Babylonian calendar
The Babylonian calendar was a lunisolar calendar with years consisting of 12 lunar months, each beginning when a new crescent moon was first sighted low on the western horizon at sunset, plus an intercalary month inserted as needed by decree. The calendar is based on a Sumerian (Ur III) predecessor preserved in the Umma calendar of Shulgi (c. 21st century BC).
The year begins in spring, and is divided into reš šatti "beginning", mišil šatti "middle", and kīt šatti "end of the year". The word for "month" was arḫu (construct state araḫ). That the calendar originates in Babylonian, and not in later Assyrian times is shown by the fact that the chief deity of the Assyrians is assigned the surplus intercalary month. During the 6th century BC Babylonian captivity of the Hebrews, the Babylonian month names were adopted into the Hebrew calendar. The Assyrian calendar used in Iraq and the Levant also uses many of the same names for its months, such as Iyyar, Tammuz, Ab, Elul, Tishri, and Adar.
'Month of the Sanctuary'
'Month of the Bull'
'Month of Tammuz'
'Month of Beginning'
(i.e. the start of the second half-year)
'Month of Laying Foundations'
'Month of the Forthcoming of Water'
'Month of Adar'
Until the 5th century BC, the calendar was fully observational, but beginning about 499 BC the months began to be regulated by a lunisolar cycle of 19 years equaling 235 months. Although usually called the Metonic cycle after Meton of Athens (432 BC), Meton probably learned of the cycle from the Babylonians. After no more than three isolated exceptions, by 380 BC the months of the calendar were regulated by the cycle without exception. In the cycle of 19 years, the month Adaru 2 was intercalated, except in the year that was number 17 in the cycle, when the month Ululu 2 was inserted. During this period, the first day of each month (beginning at sunset) continued to be the day when a new crescent moon was first sighted—the calendar never used a specified number of days in any month.
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