Babylonian Astronomy.
Babylonian Calendar. A lunisolar calendar dating back to the early second millennium BCE. It consisted of 12 lunar months with an 13th intercalary month inserted sporadically by decree. The year began with the spring equinox and the beginning of each month was determined by the observation of the new crescent Moon above the western horizon just after sunset. The names of the months (which were also adopted by the Assyrians and the Hebrews) and their Julian / Gregorian equivalents are given below:
| Number | Name | Julian / Gregorian |
|---|---|---|
| 1 | Nīsannu | March / April |
| 2 | Iyyār | April / May |
| 3 | Sīmannu | May / June |
| 4 | Du'ūzu | June / July |
| 5 | Ābu | July / August |
| 6 | Ulūlū | August / September |
| 7 | Tašrītu | September / October |
| 8 | Araḫsamna | October / November |
| 9 | Kisilīmu | November / December |
| 10 | Ṭebētu | December / January |
| 11 | Šabāṭu | January / February |
| 12 | Addāru | February / March |
| 13 | Addāru 2 | February / March |
The basic scheme of intercalation lasted until the C6th BCE and the decision to intercalate was probably based on a mix of astronomical signs, such as the heliacal risings of certain traditional stars like the Pleiades (MUL.MUL) or Sirius (MUL.KAK.SI.SA) and non-astronomical signs, such as the migration of birds or the ripening of crops.
From the C6th BCE the intercalation scheme was based on the Octaeteris, the eight year cycle of 2922 days in which there are 99 lunations. In every cycle there were three intercalated months (5 × 12 + 3 × 13 = 99). Sometime in the early the C5th BCE, the intercalation scheme was based on the more accurate Metonic Cycle of 19 years in which there were almost exactly 235 lunations with seven intercalations (12 × 12 + 7 × 13 = 235). Years 3, 6, 8, 11, 14, and 19 were given an extra Addāru in month 13, while year 17 was given an extra Ulūlū in month 7. Later, in the Seleucid Era (beginning 3rd April 311 BCE), the intercalary years were changed to 1, 4, 7, 9, 12, 15 and 18 in each 19-year cycle.
Bisextile.
