Blinded by the Viking Sunstones

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.


When he's not tired, ill or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.

6 Responses

  1. Thank you for being a voice of reason in a sea of bullshit.

  2. Like you said the first time round, “just because it could have happened doesn’t mean it did.” I keep try­ing to per­suade some engin­eer­ing friends that, while the Alexandrian Greeks inven­ted a steam engine device, they didn’t use it for min­ing or other indus­trial applic­a­tions. They simply don’t believe me.

  3. The use of a stone to find the sun is doc­u­mented in the sagas — three examples are lis­ted here: http://​www​.nord​skip​.com/​v​s​a​g​a​s​.​h​tml

    There is already plenty of evid­ence that the Vikings (or more prop­erly, Norse sea­farers) ten­ded to sail along lines of lat­it­ude, which required them being able to meas­ure lat­it­ude to stay on course. The could do this at night off the Pole Star, and dur­ing the day by sight­ing off the sun at peak height (noon). On an over­cast day, they would need the sun­stone to loc­ate the sun. And they would determ­ine noon simply by whenever the sun was highest over the horizon.

    Basically, min­eral and optical sci­ence has finally found a set of min­er­als that had the prop­er­ties required, and were sourced in Norse areas.

  4. Alun says:

    It’s a sens­ible com­ment Erich. I can see the argu­ment about whether such a stone phys­ic­ally exists, but Horvath et al. have done this at the start of the year and it’s been done many times before that. Horvath et al. did at least make ref­er­ence to the sagas in their paper, which doesn’t hap­pen in this one. In fact you’ve made a much more com­pre­hens­ive his­tor­ical argu­ment for sun­stones in your com­ment. Given they had a whole paper which was ostens­ibly about nav­ig­a­tional prac­tice in the past (Viking nav­ig­a­tion is the first keyword) I think that’s a problem.

    It’s doubly wor­ry­ing because there is noth­ing Viking in the paper. Nor is there any real dis­cus­sion of any his­tor­ical con­text. They make no case why a Tudor arte­fact is con­nec­ted to Viking navigation.

    For com­par­ison, if I presen­ted evid­ence that Walter Raleigh vis­ited North America, would that be evid­ence for Vikings get­ting there earlier? In a way it shows the ocean was cross­able, but I can’t see any his­tor­ian tak­ing that ser­i­ously. What con­vinced his­tor­i­ans that Norse set­tlers had got to the new world was find­ing the L’Anse aux Meadows site.

    I’ve read through the paper again this morn­ing and found I made an error read­ing it. I thought the tests were done on the Alderney stone. In fact they were done on a new crys­tal. The valid­ity of the optical tests depends on how close the mod­ern pre­par­a­tion is to the his­tor­ical arte­fact, so I’ll add a cor­rec­tion after post­ing this comment.

  5. Alun says:

    That’s a good spot. Yes, it’s the same paper. It gets pub­lished in dead tree form on the 8th of March 2012.