NavList:

Ed Popko, you wrote:
"For instance, there is strong evidence that the Vikings navigated on cloudy days by using sunstone, also called Iceland Spar. It’s a type of transparent calcite, that exhibits light-polarizing properties that can tell the direction of the sun within a few degrees in both cloudy and twilight conditions."

I have to do it. I have to yell it out: beetledung! beetledung! beetledung! :)
Ed, the vast majority of the people with expertise in navigation that I know who have looked into this matter seriously, and myself also, do not agree at all that there is "strong evidence" that the Vikings navigated using polarized light. The evidence is meager. This whole thing stinks of being a just so story where a neat idea has been retrofitted to history because "it would be so cool if the Vikings used this". But evidence? Not much.

You added:
"The Academy of Science paper in the reference given suggests the Dung Beetles are doing the same thing with light polarization."

Many animals can detect polarized light. When the Sun is low in the sky, the sky shows significant polarization. It's real, and obviously animals that can see polarized light can see this polarization. It's not magic.

You concluded: 
"The 1992 recovery of an Iceland Spar sunstone from the Elizabethan ship Alderney (1592) suggests that Iceland Spar and light polarization were regular navigation practice among the Vikings."

(Beetledung)·10²³!
And this time, I'm not gonna stick a smiley icon after it. When this story broke, I just couldn't believe anyone would find it even remotely convincing. This is pseudo-history. The idea that a vessel sunk in 1592 in the English channel can tell us something about Norse/Viking navigation practices is preposterous. I propose a simpler alternative theory: the lump of calcite found in the wreck was a paperweight. This falls under the "olde times is olde times" fallacy. So what if this wreck is from the end of the 16th century, and the Vikings supposedly using the "sun stone" were sailing 500 years earlier. It's all really old, right?? And so what if it the wreck is in the English Channel? France and Iceland... it's all Europe, isn't it?? Heck, wasn't Normandy named for the Norse? ...yikes.

This was not the Dark Ages. One of the great books of navigation history was first published in 1599, Edward Wright's Certaine Errors in Navigation, and there was much more happening in navigational science on the European continent. Anything as amazing as a compass that worked by light would have been widely described in this period unless we are to believe that it was kept secret as black magic. Even if we buy that, where did the "sunstone" go? The whole tale makes no sense. It's all a tall tale...

By the way, I do teach polarization of light as an actual navigation tool in some of my classes. I hand out polarizing slides, and I show students that they can find the direction to the Sun by observing the band of polarization cutting across the sky. But nearly all point out independently that they can see the Sun anyway, so what good is this? And what use is this without tables of the Sun's azimuth? The polarization of the sky by the Sun's light does yield some navigationally useful information, but it's a rare day when it would have practical use.

Frank Reed
Conanicut Island USA