NavList:

The new History Channel series "The Vikings" has this in the first episode. The hero to be, who wants to raid to the West for the first time, has a round piece of wood with a peg sticking up about 2". He explains to the friend who was worried about how to keep sailing West:
"You float the wood in water. Before you leave home you observe the shadow of the sun at noon and draw a circle around the peg matching the tip of the shadow. You sail west and, every day near noon,you observe the shadow. If it is outside of the circle, you are too far north. If it is inside the shadow,you are too far south. Match the shadow to the circle and you are sailing west."

Worried Friend: "What if you can't see the sun?"

"Ah, this is what an ??? gave me." The hero holds up a translucent crystal, moves it around on the cloudy day, and suddenly it casts a bright beam on his face. "This shows you the sun when it's cloudy." (Music swells)

(Lots of gaps and what-ifs that come to mind.)

Our hero makes it because the ads and trailers are already showing him raiding Christian churches.

Remember from the paper referenced on this subject " Transparent cleavage rhombs of calcite" and you can casually know all about the subject.
Regards, Noell
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