Vikings from Scandinavia were traders, explored new land and settled in areas far from their homeland in Northern Europe. They inhabited Iceland and southern Greenland and they even discovered North Americas long before Columbus set his foot on the land located so far away in the west. From spring to autumn the Vikings were engaging in trading trips and routinely crossed the North Atlantic in their oak ships without guidance from a magnetic compass. Viking settlements were raised as far west and north-west as south Greenland, Newfoundland and Bafﬁn Island. To reach these distant locations from Scandinavia and more closely from Iceland, it is widely accepted that they sailed along chosen latitudes using primitive Sun compasses. An eleventh-century dial fragment artefact, found at Uunartoq in Greenland has been suggested to be a fragment of a navigation instrument used by the early settlers. Instruments of this type has been tested on sea crossings and proved to be efﬁcient hand-held navigation tools, but the dimensions and incisions of the Uunartoq ﬁnd are far from optimal in this role. On the basis of the sagas mentioning sunstones, incompatible hypotheses were formed for Viking solar navigation procedures and primitive skylight polarimetry with dichroic or birefringent crystals.
|The Uunartoq artefact from Greenland. Source: Sören Thirslund.|
|The Uunatoq artefact is suggested by us to be used as a combined Sun compass and horizon board (a "twilight board") and may have been used around the clock at northern latitudes (from Bernáth et al. 2014, Proc R Soc A).|
In a recent study published in Proc R Soc A (Bernáth et al. 2014), we describe a previously unconceived method of navigation based on the Uunartoq artefact functioning as a ‘twilight board’, which is a combination of a horizon board and a Sun compass optimized for use when the Sun is close to the horizon. We deduced an appropriate solar navigation procedure using a twilight board, a shadow-stick and birefringent crystals, which bring together earlier suggested methods in harmony and provide a true skylight compass function. This could have allowed Vikings to navigate around the clock, to use the artefact dial as a Sun compass during long parts of the day and to use skylight polarization patterns in the twilight period. In ﬁeld tests, we found that true north could be appointed with such a medieval skylight compass with an error of about ±4◦ when the artiﬁcially occluded Sun had elevation angles between +10◦ and −8◦ relative to the horizon. Our interpretation allows us to assign exact dates to the gnomonic lines on the artefact and outlines the schedule of the merchant ships that sustained the Viking colony in Greenland a millennium ago.