Norse sailors found the sun's bearing from sky polarization

Viking sailors crossing open ocean to reach Iceland, Greenland and eventually North America had no compass and depended on the sun's position to hold a bearing, but the North Atlantic is reliably overcast, fogged in or snowing for long stretches — exactly the conditions in which the sun's disk becomes invisible and dead reckoning alone risks running a ship far off course on a multi-day open-water crossing.

No instrument existed for generating a bearing when the sun couldn't be seen directly; the apparent options were waiting for a break in the clouds or navigating by less reliable cues like wave patterns, bird flight and distant landmarks, none of which worked reliably mid-ocean, far from any coast.

Norse sagas, including an episode involving King Olaf Haraldsson and the navigator Sigurd, describe a 'sunstone' (sólarsteinn) used to find the sun's position on a heavily overcast day. Modern researchers propose this was a transparent calcite crystal known as Iceland spar, which polarizes light passing through it.

Read the sky's polarization pattern through a calcite crystal, which survives cloud cover and reveals the sun's true position to within a degree.

Atmospheric scattering preserves a polarization pattern across the sky even when the sun's disk is invisible.

A calcite crystal makes that invisible polarization readable by balancing a doubled image's intensity.

The polarization pattern correlates with the sun's true position, so the bearing can be derived accurately.

This proxy signal works under the same overcast conditions that block the direct signal, enabling navigation.

When the direct signal is blocked, look for a correlated property that survives the obstruction and build a tool to read it.

A 2011 study led by Guy Ropars, published in Proceedings of the Royal Society A, demonstrated the method could fix the sun's true bearing to within about one degree of accuracy, with a precision of a few degrees achievable even under dark twilight conditions; other researchers found the sky's polarization pattern remained detectable even under total overcast in Arctic Ocean conditions.