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 thirtyone- 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...