Blinded by the Viking Sunstones

A Viking Ship. Not the one you might expect, but wait and see.
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So, there’s these sun­stones that some people think Vikings could have used to nav­ig­ate to America. It’s pos­sible though the evid­ence is weak.

A Viking Ship. Not the one you might expect, but wait and see. Photo by Eoghan O’Lionnain

A few months back there was a paper where the phys­ics was sound but the his­tor­ical con­text was lack­ing. Today the news is a new paper, A depol­ar­izer as a pos­sible pre­cise sun­stone for Viking nav­ig­a­tion by polar­ized sky­light. My prob­lem with the earlier paper was that while the phys­ics made sense, there was no real attempt at his­tor­ical con­text. This paper is different.

The test crys­tal from Ropars et al. 2011.

The Alderney Stone, from Ropars et al. 2011.

A depol­ar­izer as a pos­sible pre­cise sun­stone for Viking nav­ig­a­tion by polar­ized sky­light is not a paper about Viking nav­ig­a­tion at all. It’s about tests on a sun­stone found on an Elizabethan ship by Alderney. Update 15 Nov 2001. It’s about tests on a piece of Icelandic spar that the authors have used as a sub­sti­tute for the Alderney stone. The paper doesn’t describe the meth­ods used to ensure the sub­sti­tute was a good proxy for the ori­ginal. I’ve included the images of the ori­ginal and the test stone. I can see some super­fi­cial dif­fer­ences and more dis­cus­sion of how the test stone was pre­pared could have been help­ful. The phys­ics is lovely and makes sense, but this paper on Tudor nav­ig­a­tion doesn’t cite any research on Tudor navigation.

The argu­ment is this:

  1. If you place some­thing with a small hole in front of the Alderney sun­stone two areas of light appear.
  2. By get­ting the areas to the same bright­ness you can work out where the sun is.
  3. That might have been use­ful in Elizabethan times because can­nons can deflect mag­netic compasses.
  4. But we’ve not checked any his­tor­ical records to see how Tudor sail­ors coped with that, nor if our made-up hole thing has any his­tor­ical evid­ence for it
  5. Because sun­stones means Vikings! VIKINGS I TELL YOU!

Now, if you’re inter­ested in the optics of cal­cite, this is a good paper — but why would you be inter­ested in the optics of cal­cite? The only obvi­ous reason I can think of is his­tor­ical. And a paper that tackles a his­tor­ical prob­lem by pretty much ignor­ing the his­tor­ical period your arte­fact comes from seems to me to be eccentric.

Anyway, if you were sail­ing in north­ern lat­it­udes and you couldn’t see the sun due to mist, but the light was bright enough for polar­isa­tion to be detect­able, then you could use this device to loc­ate the dir­ec­tion of the sun. The sun­stones would have to be bet­ter polar­isers than the fil­ters I use for my cam­era, because I can’t detect any notice­able polar­isa­tion in the over­cast sky today. Once you have a dir­ec­tion, with no time or alti­tude for the obser­va­tion, what are you going to do with that?

The cov­er­age I’ve seen at the Guardian and at the BBC, is cred­ited to two good sci­ence journ­al­ists, yet neither has con­tac­ted a Tudor or Viking his­tor­ian for their opin­ion. This baffles me.

Update 3rd Nov 2011: Wired / ScienceNow do report that no sun­stone has been found with a Viking ship­wreck or set­tle­ment. They also have an inde­pend­ent expert com­ment­ing on the pos­sib­il­it­ies of Tudor nav­ig­a­tion. Unfortunately it’s a bio­lo­gist on the dif­fi­culty of sight­ing from a Viking ship.

Is there some­thing clever about the paper I’ve overlooked?

Photo: Viking Line by Eoghan O’Lionnain. Licenced under a Creative Commons BY-SA licence.