Babylonian Calendar

The Babylonian calendar divided the year into twelve months, Nisannu, Ayyaru, Simanu, Duuzu, Abu, Ululu, Tashritu, Arahsamnu, Kislimu, Tebetu, Shabatu and Addaru, each running from one observed new moon to the next and each carrying its own cluster of agricultural and religious associations rather than functioning as a neutral, interchangeable unit of time. Nisannu, the first month, hosted the Akitu festival, already documented on this religion's page as Holiday 199, the most important public religious observance of the Babylonian year, centered on the temple of Marduk at Babylon and involving the recitation of the Enuma Elish creation epic and a ceremony renewing the king's authority.

Because the underlying lunar year of roughly three hundred fifty four days drifts steadily out of step with the solar, agricultural year, an extra intercalary month, a second Ululu or a second Addaru depending on where in the cycle the drift had accumulated, was periodically inserted to pull the calendar back into alignment with the seasons. In earlier periods this intercalation was announced somewhat irregularly by royal decree; by the later first millennium BCE Babylonian astronomers had worked out a much more regular nineteen year cycle for when the extra month was needed, one of the calendar's many lasting contributions to the broader history of timekeeping.

Keeping this calendar accurate required sustained, careful observation of the night sky by temple-attached scholars, and that practical need became one of the seedbeds of Babylonian astronomy and astrology as independent disciplines. Francesca Rochberg's study of Mesopotamian divination and astronomy traces how calendar-keeping observation records evolved over centuries into far more ambitious projects, including the astronomical compendium known as MUL.APIN and, by the middle of the first millennium BCE, the earliest personal horoscopes anywhere in the historical record, all built on the same underlying lunar and planetary observation practice the calendar itself required.

The Babylonian calendar's influence also extended well beyond Mesopotamia's own religious life. Its month names, in only lightly altered form, were adopted into the Jewish calendar, a debt most scholars trace to the Babylonian captivity, and its broader intercalation logic shaped the family of lunisolar calendars used across much of the later ancient Near East, a comparative history surveyed in the Oxford Companion to the Year alongside the solar and purely lunar systems it eventually stood beside.