Babylonian Astronomy and Astrology



Ancient Babylonia occupies a pivotal place in the history of modern scientific astronomy. In great part this is due to the conscientious nature of the astronomical observations that were made there and the meticulous way in which they were recorded for generation after generation. In time, the existence of a huge, cumulative database of past observations made possible the development of mathematically based rules for predicting future events. The Babylonian legacy of careful observation and recording combined with mathematical modeling went on to influence developments in ancient Greece and beyond. The other reason ancient Babylonia is so important to modern historians of astronomy is the fortunate choice of medium on which many of the ancient astronomical observations (along with many other documents) were recorded. The method used was to press wedge-shaped marks into smooth, damp tablets of clay using a stylus. Subsequently, the tablets were dried in the sun or fired in kilns for permanence. Clay tablets do not tend to disintegrate with time like (say) parchments or papyri and are unaffected by subsequent fire, so they frequently survived the looting or destruction of buildings and other cataclysmic events of history. The Babylonian cuneiform script was deciphered in the nineteenth century. In short, many high-quality records have survived, and they can be read.

The ancient city-state of Babylon lay some 90 kilometers (55 miles) south of modern Baghdad. Its power and influence came to cover all of lower (southern) Mesopotamia—the region of modern Iraq between the Tigris and Euphrates rivers down to the Persian Gulf—in the eighteenth century b.c.e., after which it followed a turbulent history under a succession of dynasties until its conquest by the Persians in 539 b.c.e. Subsequently, Babylon became part of greater empires: the Persian until 331 b.c.e., when it was conquered by Alexander the Great; then (after Alexander’s death) the Se-leucid Empire; it ultimately fell to the Romans in 63 b.c.e. The latest known cuneiform tablet dates to c.e. 75.

Most of the written evidence that comes down to us is in the form of clay tablets from the Seleucid period from 311 b.c.e. onwards. Those including astronomical data are of various types: astronomical diaries containing nightly observations; records of sightings of astronomical events such as planetary conjunctions and eclipses; and (increasingly with time) almanacs containing predictions of the length of the month, the positions of the planets among the fixed stars, and many other things. These documents demonstrate beautifully how the systematic accumulation of carefully recorded passive observations led in time to the ability to predict using mathematical models. One thing that made this development possible was the Babylonian system for representing numbers: like ours it used a fixed base, but instead of ten, the base was sixty. In other words, each “digit”—itself a set of strokes representing tens and ones—represented a value from zero to fifty-nine, with subsequent digits representing “units,” multiples of sixty, multiples of sixty times sixty, and so on. (The Maya, in contrast, used a base of twenty.)

In the Babylonian calendar, the new day began at sunset and the new month began when the thin crescent moon was first sighted in the evening sky after sunset. Back in the third millennium b.c.e., two calendars seem to have existed in parallel: an “ideal” calendar that was theoretical, with each month containing thirty days, and a common calendar based on actual lunar observations. The first of these calendars is used in early clay tablets that are essentially business documents: this is hardly surprising, since people had to agree on the day something had been signed or a commitment had been made, and this could not be dependent upon the vagaries of whether or not the new crescent moon had been seen (which could be a matter of some dispute, especially if it had been cloudy in certain places at critical times). This civil calendar needed to be the same all over Babylonia and not subject to disputes between different local officials.

Nonetheless, it seems that the actual astronomical calendar (rather than the abstract “ideal” calendar) was used for civil purposes from the second millennium b.c.e. onwards, despite the attendant problems. Gradually, the analysis of accumulated observations of first sightings of the new crescent moon enabled Babylonian astronomers to develop mathematical “rules of thumb” that permitted month lengths to be accurately and reliably predicted. Furthermore, by about the fifth century b.c.e., the nineteen-month (Metonic) intercalation cycle had been discovered and established. It provided a rule whereby the additional (intercalary) months needed to keep the lunar calendar in step with the seasonal (solar) year could be added in a mechanical, deterministic, yet reliable way. Astronomers no longer had to depend upon independent astronomical observations, such as the heliacal rising of Sirius.

One of the most remarkable consequences of the Babylonian astronomers’ attention to detail and the sheer volume of records that they accumulated over many generations was the recognition of the so-called Saros cycle: it described the fact that if a lunar eclipse occurs, others will tend to follow at regular intervals of eighteen years and eleven days for many centuries thereafter. This discovery was no mean feat, since approximately forty different Saros cycles run simultaneously, and the conditions for visibility of any particular eclipse vary considerably—many are completely invisible from any given location on the earth (if they occur during daytime, for example). They simply cannot be revealed by casual observations of lunar eclipses over a few years or even decades, even if clear skies were permanently assured.

In view of its undoubted influence on the development of modern astronomy, it is tempting to view the Babylonian tradition as the birthplace of scientific investigation of the heavens. But this would be highly misleading. The main motivation for the Babylonians’ intense interest in the skies was astrological. Different days in the common calendar were associated with different prognostications, and by the seventh century b.c.e., scholars were advising the Assyrian king of the calendrical omens. A particular concern was the issue of whether the forthcoming month would have twenty-nine or thirty days. Other concerns included the length of day and night, the heliacal events of stars, the positions of the planets, and of course the occurrence of solar and lunar eclipses. One of the most important series of celestially related clay tablets has become known as the Enuma Anu Enlil, meaning “When [the great gods] Anu and Enlil. . .” (the first line of the text). This document, which dates to around the end of the second or the beginning of the first millennium b.c.e., runs to seventy tablets, and multiple copies exist. It contains around seven thousand omens accumulated from past experience and provides advice as to whether certain celestial configurations and events—signals from the gods—would indicate their pleasure or displeasure.

This was not astrology in the sense that celestial configurations were perceived as the direct cause of terrestrial events (although this did become a widespread philosophy from the fourth century b.c.e. onwards in Hellenistic Greece) but rather that they provided portents of events that could then, if necessary, be averted by taking appropriate action. In this, astronomical predictions were used along with a variety of other forms of divination.

Ancient Babylonia was also the birthplace of modern horoscopic astrology, or at least the earliest known example of the belief in the predictive capabilities of charts recording planetary positions at the moment of a person’s birth. (Actually, the horoscopes of modern popular astrology represent a revival of this belief rather than any sort of continuity of tradition.) An important prerequisite was the division of the zodiac (through which the plan ets move) into twelve regions of equal size. Birth charts began to appear in the second half of the first millennium b.c.e. and represented a move away from the astrologers having to watch the skies passively, waiting for omens to appear, to the more active pursuit (performed on demand) of calculating where the planets would have been among the stars at a particular time. It also represented a shift away from observational astronomy toward intensive mathematics. It is ironic, in view of the way in which modern astrology is seen as the very antithesis of modern science—irrational and unscientific— that this astrological innovation in Babylonian times necessitated making full use of the most up-to-date scientific knowledge and methods that had been developed by this time.

From the late third century b.c.e. onward, two fundamentally different schools of thought emerged for generating predictions from the extensive records of existing observations. These seem to have coexisted throughout the final few centuries of ancient Babylon (until the late first century c.e.), to judge by two types of works—mathematical ephemerides and almanacs known as Goal Year Texts—that were evidently produced in parallel. The ephemerides represent the height of Babylonian scientific achievement, using sophisticated mathematical models to predict phenomena of the moon and planets with remarkable accuracy. The Goal Year Texts, on the other hand— each one a sort of astrological handbook for a given year—seem to represent an independent tradition of prediction based upon repeating cycles that had been discovered by studying the existing diaries of observations (the Saros cycle was one of these).

There is a great deal still to be learned about the nature of astronomical and astrological knowledge in ancient Babylonia and the social context in which it operated. Though about three thousand fragments of clay tablets containing astronomical information are currently known to exist, a huge amount of basic data simply remains unexplored. There are tens of thousands of fragments of clay tablets in the British Museum alone, many tens of thousands more in other museums around the world, and untold quantities still buried under the ground in modern Iraq. Since a sizeable proportion of the clay tablets that have been studied contain astronomical information, there is every reason to expect the same to be true in the future. And while many of the museum specimens are of unknown provenance, only eventually having found their way into the public domain after progressing along tortuous routes, some of those still waiting to be discovered may be excavated in a context that will yield valuable archaeological information about their broader function and significance.

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