Viking sat nav
Stories from Physics
for 11-14
14-16
Historians have speculated about how Viking seafarers were able to navigate in cloudy conditions that obscured the Sun and stars. One theory put forward is that, like some insects, they determined the direction of the Sun using the polarisation of the sky. Viking sagas refer to a ‘sunstone’, which is conjectured to have been cordierite or tourmaline. When crystals of these stones are held up to the sky and rotated, the sky appears to brighten and darken, except at polarisation neutral points (see above) which can be used to determine the direction of the Sun. However, a study has concluded that the degree of polarisation of overcast skies is so low that it is unlikely that Viking sailors would have been able to use the effect.
References
G. Horváth, A. Barta, I. Pomozi, B. Suhai, R. Hegedüs, S. Åkesson, … & R. Wehner, On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers. Philosophical Transactions of the Royal Society of London B: Biological Sciences, vol. 366, no. 1565, 2011, pp. 772-782.
J. D. Pye, Polarised Light in Science and Nature, Bristol, Institute of Physics Publishing, 2001, pp. 67-68.