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Comment by rexreed | Hacker News Reader

Exactly how would this work when the sun has set?

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.

See: http://www.livescience.com/16831-viking-sunstone-crystal-com...

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.

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!).

If you find the crystal separately, there's nothing left to print. The tool is fully functional as nothing more than a lump of crystal.

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.

OK, have a look at the Bris 'sextant'.

http://en.wikipedia.org/wiki/Bris_sextant

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.

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.

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.)

The key here is that it is within a few degrees, which make a huge difference over long distances on the sea.

Did you know that in summer toward the Arctic Circle the Sun sets but it does not get dark? It is about as light as a cloudy day, all night long.

I'm assuming they mean during twilight (i.e. after the sun has set but before it has ceased to illuminate the local atmosphere).

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