Babylonian Astronomy

This chapter discusses four Asian civilizations corresponding to four geographical areas: Mesopotamia, the Indo-Sarasvati valley, China, and Japan. The reason for giving such a brief account is that, In contrast to the history of astronomy studies based on written documents, field archae-oastronomy in Asia is less developed. For instance, various treaties of Babylonian astronomy were written on tablets and have been carefully studied, but a systematic study of the astronomical orientations of the Ziggurats, the big Babylonian pyramids built in raw bricks, is lacking. On the other hand, because of written sources, we know a lot about the development of astronomy from ancient times not only in Mesopotamia but also in India and China, and therefore it would have been impossible not to mention these countries in this book.

Mesopotamia, in the modern Iraq, is the flooding bed of the Tigris and Euphrates rivers. It is therefore very fertile even though, in contrast to the Nile, the floods of these two rivers are very irregular. As is well known, a consolidated tradition considers Mesopotamia the cradle of all civilizations because of the development of the Sumerian cities, which started in the fifth millennium BC. It is, however, hard to say how true this dogma is in reality. In Jericho, in Palestine, people already lived in a fortified city with 7-meter-tall walls many millennia earlier, and the first settlements in Anatolia and in the Indo-Sarasvati valley occurred approximately around that time as well (after all, even the term civilization is an extremely subtle one to define). The first dating of Mesopotamia itself is complicated by unclear stratigraphy and, moreover, by the fact that we do not know the real size of the Persian Gulf in

G. Magli, Mysteries and Discoveries of Archaeoastronomy, DOI: 10.1007/978-0-387-76566-2_5, 97 © Praxis Publishing, Ltd. 2009

ancient times. In any case, the Sumerians were among the first people to leave written evidence of their activities, including navigation; hydraulic structures; huge urbanization (archaeologists believe that their cities could have had up to one hundred thousand inhabitants); monumental architecture, with the building of the gigantic Ziggurat; and literature and the arts. The written evidence that the Sumerians left is quite complete, and, in addition to literary, scientific, and educational texts, we have religious rituals, lists of kings, royal decrees, and court sentences (Kramer 1963).

The Sumerian civilization, with its first traces left in Eridu around the year 5000 BC, ends in about 2300 BC, with the reign of Sargon the Great, soon to be substituted by the Assyrian domination in the north, and by the Hammurabi's kingdom around the year 1700 BC.

Differently from the Assyrian state, which will last over a thousand year, the Babylonian state is not strong enough and will suffer many dominations: by the Amorites, the Cassites, and the Caldeis. To this group belongs Nebuchadnezzar, the last great Babylonian king who reigned for 43 years during the seventh century BC; in 539 BC, Babylon was finally conquered by the Persians.

I use the term Babylonian to refer to the whole culture of the Mesopotamia area, as is usually done. Because of the many findings of texts written on clay tablets belonging to royal archives and libraries, we know Babylonian astronomy much better than the astronomical lore of the many people for whom we have only traces, such as for the Megalithic people, as we have seen, who probably did not have a written culture, or the Egyptians, of whom we do not have papyri of explicit astronomical content. It is therefore hard to decide, as Neugebauer (1975) tried to do—influencing in a negative way, at least in my view, the whole discipline of the history of ancient science—if Babylonian astronomy was really so more advanced than the other astronomies. In any case, Babylonian astronomy, as with many other ancient astronomies, was linked to astrology and divination, meaning the prediction of future events through a series of different operations (such as hepatoscopy, examining the liver of sheep; see Pettinato 1998). There was, therefore, no distinction between astronomy and astrology, a distinction that is unnecessary for us to understand in the historical context, while today it is fundamental because "modern astrology" does not have anything to do with the observation of the celestial bodies.

Babylonian astronomy was impressively complete and accurate; its development was connected with that of mathematics as well. The Babylonian number system was based on 60. It is a base that does not have an obvious interpretation (base 10 entails counting using the fingers; base 20 entails using the fingers and toes; and so forth). Teon in the fourth

Figure 5.1: A reconstruction of the Ur Ziggurat as it may have appeared in the 6 century BC

century AD supposed that the system with base 60 could come from choosing a minimum common multiple of 1,2, 3, 4, and 5. Then, however, it would be necessary to explain why such a multiple was chosen, and this invalidates Teon's explanation. Neugebauer, in turn, suggested that a system that was originally based on 10 was later modified in order to divide weighs and measurements into thirds. But it would be necessary to explain why this division into thirds was necessary, and Neugebauer's explanation is inadequate. Some further theories have been based on astronomy, but they are not proved either. Thus, we do not know why a numerical system based on 60 was used, but it was a system that worked just like any other one. Interestingly, as a consequence of this choice the Babylonians invented the measure of angles that we still use today: the full circle is divided in 360 units (the degrees), probably because 360 is the multiple of 60 closest to the number of days in a year. The degree is further divided in 60 primes (or arc minutes), and in an arc minute there are 60 arc seconds. I insist that this is an arbitrary choice (to divide an angle in 360 units is as natural as dividing it into, say, 127 units), and for us who inherited it from the Greeks and the Romans it is anything but an easy division, because our number system is decimal and therefore it forces us to repeated, unnatural mind calculations (as the introduction of the Euro currency in Europe a few years ago demonstrated, mind-computing often leads to mistakes).

The astronomical measurements of the Babylonians were extremely precise. Some of the measurements found in the Babylonians lists are indeed accurate to one arc minute. This is a noteworthy achievement. Think how small to the naked eye is an angle of 1 degree. And now divide it by 60. This would seem to be a level of precision that the unaided human eye could not achieve, also if the visual abilities of humans in the past were better than ours. Astronomers used a surveyor's cross, an instrument that helps make triangulations, and an artificial horizon (a wall with a perfectly straight top), which facilitates making astronomical observations. However, with these methods, the maximum observable accuracy is in the order of two arc minutes. It is therefore probable that the Babylonians invented an instrument to improve vision, a telescope (usually this word is preceded by the adjective rudimental, but I wonder why it should). There are indeed ancient tablets that mention astronomers' lenses supported by a golden tube to enlarge the pupil, and in Nineveh a rock crystal lens was found (Pettinato 1998). Maybe one day a new archaeological excavation will find a Babylonian telescope for the first time.

The oldest known Babylonian astronomical treatise is the Mul-apin, of which several copies on clay tablets are available. The earliest example is dated 687 BC, but there is no question that the astronomical data it reported were compiled at a much earlier date. Many dates (as early as 2300 BC) have been proposed, but a recent complete analysis has shown that the date were most likely compiled around 1370 BC at a latitude of around 35 degrees. It belongs, therefore, to the Assyrian period and geographical area (Schaefer 2007). The content of the text is divided as follows:

1. A list of celestial objects divided in three paths, or "belts," called Enlil, Anu, and Ea. The list contains 71 objects, some of which are planets, others of which are single stars and constellations. Enlil, for instance, contains the Crab (Cancer) and the Lion (Leo); Anu contains the Pleiades, Taurus, Sirius, and Orion; and Ea contains Aquarius and Scorpio.

2. A list of the heliacal rising of various stars.

3. A list of the simultaneous rising and setting of some couples of stars.

4. A list of the time passing between the risings of the same couples.

5. A list of the simultaneous culminations and risings of some other stars.

Tablets of the same period contain lists of the months of the year, and, related to them, three constellations, one for each path, with the coordinates of the brightest star in each constellation. The idea of gathering together stars and constellations, then, is documented in Babylonian astronomy around the second half of the second millennium BC (Schaefer 2002).

Actually, the division of the ecliptic into 12 constellations and the creation of the zodiac that we use today were ideas of the Babylonian astronomers. It is important to keep in mind that, although the concept of 12 zodiacal signs is so commonly accepted, there is no physical reason whatsoever to divide the star belt of the ecliptic into exactly 12 groups. Rather, it is a cultural consequence of desire of associating a constellation with each month of a 12-month year; other cultures may represent the constellations of the ecliptic in a complete different way (as we shall see, for instance, the Mayas probably had 13 zodiacal constellations, while the Indu had 27), or even measure the motion of the sun during the year by means of completely different celestial objects of reference.

More information regarding the deep interest of the Babylonians in the configuration of the celestial bodies, Jupiter in particular, associated with the main Babylonian god Marduk, and Venus, associated with the goddess Ishtar, can be found in the kudurru, large stones used to mark off land that was acquired or donated, that date from the second half of the second millennium BC. It is indeed very likely that the representation of planets, constellations, and belts existing in the kudurru was used to indicate the dates of the contracts for land or of land donations in terms of astral configurations, and therefore represent the sky on specific days, a possibility that needs further investigation (Iwaniszewski 2004).

As I mentioned before, divination had a very important place in the life of the Babylonians, and the astronomers, acting as astrologers, used specific handbooks for this purpose. Famous among them is Enuma Anu Enlil, a series of astrological tables, of which two different versions exist. The text comprises associations between celestial events, such as the eclipses and the appearance and disappearance of Venus, and various prophecies, for example, "If there will be an eclipse on the sixteenth day of the month, the king will die, the country will become a desert, and the enemy will ruin the agriculture fields." The suggestion provoked by negative omens could be so intense as to force the king to exchange places on the day in question with a royal substitute, a poor man (initially chosen within the aristocracy and, in a later period, chosen from the people) who would rule the country on the day that was considered unsafe for the king, and then would be killed.

The observations of the Babylonian astronomers were probably made observing the sky from the Ziggurats. The construction or reconstruction of these buildings went on for thousands of years, from the Sumerians until the sixth century BC. They were large pyramidal terraced buildings, with a rectangular or square base, built with raw bricks and covered with tiles or baked bricks. On top of these buildings there was a temple that could be reached by ascending suitable ramps. The Ziggurats could be large (more

Figure 5.2: An example of Kudurru (12 century BC circa) from Susa, today in the Louvre Museum in Paris. Among the many symbols representing divinities, the Moon, Venus and the Sun are immediately distinguishable on the top, and Hydra and Scorpio at the bottom. The upper band contains the symbols of Anu, Enlil and Ea.

Figure 5.2: An example of Kudurru (12 century BC circa) from Susa, today in the Louvre Museum in Paris. Among the many symbols representing divinities, the Moon, Venus and the Sun are immediately distinguishable on the top, and Hydra and Scorpio at the bottom. The upper band contains the symbols of Anu, Enlil and Ea.

than 50 meters wide and 60 meters tall), but those that remain today are unfortunately in terrible condition or underwent heavy restorations (for example, the Ur Ziggurat in Iraq). The most famous Ziggurat is certainly the one quoted in the Bible as the Babel Tower, the Babylonia Ziggurat, refurbished for the last time by Nebuchadnezzar. Even if the tower was destroyed by Persian fury, we miraculously inherited the plan of the building, inscribed on a tablet (now at the British Museum). Judging from the plan, it was a tower with a square base 90 meters wide and of equal height. It developed into seven squared towers of equal width and length, one on top of the other. The seven floors were probably associated with the five planets plus the sun and the moon and on its top there was a temple of Marduk.

The links between the Ziggurat and the sky were therefore very strong, but they are not yet completely understood, and to my knowledge there has been no reliable archaeoastronomical study of them. As a sample test, I have made some calculations using an archaeological map of Ur, where there is one of the largest Babylonian Ziggurats, first built around the year 2100 BC. This type of analysis is in danger of errors, first because it is impossible to know if north as indicated on the map is the true north, determined with a transit, or the magnetic north found with a compass, depending therefore on the magnetic declination of the day on which the map was drawn and thus not reliable for astronomical purposes; and second, because it cannot take into account the height of the visible horizon. With this caveat, the orientation of the building, according to the map, is clearly pointing to sunrise on the day of the summer solstice, indicated by the central access ramp on the front part of the building.

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