Vikings, they argue, used transparent calcite crystal - also known as Iceland spar - to fix the true bearing of the Sun to within a single degree of accuracy.The article also explained the basic process:
According to the new study, the Vikings would have relied upon the sun's piercing rays being reflected through a piece of the calcite crystal.
Researchers found that if you put a dot on top of the crystal and look through it from below, two dots will appear due to the double refraction of the Iceland spar.
Using this, they could pinpoint the sun by rotating the crystal until both sides of the double image are of equal intensity.
At that angle, the upward-facing surface indicates the direction of the sun, according to the scientists led by Dr Guy Ropars.