Comment on ‘ A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight ’

A main conclusion of this work by Robars et al. is that a piece of Iceland spar (optical calcite) can be used in a simple manner to find the direction to the Sun using polarized light, even when the Sun is obscured by clouds or just below the horizon, which supports the possible role of the renowned sunstones discussed in Viking Sagas. They propose a method of using an opaque spot on the incoming face of the crystal as a means of detecting the proper orientation of the crystal when aligned with the azimuth of the Sun. They also point out that a recent discovery of a calcite crystal in an Elizabethan shipwreck near Alderney in the Channel Islands might have been used for navigation in a similar manner. I first express my appreciation that this line of research and analysis has been brought to the attention of the scientific community and, indeed, to the public at large, as their article received much attention in the popular media, worldwide, as would most news related to Vikings. I wish to point out that the authors may have overlooked the significance of the pioneering work on this subject by the late Leif Karlsen. Although Karlsen worked on this subject extensively for the last 20 years of his life, and his work was well known among those specializing in Viking navigation or special uses of crystals, his work was not published in standard scientific journals. His main report of this work is in his book Secrets of the Viking navigators—How the Vikings used their amazing sunstones and other techniques to cross the open ocean (Karlsen 2003) and later in the Navigators Newsletter, quarterly publication of the Navigation Foundation (Karlsen 2006). It was also reported in my book Emergency navigation (Burch 2008). Karlsen’s book is in an unusual format, which could account for the oversight. In part 1, Karlsen presents a fictional account (on the basis of his extensive study of the Sagas) of a typical Viking voyage, including a narrative on navigation