Iceland Spar: the Mysterious Sunstone?
What might this crystal have been? The discovery of a crystal of a mineral called Iceland spar (a form of calcium carbonate) on a sunken Elizabethan ship suggests that this might be the mysterious sunstone. The key feature which makes Iceland spar stand out is its property of birefringence, or double refraction: this means that light passing through the crystal is split into two rays, one of which travels more slowly than the other. (You can see the effect by placing such a crystal over a single line, which will appear double.)
The combination of birefringence and polarisation of light allows, in theory at least, navigators to determine the location of the sun by rotating the crystal until the two images which it projects are of equal strength. Recently, scientists Le Floch et al have demonstrated, both theoretically and experimentally, that such successfully navigation can be achieved.
Karlsen’s Navigation Notes include a detailed description of how such a sunstone might have been used by marking a dot on one side of the crystal and holding it up to the sky. Owing to the birefringence, two dots will appear: rotating the crystal until the dots are of equal intensity gives the direction of the sun – and researchers indicate the human eye is capable of measuring this similarity to a great degree.
Of course, such successful navigation relies not only upon the accuracy of the methodology but also skills and understanding of the navigators. It’s clear, however, that the Vikings must have possessed some technique which allowed them to cross the Atlantic not once (which might have been luck) but repeatedly. And if there was a mysterious sunstone, Iceland spar certainly seems to fit the bill.
Ferreirá, Barbara Iceland spar, or how Vikings used sunstones to navigate. (2013). European Geoscience Union. Accessed March 31, 2013.
Le Floch, A., Ropars, G., Lucas, J., Wright, S., Davenport, T., Corfield, M. and Harrisson, M. The sixteenth century Alderney crystal: a calcite as an efficient reference optical compass? (2013). Proceedings of the Royal Society. Accessed March 31, 2013.
Karlsen, Leif K. Navigation Notes. One Earth Press. Accessed March 31, 2013.
Marchant, J Did Vikings navigate by polarized light? (2011). Nature. Accessed March 31, 2013.
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