Proc. R. Soc. A (2012) 468, 671–684 doi:10.1098/rspa.2011.0369 Published online 2 November 2011 A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight BY GUY ROPARS 1, *, GABRIEL GORRE 1 ,ALBERT LE FLOCH 1,2 ,JAY ENOCH 3 AND VASUDEVAN LAKSHMINARAYANAN 4,5,6,7 1 Laboratoire de Physique des Lasers URU 435, Université de Rennes 1, 35042 Rennes cedex, France 2 Laboratoire de Chimie et Photonique Moléculaires, unité CNRS 6510, Université de Rennes 1, 35042 Rennes cedex, France 3 School of Optometry, University of California, Berkeley, CA 94720, USA 4 School of Optometry, 5 Department of Electrical and Computer Engineering, and 6 Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 7 Michigan Center for Theoretical Physics, University of Michigan, Ann Arbor, MI 48109, USA Viking navigation from Norway to America in the northern latitudes remains a mystery for physicists, historians and archaeologists. Polarimetric methods using absorbing dichroic crystals as polarizers to detect a hidden Sun direction using the polarized skylight have led to controversies. Indeed, these techniques may lack in sensitivity, especially when the degree of polarization is low. Here, we demonstrate theoretically and experimentally that using the transparent common Iceland spar as a depolarizer, the Vikings could have performed a precise navigation under different conditions. Indeed, when simply rotated, such a birefringent crystal can completely depolarize, at the so-called isotropy point, any partially polarized state of light, allowing us to guess the direction of the Sun. By equalizing the intensities of the ordinary and extraordinary beams at the isotropy point, we show that the Sun direction can be determined easily, thanks to a simple sensitive differential two-image observation. A precision of a few degrees could be reached even under dark crepuscular conditions. The exciting recent discovery of such an Iceland spar in the Alderney Elizabethan ship that sank two centuries before the introduction of the polarization of light in optics may support the use of the calcite crystal for navigation purposes. Keywords: Viking navigation; sky polarization; Iceland spar 1. Introduction Although the first observation of the polarization of the skylight was performed only in 1809 by Arago (Arago 1811), earlier uses of the phenomenon have been suggested and discussed, namely in the Viking navigation nearly 1000 years before (Ramskou 1967; Können 1985; Mills 1992). Following the Icelandic sagas, *Author for correspondence (guy.ropars@univ-rennes1.fr). Received 9 June 2011 Accepted 3 October 2011 This journal is © 2011 The Royal Society 671 Downloaded from https://royalsocietypublishing.org/ on 22 February 2022