Vikings used 'sunstones' as optical compasses to navigate the foggy North Atlantic.

The legend of the sunstone appears in medieval Icelandic sagas, such as the 'Rauðúlfs þáttr,' which describes King Olaf using a stone to find the sun in a snowy sky. For decades, historians debated if these stones were real until a 2011 study published in the journal 'Philosophical Transactions of the Royal Society B' demonstrated their effectiveness. Researchers found that crystals like calcite, cordierite, or tourmaline can detect the polarization pattern of sunlight, which remains visible even when the sun is below the horizon or hidden by clouds.In 2013, a significant archaeological breakthrough occurred when a calcite crystal was discovered in the Alderney shipwreck, an Elizabethan vessel from 1592. Although the ship sank centuries after the Viking Age, the presence of the crystal alongside navigational tools suggests that 'optical compasses' remained a backup technology even after the introduction of magnetic compasses. The crystal, known as Iceland spar, works through double refraction, splitting incoming light into two separate beams.By observing the relative brightness of these two beams, a navigator can determine the sun's position within a margin of error of just one degree. This precision was vital for the Vikings, who navigated the 'White Zone' of the North Atlantic where magnetic interference and heavy fog made traditional celestial navigation nearly impossible. This discovery provides a scientific basis for how Norse explorers successfully crossed thousands of miles of open ocean to reach Newfoundland, Canada, around 1000 AD.