How the Vikings Might Have Used Iceland Spar Crystals for Navigation

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This is an example of an Iceland spar (calcite) crystal, the mythical ‘sunstone’  Vikings used for navigation. Image credit: Parent Gery

Natural navigation: ah, that’s easy isn’t it? Moss on the north side of trees (or the south in the southern hemisphere)… finding the poles via the sun… using the stars at night. That’s all fine and dandy – up to a point. But on a foggy day in the North Atlantic, neither sun nor stars nor trees are going to be a lot of use.

Yet somehow, in the days before compasses, the Vikings successfully navigated their way across north of this tempestuous and often foggy ocean to reach Iceland, Greenland and North America. Exactly how they did so has been a matter of debate, not to mention controversy, for years, with the Viking sagas referring to a mysterious and possibly magical crystal called a sunstone. The discovery of an Elizabethan shipwreck with just such a crystal on board has raised the possibility that such a sunstone may have existed – and that it could have been the key to the Vikings’ successful navigation.

Optical Mineralogy: Polarisation and Birefringence

Minerals are complex things, their properties differing according to the structure of the molecules within them. It’s obvious to anyone who’s ever stared in awe at a display of jewellery that some of these minerals are perfectly clear – in itself a reflection of the molecular alignment. But the optical properties of minerals go far beyond the clarity of a diamond.

In terms of navigation, it’s important to understand something of the properties of light. Generally speaking, as light travels, it emits energy at right angles to the direction of travel. As it travels through the atmosphere, however, some of that energy is absorbed and the light becomes polarised. In other words, it has a strong focus in a single direction – in this case, in a particular direction relative to the sun.

How did this help the Vikings on a cloudy day? Well, as modern-day navigator Leif Karlsen puts it in Navigation Notes: “The earth’s atmosphere serves as a polarizer. If the sunstone is placed horizontal when the sun is on or near the horizon, it … provides a directional reference during twilight, or when the sun is near the horizon in a fog bank, or behind an island.” So, in certain circumstances, with the right crystal and a degree of know-how, the smart sailor can find his (or her) way without the sun.

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