Swedish astrophysicist Göran Henriksson of Uppsala University finds that the pagan inhabitants of Old Uppsala used an exact lunisolar calendar to fix the date of the great winter sacrifice

Swedish astrophysicist Göran Henriksson of Uppsala University has established that the pagan inhabitants of Old Uppsala used an exact lunisolar calendar to fix the date of the great winter sacrifice. In the pre-Christian era Old Uppsala was the principal religious and political centre of the Svear, one of the peoples who inhabited the territory of Sweden. Three large "royal" mounds survive there, raised between 450 and 550 AD.

The sacrifice was described in detail by the German chronicler Adam of Bremen around 1075. According to him, the rite took place once every nine years. The ancients counted years inclusively, so by modern reckoning the rite recurred every eight years. The festival lasted nine days, and on each day the priests sacrificed one man and seven male domestic animals. The bodies of all 72 victims were hung in the sacred grove.

The sacrifice coincided with the Disting. This was the name given at once to the sacrificial festival, to the general assembly of the inhabitants of the region, at which political decisions were taken, and to a great fair. Attendance at the Disting was obligatory. The festival began at the full moon that fell between 28 January and 26 February by the Gregorian calendar. The sixteenth-century Swedish historian Olaus Magnus explained that the full moon gave light for winter journeys to the place of assembly.

Henriksson points out that eight years is the shortest interval after which the phases of the Moon recur on roughly the same calendar days. He has calculated that one of the great sacrifices fell in the year 852. In that year the missionary Ansgar, known as the Apostle of the North, paid his second visit to the trading town of Birka.

The people of Uppsala marked the necessary directions on the ground. Two long rows of posts indicated the northern limit of the rising of the full Moon and the direction of the sunrise. One row, about 850 metres long, was oriented on the rising of the winter full moon, and the other, about 600 metres long, on the sunrise of 6–8 February. The mounds, in Henriksson's view, also served as markers. Viewed from the thing mound of Tingshög, the Sun sets over the mounds on the days that marked the limits of the Disting, 28 January and 26 February. The middle mound, possibly the grave of the legendary king Aun, lines up with the direction of the sunset on 26 February. Once every 19 years the full moon falls exactly on these sunsets. Nineteen solar years are almost exactly equal to 235 lunar months, so after that interval the phases of the Moon return to the same dates.