"In particular, at twilight when the sun is no longer observable being below the horizon, and the stars still not observable, this optical device could provide the mariners with an absolute reference in such situation," the researchers wrote online this week in the journal Proceedings of the Royal Society A.
No such crystals have been found yet at Viking sites. The team notes that archaeologists are unlikely to find complete crystals as part of a group of grave goods, since the Vikings often cremated their dead.
But recent excavations turned up the first calcite fragment at a Viking settlement, "proving some people in the Viking Age were employing Iceland spar crystals," the researchers wrote.
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