Sources, Rediscovery, and Historiography

Roughly five thousand cuneiform tablets are known which contain Babylonian or Assyrian astronomical and astrological texts. These tablets mostly date to the 1st millennium bce. The majority of the known tablets were recovered from the sites of Babylon, Nineveh, and Uruk between the middle of the 19th century ce and the early part of the 20th century ce, both from official, but often not very scientific, archaeological excavations carried out on behalf of museums in Europe and via the antiquities market. Beginning in the 1870s, scholars started to identify and investigate astronomical texts. In particular, Joseph Epping SJ in the 1870s and 1880s and Franz Kugler SJ between the 1890s and 1920s made pioneering investigations of Babylonian astronomy in which they were able to decipher the astronomical terminology, identify the basic types of Babylonian astronomical text, provide identifications for many Babylonian stars, and, most significantly, demonstrate that Babylonian astronomy encompassed detailed observation of the night sky and the development of complex mathematical methods for predicting future astronomical phenomena.¹ Their discoveries demonstrated that Babylonian astronomy was the equal of, but at the same time very different from, the better-known Greek astronomical tradition. Furthermore, Kugler showed that several aspects of Greek astronomy could be traced back directly to Babylonian predecessors.

For much of the 20th century, the study of Babylonian astronomy was focused, as it had to be, on the detailed study and analysis of individual or groups of astronomical texts, rather than on attempts to write overarching “histories” of Babylonian astronomy. Furthermore, following the death of Kugler in 1929, much of the study of Babylonian astronomy narrowed its focus to the investigation of the technical details and internal structure of the mathematical astronomy of the last few centuries bce.² Although this extremely internal and technical approach to the history of Babylonian astronomy meant that the topic was largely ignored by the wider fields of history of science and ancient studies, it provided the essential foundation on which broader historical investigations of Babylonian astronomy could to be made, a process that started in the 1990s.³ These broader types of investigation include studying the cultural locus of astronomical practice, the scholars who wrote astronomical texts, the archival context within which astronomical texts were written and stored, the cognitive and philosophical background of astronomy and astrology, and the connections between astronomy and astrology and other forms of Babylonian scholarship.

The Third Millennium bce and Earlier

Some of the earliest known cuneiform tablets already attest to the use of a lunisolar calendar with months whose beginning was governed by the first appearance of the new moon crescent and extra “intercalary” months added into some years in order to keep the calendar in line with the seasons.⁴ This calendar relied upon a basic knowledge of the cycle of the moon and the seasons and upon keeping a regular watch for the new moon and almost certainly dates back to the period before the development of writing. The calendar served both administrative and cultic roles, providing a framework both for keeping track of work done and rations owed, for example, and for the performance of rituals.

The use of Sumerian names for some of the constellations in later periods suggests that at least some of the constellations were created and named in the 3rd millennium bce.⁵ Once more, it is quite likely that some of the constellations had been named and the planets had been distinguished from the fixed stars before the development of writing.

The Second and Early First Millennium bce

Very few 2nd or early 1st millennium bce astronomical or astrological cuneiform tablets have been identified. Current knowledge of early Babylonian astronomy and astrology is, therefore, reliant on later copies of works which it is believed were composed during this period. By definition, however, what is preserved in later copies is what someone during this later period thought was worth copying. As a consequence, although what we can reconstruct of early Babylonian astronomy and astrology provides a more or less self-consistent portrait of these practices, it may only be part of the picture. In short, early Babylonian astronomy and astrology may have included a range of activities that we know nothing about. This fact must be kept in mind when reading the following summary.

As far as our sources attest, there were five main aspects of early Babylonian astronomy and astrology: describing the creation and structure of the universe (cosmology); listing the names of stars and constellations, often in meaningful orders; discussion of rules for knowing when to intercalate an extra month; the development of simple mathematical schemes to represent the variation in certain astronomical phenomena such as the length of day and night, the duration of visibility of the moon, and the length of a shadow cast by a gnomon; and the creation and listing of celestial omens.

The creation epic Enūma Eliš describes how the Earth and the heavens were created following the vanquishing of Tiamat by the god Marduk. Tiamat’s body was spilt in two and one half used to form the heavens. Marduk then ordered the stars into constellations, assigning three constellations to each month of the year. He then assigned the moon to mark the passage of the months through its cycle of phases and the sun to mark the year. An important aspect of this creation story is that the heavens are set up with an order based upon repeating cycles tied to the calendar. The cyclical repletion of astronomical phenomena is a key concept that underlies much of Babylonian astronomy.

Star lists are usually organized according to the order of the rising of the stars across the eastern horizon. This order also corresponds to the sequence in which the stars make their first appearance (heliacal rising) over the course of the year. Some star lists include the dates on which the stars make their first appearances. These dates are given in a schematic 360-day calendar in which it is assumed that each month has 30 days and there are always 12 months in the year. This calendar, which differs from the true lunisolar calendar of everyday life, is used in early astronomy to provide a simplified schematic framework within which to operate. The same 360-day schematic calendar was used in administrative and other non-astronomical contexts in order to simplify computation.⁶ In addition to ordering the stars by their rising, some star lists divide the sky into three regions named after the gods Enlil, Anu, and Ea. These regions correspond to bands of declination: the Enlil stars are in the north, the Anu stars are in a band centred on the celestial equator, and the Ea stars are in the south. Finally, some star lists give details of the culmination of stars at the meridian. Intercalation rules found in early works often rely simply on intercalating when an observed first appearance of a star occurred one month later than the date given in a star list.

Mathematical schemes developed to represent the change in the length of day and night over the year, and the change in the duration of visibility of the moon over the course of a month made use of a simple arithmetical device known to modern scholars as a “zigzag function.” The value of a zigzag function increases by a constant amount until it reaches the maximum, at which point it decreases by the same constant amount until it reaches the minimum, after which it again changes direction and increases again by the same constant amount. In early Babylonian astronomy, zigzag functions always operate within the framework of the schematic calendar. Furthermore, these zigzag functions always have an integer period, which implies that the value of the function hits the maximum and minimum each cycle.

Celestial divination is part of a wider Babylonian practice of divination. Babylonian omens, both celestial and other, are written in the form of “if … , then …” statements. In celestial omens the “if” part of the statement can refer to the appearance or some phenomenon of the moon, sun, a planet, or a star or constellation, or certain metrological phenomena. The “then” part of the omen always refers to some outcome on Earth such as war, famine, a plentiful harvest, or the death of the king. Celestial omens always concern the land as a whole, never an individual, with the exception of the king.

Three particularly important texts epitomize early Babylonian astronomy and astrology. The “Three Stars Each” texts contain star lists and values for the length of daylight following a simple mathematical scheme. Enūma Anu Enlil is a large compilation of celestial omens. In its full form, Enūma Anu Enlil was written across about seventy tablets. In addition to omens, one tablet of Enūma Anu Enlil contains mathematical schemes for the length of day and night and the duration of visibility of the moon. Finally, MUL.APIN contains star lists, intercalation rules, mathematical schemes, and a small set of celestial omens. The date of composition of these three texts is not known, but they were all widely copied in later periods.

The Late Neo-Assyrian Period (c. 725–650 bce)

A large number of cuneiform tablets relating to astronomy and astrology are preserved from the Neo-Assyrian cities of Nineveh, Assur, and Calhu. These tablets include copies of earlier astronomical works such as MUL.APIN and Enūma Anu Enlil as well as a large body of correspondence sent to two Assyrian kings, Esarhaddon and his successor Ashurbanipal, by their scholarly advisers. This correspondence concerns the interpretation of ominous events and what measures the king should take in response to the omens.⁷ Celestial omens feature prominently in this material. The correspondence comprises two types of texts: brief reports of celestial observations quoting the relevant omen, and longer letters from scholars to the king providing further information on the interpretations of omens and giving their advice.

Among the observed celestial phenomena reported in this correspondence are lunar and solar eclipses, the appearance of the new moon crescent, the appearance of the moon, the sun, or a planet, haloes, the movement and phenomena of the planets, comets, and meteors. The omens are often, but not always, quoted from Enūma Anu Enlil. Further interpretation of the omens was quoted from established written commentaries, from oral tradition, or from a scholar’s own expertise. In some cases, the scholars would advise the king that it was necessary for a particular ritual to be performed in order to ensure that no harm would come to him or to the land because of the omen.

The Late Babylonian Period (c. 750 bce–100 ce)

A large number of cuneiform tablets from the Late Babylonian period attest to a wide variety of astronomical and astrological practices. The largest number of these tablets, several thousand, come from the city of Babylon in central Babylonia; a smaller number of tablets, a few hundred, were found at the city of Uruk in southern Babylonia. Many of the tablets can be connected directly or indirectly to small groups of scholars who were associated with the temples in these cities.⁸

Central to Late Babylonian astronomy was a tradition of regular and systematic observation. These observations were recorded in texts known to modern scholars as “Astronomical Diaries” (their Akkadian name translates as “Regular Watching”).⁹ Most of the reports found in the Diaries concern cyclical astronomical phenomena that may in principle be predicted in advance such as the first and last appearances, stations, and acronychal risings of the planets, the date on which a planet makes its closest approach to a star and its distance “above” or “below” the star, eclipses of the sun and moon, and the time interval between the moon and the sun rising or setting measured on six specific occasions during the month (named by modern scholars the “lunar six”).

The observations recorded in the Diaries were used to develop two types of methods for predicting future astronomical phenomena. The first method, called by modern scholars “goal-year astronomy,” uses planetary and lunar periods to predict future astronomical events from previous observations. For example, the Babylonian scholars discovered that after forty-six years the phenomena of Mercury occurred on more or less the same day in the Babylonian calendar. It is, therefore, possible to predict Mercury’s phenomena for a coming year by simply taking the observations made forty-six years earlier and making small corrections to take into account the known inaccuracy of the forty-six-year period and, if necessary, intercalation. The same method but with different numbers of years can be used for the other planets.¹⁰ Related, but slightly more complicated methods were developed for predicting eclipses and the lunar six.¹¹

The second method of prediction is called by modern scholars “mathematical astronomy.” In mathematical astronomy, complex arithmetical functions, often the sum of two or more other functions, are used to compute planetary and lunar phenomena independently from direct observational input. Two principal methods were developed: “System A,” which uses a step function to compute the difference in longitude between two successive occurrences of a phenomenon, and “System B,” which uses zigzag functions with non-integer periods to compute the same thing. The lunar systems in particular display a high level of both astronomical understanding and sophisticated mathematical thinking.

A crucial development in Babylonian astronomy took place in the 5th century bce with the creation of the zodiac as a uniform division of the path through which the sun, moon, and planets move into twelve equal-length parts, each of which contain 30 UŠ or degrees.¹² The signs of the zodiac were named after constellations which fell within them. Many of these names remain in use today. For example, the Twins is our Gemini, the Crab is our Cancer, the Lion is our Leo, etc.

Alongside observation and predictive astronomy, new forms of astrology emerged in Babylonia during the Late Babylonian period, including the prediction of the life of an individual based upon the configuration of the sun, moon, and planets at the moment of birth (horoscopy), schemes relating dates, signs of the zodiac and the ingredients used to make medical remedies (astral medicine), and astrological methods for predicting increases and decreases in the values of basic commodities and the weather.

Legacy

Many aspects of Babylonian astronomy and astrology circulated to other ancient cultures. For example, Hipparchus and Ptolemy had access to and used some Babylonian records of lunar eclipses,¹³ and Ptolemy used some Babylonian planetary observations.¹⁴ Babylonian mathematical astronomy appears to have been fairly widely known in Graeco-Roman Egypt, and several Greek astronomers, including Hipparchus, Geminus, and Ptolemy had access to at least some of the parameters of the lunar System B. Many aspects of Babylonian astrology also circulated to the Graeco-Roman world.¹⁵ Some parts of Babylonian astronomy and astrology were also incorporated into Indian traditions.