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Although Vikings did not have a magnetic compass, they had ruled the northern Atlantic Ocean for three centuries between about AD 900 and 1200. Their sailing success is attributed to the use of a genuine sun compass with which they could determine the geographic north direction.

When the sun was hidden by clouds or thick fog, the navigator had to determine first the position of the invisible sun...According to a famous hypothesis, Viking sailors could navigate along the latitude between Norway and Greenland by means of sky polarization in cloudy weather using a sun compass and sunstone crystals. 

Using data measured in earlier atmospheric optical and psychophysical experiments, here (in a study entitled "Success of sky-polarimetric Viking navigation: revealing the chance Viking sailors could reach Greenland from Norway") we determine the success rate of this sky-polarimetric Viking navigation. 

Simulating 1,000 voyages between Norway and Greenland with varying cloudiness at summer solstice and spring equinox, we revealed the chance with which Viking sailors could reach Greenland under the varying weather conditions of a three-week-long journey... if they analysed sky polarization with calcite, cordierite or tourmaline sunstones.

A hunk of calcite "sunstone." Photo by Arni Ein via Wikipedia

A hunk of calcite "sunstone." Photo by Arni Ein via Wikipedia

Examples of voyage routes are also presented. Our results show that the sky-polarimetric navigation is surprisingly successful on both days of the spring equinox and summer solstice even under cloudy conditions if the navigator determined the north direction periodically at least once in every 3 hours, independently of the type of sunstone used for the analysis of sky polarization. 

This explains why the Vikings could rule the Atlantic Ocean for 300 years and could reach North America without a magnetic compass. Our findings suggest that it is not only the navigation periodicity in itself that is important for higher navigation success rates, but also the distribution of times when the navigation procedure carried out is as symmetrical as possible with respect to the time point of real noon.