Iceland, an island in the North Atlantic, was settled by the Vikings around 900 A.D. Their initial way of keeping track of time was counting months by the change of lunar quarters. In northern latitudes nights are bright, and the moon is rarely seen from April to late August. Time during summer was therefore counted in weeks. A primitive calendar, counting 52 weeks or 364 days in the year, was in use from the establishment of the Icelandic parliament, Althingi, in 930 in order to set the time for the yearly parliamentary gathering. The 52 weeks were divided into two semesters or misseri, 26 weeks of summer and 26 weeks of winter. This system may have been tried out in the short term, quickly revealing the need for a more reliable system of time-computing. Only 25 years later it had become clear that the summer "moved back to the spring", i.e. the summer began earlier and earlier according to computations by this calendar. This was inconvenient, as the parliamentary gathering had to assemble at a time during the short Icelandic summer after the completion of certain necessary farming tasks, and before others were due to begin. The error seems to have been realized around the year 955, from an observation of the location of the sunset, which in northern areas moves rapidly clockwise along the horizon before the summer solstice, and subsequently anti-clockwise. This method would not be as suitable at lower latitudes, and is most likely an Icelandic invention.

To correct the error, one week, known as summer's extra week, was inserted every seventh year in the week-calendar, making the year 365 days. After the introduction of the Christian faith in the year 1000, the system was adapted to the Julian calendar of the church, by inserting a week every sixth year, and every fifth year if there were two leap years in between. This method is explained in one of the most reliable sources in Icelandic medieval literature, written in the period 1122-1133. In spite of the introduction of the Julian calendar, the Icelandic week-based
calendar remained in use up to the 20th century in farming society.

A special technique to count time on the fingers, a so called "finger-rhyme", was developed by the Church and practised. In 1739 a bishop of the Evangelical Lutheran faith published a book, Fingra-Rím/Finger-Rhyme/Dactylismus Ecclesiasticus, where he explained how to compute the recently-introduced Gregorian calendar on the fingers, as well as the secular misseri calendar. A so-called golden number of the year, computed according to a cycle of 19 years, is of special importance for computations of dates of ecclesiastical feasts, such as Easter, as well as is the dominical letter of each year, denoting the positions of Sundays of each year. The first day of summer in the misseri calendar was adapted to the ecclesiastical calendar, and could be computed on the fingers according to dominical letters, while the computation of first day of winter was complicated by the insertion of summer's extra weeks, called rhyme-spoilers. The misseri calendar was thus practised in consensus with the Church, which registered births and deaths, reported by the general public according to the misseri calendar.

So many remnants of the old system have survived to the present day that most grown-up Icelanders are familiar with it. Counting the time through the difficult winter months to the beginning of the summer misseri is especially important. It marks the beginning of the light period, the birth of livestock, turning animals out to pasture after feeding them through the winter, and the expected time of fish shoals entering the fishing grounds.

In the presentation the features of the Icelandic mixture of week-based and lunar misseri calendar will be explained, and its ethnomathematical origin and status clarified.

Sources

D'Ambrosio, U. (2001). Ethnomathematics. Link between Tradition and Modernity.
Rotterdam / Taipei: Sense Publishers.

Richards, E.G. (1998). Mapping Time. The Calendar and its History. Oxford: Oxford
University Press.

Thorkelsson, Th. (1928). Misseristalið og tildrög þess. Skírnir 102, 124-144.