Before the invention of modern navigational equipment, ancient sailors crossed the seas largely through their knowledge of the positions of the Sun and stars. So how did Vikings cross from Europe to America in the often foggy, cloudy northern Atlantic?
There's no real way that Vikings could have crossed such tremendous distances - they were probably second only to the Polynesians as far as ancient navigators go - without being able to consistently rely on knowing where the Sun and stars were. Viking legends speak of a "sunstone" that allowed their sailors to figure out celestial positions even in deep fog. However, archaeologists have long struggled to find proof of any such device...until now.
Researchers at the University of Rennes in France have put together all the experimental and theoretical evidence they could muster, and they're pretty sure they know exactly what this sunstone was. It was, in fact, a transparent calcite crystal known as Iceland spar. It's found all over its namesake country, and Vikings could have used it to depolarize light, which means the crystal is able to split light along different axes.
How is that useful for navigation? Well, Viking sailors simply had to place a dot on the top of the crystal and then look up at it from below. The incoming light would hit the dot and seemingly duplicate it. That optical effect, amazingly enough, was all ancient navigators needed to locate the Sun, even when it was completely hidden from view.
Lead researcher Guy Ropars explains:
"Then you rotate the crystal until the two points have exactly the same intensity or darkness. At that angle, the upward-facing surface indicates the direction of the Sun. A precision of a few degrees can be reached even under dark twilight conditions...Vikings would have been able to determine with precision the direction of the hidden Sun."
The human eye is well-suited to using this compass, since it can make extreme fine distinctions about the relative brightness of the two dots. The fact that the crystal was found all over Iceland makes it likely that the Vikings did indeed use the crystal, and there's also some proof that later navigators made use of it as well. A British shipwreck dating back to the reign of Queen Elizabeth I had an Iceland spar on board.
The researchers suspect that, in the early days of compasses, sailors still found the crystals more trustworthy than the new invention...and with good reason:
"We have verified ... that even only one of the cannons excavated from the ship is able to perturb a magnetic compass orientation by 90 degrees. So, to avoid navigation errors when the Sun is hidden, the use of an optical compass could be crucial even at this epoch, more than four centuries after the Viking time."