties of the transparent crystal feature such as reflectiveness, refractiveness and average opacity, as well as atmospheric conditions encountered in the seas ancient European seafaring peoples are known to have sailed.
I think the claim that this could be used at dusk and twilight is a little silly. Maybe it could have, but then again, figuring out where the sun just set, even under cloud cover, doesn't exactly require a "sunstone". (Hint: it's that glowy part of the sky.)
So it depends on polarizing the sun light, right? Could one build something like this from a typical polarizing camera filter? Would it need a linear or circular filter? I guess the former.
A little off topic, but Iceland Spar features prominently in Pynchon's "Against the Day"[1]. It is a pretty amazing book that might appeal to this audience.
