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Dating is one of the banes of an archaeologist’s life and work, mainly because of the various (and unsatisfactory) dating methods used by the people of antiquity. These suffered from two major flaws: in the first place most calendars were lunar rather than solar, and they were calculated according to the reign of the country’s monarch.
In absolute terms – as seen by someone outside our solar system, for example – the moon circles our earth every 27.321 days. From here on earth, however, what with equinoxes precessing and seasons slipping (to say nothing of tilted axes!) the average lunar month consists of 29.531 days. This unfortunate number does not divide evenly into the 365.256 days of the solar year, a fact that led to endless confusion in ancient chronologies.
One of the better known attempts to deal with these problems is the Jewish calendar, which combines a 19 year lunar cycle with a 28 year solar cycle. Ten of the twelve months have fixed lengths of either 29 or 30 days, the remaining two vary depending on where they come in the solar cycle. This is to prevent 1st Tishri, the Jewish New Year, falling on inauspicious days like Sunday, Wednesday or Friday. In addition, seven times in the lunar cycle an extra month of thirty days is added in, to keep the seasons in their proper places in the calendar. This extra month is added in years 3, 6, 8, 11, 14, 17 and 19.
Our modern calendar is based on the work of Julius Caesar who, in 46 BC, changed the Roman calendar from lunar to solar. Plutarch, in his Life of Caesar, described Caesar’s achievement like this:
“His reform of the calendar, however, and the corrections made in the irregularity of reckoning time were not only studied by him with the greatest scientific skill, but were brought into effect and proved extremely useful. In very anciet times there had been great confusion among the Romans with regard to the relation of the lunar to the solar year, with the result that festivals and days of sacrifice gradually got out of place and finally came to be celebrated at the very opposite seasons to what was originally intended. Nor was the confusion confined to the remote past. Even at this time most peple were completely ignorant on these subjects; only the priests knew the proper time and they, without giving any notice, would suddenly insert in the calendar the intercalary month known as Mercedonius. It is said that this month was first put in by King Numa, who thus managed to find an unsatisfactory and short-lived remedy for the error in the adjustment of the sidereal and solar cycles. I have dealt with this subject in my Life of Numa. Caesar, however, put the problem before the best scholars and mathematicians of the day and, out of the various methods of correction already in use, he formed a new method of his own which was more accurate than any of them. It is the one still used by the Romans and it seems that they, better than all other people, have avoided the errors arising from the inequality between the lunar and solar years. Yet even this gave offence to those who looked at Caesar with envious eyes and resented his power. Certainly Cicero, the orator, when someone remarked that the constellation Lyra would rise next day, remarked: ‘No doubt. It has been ordered to do so’ – implying that even the risings of the stars was something that people had to accept under compulsion.”
Plutarch, Fall of the Roman Republic, (tr. Rex. Warner) London: Penguin Books Ltd, 1972. p. 298
One of those consulted was the astronomer Sosigenes, on whose advice Julius Caesar set up the familiar cycle of one leap year every four years. He added 90 days to the year 46 BC in order to get the seasons back into their proper place, but decided to make the year begin on January 1 instead of mid-March. The major difference was that February had 29 days, with 30 in a leap year. It was Augustus who reduced February by one day, in order, so people said, to make his own month – August – the same length as Julius’ month, July.
Unfortunately Julius Caesar forgot to explain matters properly to the priests who guarded the calendar (or perhaps those worthies indulged in a bit of deliberate sabotage). For the next 36 years they used the normal Middle Eastern method of counting, which was inclusive. The fourth year of one cycle was also counted as the first year of the next (compare this with the “three” days that Jesus was supposed to have remained in the tomb) so you had leap years every three years. Someone noticed this in 10 BC and so leap years were omitted until 4 AD just to get things back in line again.
A subsequent minor alteration by Pope Gregory in 1582 – rejected for many years by most Protestant countries as a sinister papal plot – gave us our modern calendar which, on the whole works quite well. At least, it works for those of us in the northern hemisphere. Somehow, in the southern hemisphere the seasons seem to have slipped, with spring coming in September and Christmas being celebrated in the heat of summer. Julius Caesar and his attendant augurs must be turning in their graves!