Babylonian astronomy plays a critical role in
the development of Greco-Roman astronomy, which is in turn essential for
establishing a reliable chronology of the ancient world. Among other
things, it is from the Babylonians that we derive our sexagesimal system
for minutes and seconds. We are very fortunate that we are in a position
to confirm independently the chronology found in major works like the Almagest. Thanks to excavations of
numerous cuneiform tablets we have abundant evidence of the Babylonian
calendar, the regnal dates of their rulers, and their astronomical
The Babylonian calendar was
lunisolar, which means that periodic leap months were required to keep the
lunar and solar years in synchronization. The months began at the first
visibility of the new crescent at sunset. In later Babylonian times, the
new moon was determined not by direct observation but by a complex
mathematical rule, which in fact yielded a very close result.
The intercalary month was
inserted either after Ululu
and it was simply called Second Ululu,
or Second Addaru.
There is some evidence that by the reign of Nabonassar (747 BCE)
Babylonian astronomers had discovered the Metonic 19-year cycle, but until
the 4th century BCE, there is no evidence that a 19-year cycle was used to
assign fixed intercalary years within the cycle. In its fully developed
form, years 3, 6, 8, 11, 14, and 19 had a second Addaru,
and year 17 had a second Ululu.
For earlier Babylonian
history, years are reckoned by the regnal year of the ruler. After
Seleucus I conquered Babylon, scribes began to record dates in the Selucid
Era (SE), a continuous count of years that did not stop with the death of
Seluceus. Year 1 SE corresponds to 312/11 BCE, a correspondence that can
be confirmed by records of astronomical observations dated in this era.
After the Parthians
conquered Mesopotamia, the western part of the Selucid empire switched the
beginning of its year from spring (Nisanu)
to fall (Tashritu),
under Greek influence. The Parthians kept Nisanu as the beginning of the year.
The seven-day cycle makes its earliest
appearance in Babylonian documents of the 7th century BCE. It is not quite
yet the week as we know it, however. In origin, it seems to have been one
fourth of the approximate time in a month the moon was visible. In short,
it does not include the days around the new moon, and is not therefore a
continuous cycle. To picture what this "week" was like, imagine
one of our months with four regular weeks, and then a few epagomenal days
at the end of the month, which do not belong to any week.