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Misseri-calendar - Discovery of Errors in the Christian Calendar

Kristín Bjarnadóttir (University of Iceland)

Icelanders agreed to accept the Christian Faith in the year 1000. The Icelandic Christian Church as an institution was established in the late eleventh and early twelfth century. The Church introduced into Iceland the Julian calendar, with one extra day added to the usual 365 days every fourth year, the leap year. The Julian calendar was introduced by Julius Caesar in 46 BCE. It was the predominant calendar in the Roman world, in most of Europe, and in later European settlements in the Americas and elsewhere.

In order to make the misseri-calendar fully compatible with the Roman calendar, the Summer’s Extra Week was inserted into the misseri-calendar every sixth year, or every fifth year if two leap years were in-between. It was inserted in midsummer, beginning on Sunday after 13 weeks, and was adjusted so that summer always began on Thursday between April 9 and 15.

By adding a day to 365 days every fourth year, the average length of the year became 365.25 days, while in reality it is approximately 365.2422 days. The Julian calendar assumed the summer solstice to be on June 21, as decided in Nicaea in 325 CE. In the twelfth century, it fell on June 15. This was due to the addition of six too many leap-year days which would have been skipped at years 500, 600, 700, 900, 1000, and 1100 according to a later correction of the calendar, called the Gregorian calendar.

In the first half of the twelfth century, a farm-worker, Oddi Helgason (1070/80–1140/50), called Star-Oddi, made observations of the annual motion of the Sun. An account of his observations is found in the ancient treatise Odda-tala [Oddi’s Tale], contained as a separate treatise in the oldest part of the manuscript GKS 1812, 4to, written around 1192 [9]. The Icelandic week-based misseri-calendar was adjusted to the Julian calendar in the early twelfth century, during Oddi's time. Oddi observed the summer solstice and the winter solstice to be earlier than the official date, i.e. on June 15 and December 15 instead of June 21 and December 21 [10]. The author(s) of the Icelandic calendar treatise, Rím II [rím: rhyme, meaning calendar] [11], writing in the late thirteenth century, compared the summer solstice in Iceland with that "in the middle of the world," the Mediterranean [Mid-Earth] Sea, as follows [12]:

Solstice in summer is four nights before the mass of John the Baptist ... It is so in the middle of the world. Some men say that it is close to a week earlier in Iceland.

The error of the Julian calendar had thus been discovered in Iceland in the twelfth century, but its size seems to have been believed to depend on latitude. No amendments were made to the Icelandic misseri-calendar until year 1700 when the Gregorian calendar was adopted in the Danish realm, Iceland included. From then on, the First Summer Day has fallen on Thursday between April 19 and 25. The two calendars were printed parallel to each other in Dactylismus [13], where Bishop Jón Árnason in 1739 cleverly adjusted the misseri-calendar to the Gregorian calendar.

Figure 8. Celebration of First Summer Day. (Photographer unknown)

[9] GKS 1812 4to [quarto]. The Árni Magnússon Institute for Icelandic Studies, manuscript collection, Reykjavík.

[10] Vilhjálmsson, Th. 1991. Time-Reckoning in Iceland before Literacy. In C. L. N. Ruggles (Ed.), Archaeoastronomy in the 1990s, 69–76. Loughborough, UK, Group D Publications. Available at

[11] Beckman, N., and Kr. Kålund (Ed.). 1914–1916. Alfræði Íslenzk. Islandsk encyklopædisk Litteratur. II Rímtöl, pp.83–178. Copenhagen: STUAGNL.

[12] Beckman and Kålund, 1914–1916, p. 121.

[13]  Árnason, J. 1838; 1946. Dactylismus Ecclesiasticus eður Fingra-Rím. Originally printed in 1739, reprinted in 1838, photocopied in 1946. Copenhagen: P. Jónsson. Available at HathiTrust Digital Library:

Kristín Bjarnadóttir (University of Iceland), "Misseri-calendar - Discovery of Errors in the Christian Calendar," Convergence (December 2016)