On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers

Abstract

Between AD 900 and AD 1200 Vikings, being able to navigate skillfully across the open sea, were the dominant seafarers of the North Atlantic. When the Sun was shining, geographical north could be determined with a special sundial. However, how the Vikings could have navigated in cloudy or foggy situations, when the Sun's disc was unusable, is still not fully known. A hypothesis was formulated in 1967, which suggested that under foggy or cloudy conditions, Vikings might have been able to determine the azimuth direction of the Sun with the help of skylight polarization, just like some insects. This hypothesis has been widely accepted and is regularly cited by researchers, even though an experimental basis, so far, has not been forthcoming. According to this theory, the Vikings could have determined the direction of the skylight polarization with the help of an enigmatic birefringent crystal, functioning as a linearly polarizing filter. Such a crystal is referred to as 'sunstone' in one of the Viking's sagas, but its exact nature is unknown. Although accepted by many, the hypothesis of polarimetric navigation by Vikings also has numerous sceptics. In this paper, we summarize the results of our own celestial polarization measurements and psychophysical laboratory experiments, in which we studied the atmospheric optical prerequisites of possible sky-polarimetric navigation in Tunisia, Finland, Hungary and the high Arctic.

How could the Viking Sun compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment

“…(e) Estimation of the solar azimuth and elevation angles with sunstones Several atmospheric optical phenomena can be used to estimate the position of the occluded Sun, but the Vikings have been hypothesized to rely on the polarization pattern of the sky [1,2,4,5,[8][9][10][11]. Skylight is formed predominantly by first-order Rayleigh scattering of sunlight on atmospheric molecules and aerosols, and is partially linearly polarized (figure 6).…”

“…No authentic description of its nature and use is known, but the high value of a sunstone is indicated by its mentions in medieval treasure inventories. It is a widely accepted hypothesis that it was a birefringent crystal with which Vikings performed crude skylight polarimetry and located the occluded Sun to add sky polarimetric navigation to solar navigation (figure 1) [2,4,5]. The basic idea of this hypothesis was first introduced by Ramskou [4].…”

Orientation with a Viking sun-compass, a shadow-stick, and two calcite sunstones under various weather conditions

“…This pattern exists even under thick clouds, although the degree of polarization of skylight is very low in such situations [10][11][12][13]. By measuring the direction of polarization of skylight in two or more celestial points, one can mark out celestial great circles (perpendicular to the local direction of polarization), the intersection point of which provides a good estimation of the position of the occluded sun [8].…”

“…Theoretically, Vikings could use such a primitive skylight polarimetry to locate the occluded sun and use this information for navigation [6,8,15,16]. In modern astrophysics, comparison of the irradiances of ordinary and extraordinary beams in birefringent calcite is used to detect extremely weak polarized light produced in the atmospheres of exoplanets [17].…”

“…Navigation by means of the polarization pattern of the sky is a natural ability of several animal species (e.g., insects and birds), but it was also applied on board trans-arctic flights of the 1960s [6][7][8]. Skylight origins mainly from the Rayleigh scattering of sunlight on atmospheric particles and is predominantly partially linearly polarized.…”