http://news.yahoo.com/magic-viking-sunstone-just-natural-crystal-004406011.html

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Before the invention of the magnetic compass, navigating with a sundial would have been difficult, particularly on overcast days. Ancient Viking lore suggests that they had a magical tool to find the sun, even when the star was hidden.

Researchers have now discovered the crystal that would have made such a magical apparatus possible. The Vikings could have used a common calcite crystal, called an Icelandic spar, to find the sun in the high latitudes where they would have had to battle long twilights and cloudy skies to navigate. This special "sunstone" could find the direction of the sun even when it was out of view because it plays a trick with the light.

To use the crystal, the Vikings would have held the stone up to the center of the sky (from their perspective). When sunlight hits the crystal, that light gets polarized and broken into an "ordinary" and an "extraordinary" beam.

On a clear day, the Vikings would have rotated the crystal until the two beams lined up. Since these two beams line up and have the same brightness at only one angle, by noting where the sun is when this happens the Vikings could establish a reference point that could be used even when the sun wasn't visible.

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Me, I am just want to learn how to celestial navigate... but what the heck, probably wouldn't hurt to carry a calcite crystal.