That's cool, where can you get one? And how do they know the Vikings used one? When the sun reaches its highest point in the sky, local noon, it is due south exactly, I believe. Pretty helpful.
This is an interesting question - one that is easily answered.
Ever wonder how sightings were taken at sea when there was a heavy overcast or a storm and the sun was NOT visible?
At High Noon, the sun will be at its highest point and no matter how bad the cloud cover is, the sun will ALWAYS be visible at the noon hour, briefly perhaps, slightly dimmed by clouds, but still with enough resolution to make a sighting.
Ask a sailor.
Forget the Mystic Crystal Revelations..