The Babylonian calendar: lunar months and intercalary months in Mesopotamia

In Mesopotamia in the first millennium BC, the month began on the evening when, after sunset, the first thin lunar crescent appeared in the west. The Babylonian calendar was lunisolar: it followed the Moon for its months, but it had to stay anchored to the seasons, because festivals and farm work depended on river floods and harvests. From this compromise between two heavenly bodies came one of the most influential timekeeping systems of antiquity, inherited in part by Jews, Persians and Greeks.

The twelve months and the first crescent

In their standard Babylonian form the months were called Nisannu, Ayyaru, Simanu, Du'uzu, Abu, Ululu, Tashritu, Arahsamnu, Kislimu, Tebetu, Shabatu and Addaru. The year began in spring with Nisannu, the month of the great Akitu festival celebrated in Babylon in honour of Marduk. Each month lasted 29 or 30 days: if the crescent was not seen on the evening of the thirtieth day, the next month began anyway, because a lunar month cannot exceed thirty days.

Watching for the crescent was no mere formality. Scribes measured the interval between the setting of the Sun and that of the Moon and, on the tablets we now call astronomical diaries, most of them kept in the British Museum, they recorded night after night the position of the Moon and planets, the weather and even the prices of goods. The oldest securely dated diary goes back to 652 BC, and the series continues into the first century BC.

The problem of the extra month

Twelve lunar months make about 354 days, eleven fewer than the solar year. Without corrections, Nisannu would have slipped from spring into winter within a few decades. To prevent this, a thirteenth month was added from time to time, a second Addaru at the end of the year or a second Ululu in the middle. In the Old Babylonian period the decision lay with the king: among the letters attributed to Hammurabi is one announcing the insertion of a second Ululu.

For centuries intercalation remained empirical, tied to observation of the sky and in particular to the position of the Moon relative to the Pleiades or to the rising of certain stars. The astronomical compendium known as MUL.APIN, whose composition goes back at least to the beginning of the first millennium BC, contains rules of exactly this kind, which made it possible to tell whether the year was lagging behind the Sun.

The nineteen-year rule

Between the sixth and fourth centuries BC Babylonian astronomers arrived at a fixed scheme: seven intercalary months every nineteen years, almost always a second Addaru and, in one specific year of the cycle, a second Ululu. The scheme exploited a remarkable numerical coincidence: 235 lunar months last almost exactly as long as 19 solar years. In Greece the same relationship is linked to the name of Meton of Athens, but we do not know for certain whether the Greeks drew it from Mesopotamian sources.

This regularity made it possible to date documents and to predict festivals years in advance. Astronomers of the Seleucid age, working in the temples of Babylon and Uruk, built on it mathematical methods for calculating the visibility of the crescent and eclipses, which are studied today as one of the earliest forms of predictive astronomy in history.

The legacy of that system is still visible. The names of the Hebrew months, such as Nisan, Sivan, Tammuz, Av, Elul and Tishri, derive directly from the Babylonian ones, adopted by the Jews during and after the exile. And the choice to entrust the start of the month to the sight of the first crescent survives in the Islamic calendar. Looking for the new Moon at sunset is a gesture the scribes of Babylon would have recognised at once.