Babylonian Lunisolar Calendar

When ancient Israel fell under the dominion of great pagan empires, its calendar was radically changed. In the Hebrew Bible, the months of the year are mostly numbered prior to the exile to Babylon. But after that, Babylonian-like month's names appeared in the books of Zechariah, Esther, Ezra, and Nehemiah. The Babylonian-like months of Nisan, Sivan, Elul, Kislev, Tebet, Shebet, and Adar are used either on their own or along with the numbered months. Nisan was consistently equated with the first month of Exodus (Exodus 12:2) while Nisannu was the first month in the Babylonian calendar.

The Babylonian calendar originated in Babylon (Southern Iraq) in the early second millennium BC and circulated throughout the rest of Mesopotamia in the late second millennium BC. In the first millennium BC, the calendar became the official calendar of the great empires of Assyria, Babylonia, and Persia. After the Jewish Hasmonean state was separated from its Hellenistic Seleucid rulers in the mid-second century BC, the Jews adopted only Babylonian-like month's names but used their own Hebrew lunisolar calendar for the Festivals of God. Since Zechariah, Ezra, and Nehemiah were servants of God, we believe that they also used the Hebrew lunisolar calendar rather than the Babylonian one for the Biblical festivals while under the dominion of the great pagan empires.

The Babylonian calendar was also intercalated every two or three years by the addition of a 13^th month (usually by duplicating the 12^th month, Addaru, and less frequently by duplicating the sixth month, Ululu). This calendar may be quite similar to the original Hebrew one, but there are significant differences in the intercalation rule and in the definition of the first day of the month.

As mentioned in Chapter 2, the Hebrew calendar is intercalated only in the month of Adar, but not Elul. In the Hebrew calendar, Adar II is intercalated when the full moon after the 12^th month falls before the Vernal Equinox. In contrast, the first month of the Babylonian calendar always falls on or after the Vernal Equinox, so only some of the first months in the Babylonian calendar coincide with the first months in the Hebrew calendar.

The first day of a Babylonian month is a day when a new crescent is seen in the western sky after sunset. Within this criterion, the full moon falls mostly on the 14^th day of a month. That is why the recorded lunar eclipses by Babylonians mostly took place on the 14^th day and the recorded solar eclipses on the last day of a month. The same criterion has been used in the Islamic pure lunar calendar. This definition may be associated with the pagan worship to the moon in the first day of a month when a new crescent is seen in the western sky. In contrast, the God of Israel does not allow His people to worship any visible object, so the first day of a month corresponds to the dark-moon phase, making it impossible for them to worship the visible moon on that day. Therefore, the first day of a Hebrew month is one or two days earlier than that of a Babylonian month.

It is worth noting that in the post-Seleucid Macedonian calendar (see Table II), the first month of a year starts from Xandikos, which is equivalent to Nisannu in the Babylonian calendar and to Nisan in the Hebrew calendar. But in the Seleucid Macedonian calendar, the first month of a year starts from Dystros, corresponding to Addaru in the Babylonian calendar and to Adar in the Hebrew calendar. The month Dystros is the month just before the Vernal Equinox. The new-year month in the Seleucid Macedonian calendar is the same as that in the Chinese lunisolar calendar.