The Vikings used a so-called sunstone to magically determine the position of the sun, even on cloudy days or when it was below the horizon line. But a new study says that sunstone wasn't so much magic as real, reports the Telegraph. The stone was a kind of transparent, calcite crystal—called an Icelandic spar—and when held to the sky, it could polarize light, allowing the user to know the position of the sun within a few degrees.
The sunstone, which has been found in one shipwreck that dates back to 1492, would have been very useful before Europeans started using magnetic compasses in the 12th century. This is especially true for northern Vikings, whose oceanic routes were often full of fog, and whose short winter days meant the sun was often below the horizon. "It looks very promising to find Iceland spars in other ancient shipwrecks," says the study.