Magical Viking stone may be real

Cool little bit of optics here. Scattered light gets partially polarized, depending on the angle, and will be maximally polarized at 90º from the source. Of course, light can scatter more than once, so this will not result in perfect polarization from the atmosphere.

The researchers said such sunstones could have helped the Vikings in their navigation from Norway to America before the discovery of the magnetic compass in Europe.
They would have relied upon the sun’s piercing rays reflected through a piece of the calcite. The trick is that light coming from 90 degrees opposite the sun will be polarised so even when the sun is below the horizon it is possible to tell where it is.
They used the double refraction of calcite to pinpoint the sun by rotating the crystals until both sides of the double image are of equal intensity.

Here’s a view of the sky at roughly 90º from the sun, looking through a linear polarizer at two orientations — one that blocks the most amount of light and one the blocks the least. You can tell the light is definitely polarized.