A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Wolfgang Köberer
Date: 2018 Apr 9, 00:12 -0700
This is one of the zombie ideas that keep popping up from time to time even after they have been killed.
There are 2 questions to be asked in that respect:
1 - Can the object be used in the way the theory posits?
2 - Is there evidence that it was used in this way in navigation?
Apparently the answer to the first question is yes - these researchers have put a lot of effort into finding this out.
The answer to the second question is no; and there is no way to use the first answer to get a positive answer to the second question. I might as well presume that the Vikings used sextants to navigate by Polaris. Obviously that is possible, but that does not mean that they really did that - same for instance with the compass.
But there is ample evidence in the sagas that they got lost (this state called 'hafvilla'). And not a single sun stone has been found so far (and to posit the explanation that this may be due to Viking burial mounds being robbed of them introduces another point which must be proved before we can use it in the discussion - greetings from Occam).
The sagas do not contain a single mention of sun stones used at sea; the one instance where such a thing is mentioned happens on land - a fact that the sun stone enthusiasts don't mention even if they don't falsify the wording. If you want to know more about it read Schnall, Navigation der Wikinger, still the best treatment of this question (in German, alas!).
Finally: there is recent research on the reliabilty of dead reckoning in these waters without instruments which to me seems to show that the Vikings were able to make the crossings. Let me know if you are interested in it.