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Viking origins

Traditionally, the Vikings originating in Scandinavia in the early Middle Ages are associated with violence and brutal force. However, the views of modern scholars paint a less mono-chromatic picture. Many of the activities of the Vikings required and produced knowledge of time-reckoning and of what we would nowadays classify as astronomy. For example, their extensive travelling and trade must have involved some knowledge of astronomy. The necessity of such knowledge is generally recognized in the case of coastal navigation, but also holds for inland travel through previously unknown areas, such as the vast lands of Eastern Europe.

Inland travel and coastal navigation is one thing, but regular trans-oceanic traffic is quite another. Yet such traffic was required to support the Scandinavian settlement of Iceland and Greenland, around the years 900 and 1000 respectively, at a time when the people of Europe knew nothing of the compass or the sextant. Even with good luck the oceanic voyage would take about a week, and without it land might not be sighted for several weeks. The navigational methods used included both terrestrial and celestial observations. There is hardly any doubt that the knowledge written down on vellum in Iceland in the twelfth and thirteenth centuries derives to a high degree from these observations and this experience.

Why did they need a calendar?

In 930, the Icelanders decided to establish the Althingi, a kind of parliament where an important part of the population gathered once a year for purposes of legislation and justice. Those who went there would spend two to five weeks away from home at a precious time of the year. The farms were scattered at long distances and the landscape often barely passable. Therefore the traditional Scandinavian method of summoning meetings by message was not viable—they needed a simple and reliable calendar to help people know when to start from home so as to arrive at the same time as the others. Moreover, since the Icelandic summer is short, it was a matter of primary concern to utilize summer time as well as possible, and date the parliament at the time of summer when the loss of domestic labor was least harmful.

To understand the need for a calendar we may also look at the agriculture itself and its annual cycle. Certainly, the caprices of Icelandic weather and nature are such that the calendar may often be a bad guide for action. In deciding when to let cattle and sheep out on grass or when to start hay-making it is better to observe the actual signs of nature than the calendar. But there are certain kinds of annual operation where the calendar proves superior: for example, in determining when to sow the grain, something which people had tried with little success in the first centuries of settlement in Iceland. Another good example is that of deciding when to let the ram to the ewes. It is important to do this at the right time in the winter so that the lambs have the best possible prospect of growing in the short summer, without too much risk of interludes of bad weather in the spring just after they are born. When the individual farmer makes his decision on this at some point around Christmas time, he has no clear natural signs of a terrestrial nature to go by.

How similar was it to the Julian calendar?

In the brief history of Iceland called ?slendingab?k (The Book of the Icelanders, Libellum Islandorum), written by Ari the learned in the period 1122-33, we have a report on a calendar reform of about 955:

This was when the wisest men of the country had counted in two semesters 364 days or 52 weeks-then they observed from the motion of the sun that the summer moved back towards the spring; but there was nobody to tell them that there is one day more in two semesters than you can measure by whole weeks, and that was the reason.

There was a man called Thorsteinn the black, a very wise man. When they came to the Althing he sought the remedy that they should add a week to every seventh summer and try how that would work.

By a correct count there are 365 days in a year if it is not a leap year, but then one more; but by our count there are 364. But when in our count a week is added to every seventh year, seven years together will be equally long on both counts. But if there are 2 leap years between the ones to be augmented, you need to add to the sixth.

How did Thorsteinn the Black determine his intercalation? His farm was favorably located in the country to utilize the so-called mountain circle method, that is, to follow the annual motion of sunrise and sunset near the horizon where he would have suitably distant mountains and other reference points in the landscape to make fairly exact observations possible. At high latitudes the points of sunrise and sunset move so fast that this method could easily be used to determine the length of the year to within a day.

According to this, people started by counting 52 weeks or 364 days in the year. When they realized the insufficiency of this they tried the remedy of intercalating one week every seventh year (sumarauki), thus making the average year 365 days. The method chosen may seem strange to us but it is a natural consequence of the important role of the week in the original calendar.

So far the interpretation of the text seems straightforward. However, the text continues to describe the relation and adaptation of the Icelandic calendar to the Julian one, which must have been gradually introduced in Iceland in the eleventh and twelfth centuries, following formal Christianisation of the country in the year 1000. The text says that if there are two leap years between the years to be increased by a week, then the sixth year (instead of the seventh) should be increased. This is plainly wrong and would yield a worse approximation than the more simple rule of intercalating a week every sixth year. Scholars find this confusing, except by assuming the Latin meaning of the numerals. Thus ‘septimo quoque anno’ actually means ‘every sixth year’ by our count. In this way Ari’s text can be interpreted so as to coincide with practice in his time, as seen from almost contemporary Easter tables. Also, he would escape Occam’s razor, since his formula would otherwise be more complicated than necessary for its accuracy.