A lunisolar calendar. The Akitu festival. The thirty-six stars of Astrolabe B. Iqqur ipuš hemerology. An interactive reconstruction of the system in which astrology was born.
The twelve-sign zodiac we use today, the gods assigned to the seven planets, the concept of planetary exaltation, even the seven-day week — all of them trace back to Babylon. Hellenistic astrology owes its operational machinery to Egypt, but its conceptual spine was woven here. Marduk was Jupiter, Ishtar was Venus, Sin was the Moon; these correspondences were made here first, then carried to the Greek world, and from there to India, Persia and the Arab realm.
The Babylonian calendar is lunisolar: it tracks both the Moon and the Sun. Each month begins with the first crescent observed at the western horizon after sunset. Twelve such months total around three hundred fifty-four days; to close the eleven-day gap with the solar year, a thirteenth month is added in seven years out of a nineteen-year cycle. By the time the Greek astronomer Meton "discovered" this cycle in 432 BCE, the Babylonians had been using it for centuries.
This tool brings that system back to life through a modern lens. For the year you choose, it calculates the first crescent after the spring equinox astronomically and fixes Nisanu 1. For each month it presents the star commentaries of Astrolabe B, the heliacal risings of MUL.APIN, the favorable-days list from BM 50634, and month-specific omens from Iqqur ipuš.
February, April, September, Tammuz — four words in modern calendars that still carry the language of Babylon. Most of us never notice.
Each month begins on the evening the new crescent appears at the western horizon. Click a month to see its Astrolabe B commentary, MUL.APIN stars, festivals and Iqqur ipuš omens.
The first twelve days of Nisanu. The annual restaging of creation: Marduk's victory over Tiamat enacted each spring as the renewal of the cosmos.
The eighteen constellations along the Path of the Moon in MUL.APIN were reduced, around the fifth century BCE, to twelve equal thirty-degree segments. The birth certificate of the modern zodiac.
| Sumerian | Meaning | Modern | Greek |
|---|---|---|---|
| LÚ.ḪUN.GÁ | Hired Man | Aries | Krios |
| GU₄.AN.NA | Bull of Heaven | Taurus | Tauros |
| MAŠ.TAB.BA.GAL.GAL | Great Twins | Gemini | Didymoi |
| AL.LUL | Crayfish | Cancer | Karkinos |
| UR.GU.LA | Lion | Leo | Leon |
| AB.SIN | Furrow (Shala's ear of barley) | Virgo | Parthenos |
| zibanītu | Scales | Libra | Zygos |
| GIR.TAB | Scorpion | Scorpio | Skorpios |
| PA.BIL.SAG | Pabilsag (Sumerian deity) | Sagittarius | Toxotes |
| SUḪUR.MAŠ | Goat-fish (Ea) | Capricorn | Aigokeros |
| GU.LA | The Great One | Aquarius | Hydrokhoos |
| KUN.MEŠ | Tails (+ Anunitum) | Pisces | Ikhthyes |
The Hellenistic concept of hypsoma comes directly from Babylon. Bīt niṣirti or ašar niṣirti: "house of secrecy" or "place of secrecy" — the point on the ecliptic where a planet produced a favorable omen.
This tool is an astronomical reconstruction of the Babylonian civil calendar. Each month is computed from the visibility of the first crescent at the latitude of Babylon (32.46°N). Small deviations from historical records are expected — they arise from the observing conditions of Babylonian priests (āšipu, bārû), cloud cover and human judgement.
Sources: MUL.APIN (Hunger-Steele 2019), Astrolabe B (Horowitz 2014), Iqqur ipuš (Labat 1965), BM 50634 (CCP 2017), Enuma Anu Enlil (Rochberg 1988), Babylonian Chronology (Parker-Dubberstein 1956).
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About the Babylonian calendar
The Babylonian year began with the month of Nisannu, which started at the first observed lunar crescent after the spring equinox. The Akitu festival was an eleven-day royal and cultic celebration during which the statue of Marduk was carried in procession and the Enuma Eliš was recited, ritually renewing the kingship and the cosmic order. A second, smaller Akitu was also held in Tashritu (the autumnal equinox month) in several cities of southern Mesopotamia.
MUL.APIN ("the Plough Star") is a two-tablet cuneiform compendium of Babylonian astronomy compiled around 1000 BCE that catalogues stars, constellations, heliacal risings, planetary periods, and the ideal 360-day schematic year. It records the path of the Moon along eighteen constellations - a precursor to the later twelve-sign zodiac - and provides the first systematic intercalation rules. The standard modern edition is Hunger and Steele (Routledge, 2019).
The Babylonian calendar was lunisolar, so an extra month - either a second Addaru (Addaru II) or a second Ululu (Ululu II) - had to be inserted roughly every two or three years to keep Nisannu close to the spring equinox. From the fifth century BCE onward, scribes adopted a fixed nineteen-year cycle in which seven intercalary months are inserted at predictable positions; this is the same Metonic cycle that later entered Greek and Hebrew astronomy. Earlier, intercalation was decreed ad hoc by the king on the advice of the scholarly bārû and ṭupšarru.
Iqqur ipuš ("He demolished, he built") is a series of cuneiform tablets, edited by René Labat in 1965, that organises favourable and unfavourable days for human activities month by month - building a house, marrying, sowing, travelling, taking medicine, or performing rituals. Each entry pairs an action with the conditions under which it should or should not be undertaken in each of the twelve Babylonian months. The text belongs to the broader genre of hemerologies and was used by āšipu and bārû priests as a practical liturgical calendar.
By the fifth century BCE, Babylonian scribes had standardised the ecliptic into twelve thirty-degree signs - a development first attested in Late Babylonian astronomical diaries and in texts such as TCL 6 11. This twelve-sign zodiac, along with the doctrine of planetary exaltations and the concept of the horoscope (Akkadian ṭersītu), was transmitted to the Greek world during the Hellenistic period and forms the immediate foundation of Western astrology. Francesca Rochberg's work, especially The Heavenly Writing (2004), traces this transmission in detail.
The visibility of the first crescent depends on the Moon's lag time, altitude, and the atmospheric clarity of southern Mesopotamia, and Babylonian observers achieved remarkable consistency from the latitude of Babylon (about 32.46° N). Astronomical diaries from the seventh century BCE onward record both observed and predicted first visibilities, and modern reconstructions match these records to within one day in the great majority of cases. Occasional discrepancies arise from cloud cover, atmospheric refraction, and human judgement, which is why the calendar can drift slightly from any purely arithmetical reconstruction.