Calibrating Calendars Using Eclipses

When we know how to convert the dates of ancient eclipses to our modern calendar, the records provide useful information about the Earth's spin history, and the trend over the past 2,700 years is now well delineated. But the converse is also true. If one has a definite report that an eclipse was recorded from a certain place in a certain year then one can calibrate the calendar the locals were using.

Consider the calendar of the Roman Republic. After about 400 B.C. the Romans were using a scheme whereby in most years there were 12 months adding up to 355 days, rather than the actual solar year which averages close to 365.25 days. That leaves a deficit of over ten days a year. Every so often the leaders of the Senate were supposed to declare an additional month of 22 or 23 days, to be inserted into February, but they were quite lax in this regard for various reasons. One was that the thirteenth month was considered unlucky, hence the common fear of the number 13 (triskaidekaphobia). The major reason, though, was that they were able to manipulate the year length to their own advantage for taxation or electoral purposes. The outcome was that the date on the Roman calendar seldom bore any clear relation to the season. The end of May would come, but still it was winter. Looking back it would be almost impossible for historians to be able to say what occurred when, if it were not for eclipses.

In 168 B.C. the Romans defeated the Greeks in the Battle of Pydna, a town on the western side of the Gulf of Salonika. The battle was pivotal for the eventual Roman control of Greece because it quelled the Macedonians. (These were the people who in the latter half of the fourth century B.C. had produced Alexander the Great, and under him conquered an empire stretching from the eastern end of the Mediterranean all the way to India.) The Romans recorded this battle as occurring, on their haphazard calendar, on September 3. From this, one might imagine that it took place in early fall. In fact we know that the Battle of Pydna was fought near midsummer's day. The great writers Livy and Pliny recorded that a lunar eclipse was predicted by the tribune Sulpicius Gallus and seen on the night before the conflict, giving courage to one side while the other was filled with dread. Perhaps the Roman generals chose the date on the basis of the prediction, telling their troops ahead of time that there would be an eclipse as a sign of divine favor.

Knowing that the year was 168 B.C. we are able to back-calculate the date of the eclipse and check its visibility from Greece. Using the regular calendar introduced by Julius Caesar over a century later, and projecting it backwards, the eclipse was on June 21. This nicely confirms the basis of the story related by Livy and Pliny: on a sensible, well-regulated calendar the battle was indeed fought close to midsummer's day.

From that eclipse we find that the Roman republican calendar in that year was 74 days out of synchronization with the seasons. The discrepancy at times had been even greater. A solar eclipse in 190 B.C. shows that the calendar then was ahead of the seasons by 119 days. When Julius Caesar introduced his eponymous calendar to begin in 45 B.C. (our modern calendar is the same except with a slight adjustment to the frequency of leap-year days), he had to add 80 days to 46 B.C. to make up for the shortcomings of his predecessors.

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