The Babylonian Luni-Solar Calendar. The Mesopotamian calendar was a luni-solar calendar, based on the lunar month and the solar year and day. The basic unit was the month, which began on the evening of the first sighting of the new moon. It lasted twenty-nine or thirty days, depending on when the first crescent of the moon became visible again. There is no evidence for a thirty-one-day month, so it is likely that if thirty days had passed since the previous first visibility, a new month was begun even if weather conditions prevented sighting of the crescent. By the fifth century b.c.e., the beginning of the month could be determined by computation, but cuneiform texts suggest that actual observations of first visibility were still relied on well into the Seleucid period (311–129 b.c.e.). The year began in the spring, in the month of Nisannu (comparable to March/April). Of course, the calendar of lunar months did not stay in line with the solar year. Twelve revolutions of the moon around the earth (twelve lunar, or synodic, months) are about eleven days shorter than the solar year, the time it takes the sun, moving eastward, to make a full revolution of travel from its starting point and back again. Thus, Mesopotamians had the problem of keeping the twelve-lunar-month year in step with the solar agricultural year, so that the seasons fell at more or less the same point of the calendrical year every year.
Intercalation. From at least the late third millennium b.c.e., when it was felt that the seasons had become sufficiently displaced from the calendar, a thirteenth, or intercalary, month was added to the year at any time. Somewhat later, in the early second millennium b.c.e., only months VI and XII were intercalated. This step was taken according to royal command or on the advice of temple officials. From about 500 b.c.e. on, a nineteen-year cycle of intercalations began to be used consistently. This cycle is often attributed to the late fifth-century Greek astronomer Meton of Athens, and it is commonly called “the Metonic Cycle,” but it was known and used by the Babylonians long before his time. In this scheme, additional months were added according to specific guidelines, and the pattern was repeated every 19 years, or 235 months. The traditional Jewish calendar was taken from the Mesopotamian calendar, the only difference being that the Babylonian year began in the spring, while the Jewish year begins in the fall. Most Jewish month names are similar to their first millennium b.c.e. Babylonian counterparts.
Procedure for Intercalation. The 19-year cycle of intercalations called for the addition of 7 intercalary months throughout every period of 19 years. The 19 years were composed of 12 years with 12 months each plus 7 years with 13 months each. These 7 years with intercalary months were made up of 6 years with a second month XII, called Addaru II, and 1 year with a second month VI, known as Ululu II.
Asger Aaboe, Episodes From the Early History of Astronomy (New York, Berlin & Heidelberg: Springer, 2001).
John Britton and C. B. F. Walker, “Astronomy and Astrology in Mesopotamia,” in Astronomy Before the Telescope, edited by Walker (London: Published for the Trustees of the British Museum by the British Museum Press, 1996), pp. 42–67.
Francesca Rochberg, “Astronomy and Calendars in Ancient Mesopotamia,” in Civilizations of the Ancient Near East, 4 volumes, edited by JackM. Sasson (New York: Scribners, 1995), III: 1925–1940.
Ronald Wallenfels, “30 Ajjaru 219 SE = 19 June 93 BCE,” Nouvelles Assyri-ologiques Brèves et Utilitaires (1992): 37, no. 46.