How Viking sunstones revealed the sun

Viking sunstones were birefringent or dichroic crystals like Iceland spar that revealed the sun’s position by detecting light polarisation in the atmosphere. By rotating the stone to match the intensity of its double images or observe specific light patterns, navigators could pinpoint the sun's exact horizontal direction. This tool allowed for accurate navigation during heavy cloud cover, fog, or twilight when the sun was otherwise invisible to the naked eye