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 (Third Dynasty of Ur) predecessor preserved in the Umma calendar of Shulgi (c. 21st century BC). The year begins in spring, and is divided into 12 months. The word for "month" was arḫu (construct state araḫ). The chief deity of the Assyrians is assigned the surplus intercalary month, showing that the calendar originates in Babylonian, and not later Assyrian times. During the 6th century BC Babylonian captivity of the Jews, the Babylonian month names were adopted into the Hebrew calendar. In Iraq and the Levant the Gregorian solar calendar is used with these names replacing the Latin ones as Arabic names of Roman months. The Assyrian calendar used in is an example, also the Turkish months. These were inspired by the Ottoman Rumi calendar, itself derived from the Roman Julian solar calendar. Despite appropriating the mere Babylonian names, their months are not equivalent to those of the Hebrew and Babylonian lunisolar calendar. 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 Ulūlu 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.