Abstract: This paper outlines the historical background of the ancient sidereal zodiac of the Babylonians. Sidereal means “of the stars”. Both the ancient Babylonian zodiac and the modern astronomical zodiac are sidereal, i.e., defined in relation to the stars belonging to the zodiacal belt. Whereas the Babylonian zodiac comprised twelve equal constellations, each 30 degrees in length, the astronomical zodiac is made up of twelve unequal-length constellations. Following the definition of the Greek astronomer Hipparchus (second century B.C.), where 30-degree constellational divisions are called signs, the constellations of the Babylonian zodiac were also zodiacal signs. In the ancient Babylonian zodiac each of the zodiacal signs/constellations were 30 degrees long. This paper considers the historical background to the sidereal zodiac of the Babylonians and how the signs/constellations of the Babylonian zodiac were defined in relation to 1st magnitude stars belonging to the zodiacal belt. A subsequent paper “Greek Star Catalogs and the Modern Astronomical Zodiac” explores the origin of the modern astronomical zodiac.
Key words: Aldebaran-Antares axis; ancient astrology; ancient astronomical observations; ancient mathematical astronomy; Babylonian star catalog; constellations; cuneiform texts; ecliptic; fiducial axis; fixed stars; history of the zodiac; mul-APIN; normal stars; zodiac; zodiacal belt; zodiacal signs.
The belt of stars marking the background of the orbits of the Sun, Moon and planets has been of interest for thousands of years. Ever since human beings began to take an interest in observing the world around them, they could not fail to notice that the Moon and the planets move against the background of fixed stars not in a random way, but in such a way as always to pass before the same fixed stars. Hence, the particular fixed stars or rather belt of fixed stars against which the Moon and planets are seen to progress, acquired a special significance. This belt of stars, known as the ?zodiacal belt,? became distinguished from the rest of the sphere of fixed stars because the paths of the Moon and the planets ? and also the Sun, although of course the apparent passage of the Sun against the background of the fixed stars generally cannot be seen with the naked eye ? all lie within the zodiacal belt. In modern astronomy the zodiacal belt is defined in relation to the apparent path of the Sun through the fixed stars, which path is taken as the middle of the belt, so that the zodiacal belt, usually taken to be 16 degrees wide, by this definition extends 8 degrees north and 8 degrees south of the apparent path of the Sun. The zodiacal belt is thus a belt of fixed stars along the middle of which runs the apparent path of the Sun and contains also the paths of the Moon and the five planets known to the ancients through naked eye observation ? Mercury, Venus, Mars, Jupiter, and Saturn. The orbits of the more recently discovered planets, Uranus and Neptune, also lie within the zodiacal belt, as defined here; but Pluto, which was discovered more recently (in 1930), since its orbital inclination is 17 degrees, periodically strays beyond the limits of 8 degrees north and 8 degrees south latitude.
The path of the Sun ? actually the apparent path of the Sun ? against the background of fixed stars is called the ?ecliptic.? This word is derived from the same root as the term ?eclipse?. Thus, when there is a total eclipse of the Sun, sometimes its position can be observed in relation to the background of fixed stars, provided that the astronomical conditions are right and a suitable location for observation is possible. Similarly, eclipses of the Moon take place only when the Moon is on, or very near, the ecliptic. Theoretically, therefore, the Sun?s apparent path, which specifies the ecliptic, could be determined in relation to the fixed stars through observation of solar and lunar eclipses.
It is important to clearly distinguish between the ecliptic and the zodiacal belt. The ecliptic is specifically the Sun?s apparent path against the background of the fixed stars, while the zodiacal belt embraces the paths of all the planets (with the exception, periodically, of Pluto)including the Sun?s path. The ecliptic is a line, a circle, through the celestial sphere, and the zodiacal belt is a region, a belt, around the celestial sphere. The relationship between them is given by the fact that the circle of the ecliptic passes through the middle of the zodiacal belt.
Ancient astronomy was concerned with observing the movements of the heavenly bodies, for which purposes the zodiacal belt ? although not necessarily defined as above ? was used as the natural frame of reference. Indeed, the zodiacal belt was the frame of reference originally used by Babylonian astronomers, who were the first to make systematic astronomical observations of the movements of the planets against the background of the fixed stars. According to the Greek astronomer Claudius Ptolemy (second century A.D.), systematic observations by Babylonian astronomers began in the first year of the reign of King Nabonassar of Babylon (747 B.C.).1 Ptolemy?s statement has been confirmed by the excavation of cuneiform texts from Babylon and Uruk, some of which contain recorded astronomical observations of eclipses going back to the eighth century B.C., or the early years of the era Nabonassar. ?Diaries? of astronomical observations by Babylonian astronomers, dating back to the seventh century B.C., give the positions of the Moon and planets within a zodiacal belt extending between 10 degrees north and 7½ degrees south of the ecliptic.2 In these early astronomical texts, the positions of the Moon and planets are given with respect to a set of 31 reference stars, called Normal Stars,3 the more prominent stars belonging to the zodiacal belt. All 31 Normal Stars used by Babylonian astronomers have been securely identified, and have been found in a zone between 10 degrees north and 7½ degrees south latitude.4 In the texts the position of the Moon or of a planet is given by stating that it is ?in front of? a Normal Star (which means to the west of the star), or that it is above or below the star, often in terms of the Babylonian units ?cubit? and ?finger?.
The primitive system of Normal Stars, although used throughout the period of Babylonian astronomy, until roughly the beginning of the Christian era, became superseded for most practical purposes by a new system in which the positions of the Sun, Moon, and planets came to be given in terms of zodiacal signs. The first recorded use in Babylonian astronomy of the new system, a list of solar eclipses given in signs of the zodiac, belongs to the first half of the fifth century B.C., the eclipses extending from 475 to 457 B.C.5 In this system the zodiacal belt is divided into twelve equal sectors or signs, each 30 degrees long, where the signs are defined with respect to fixed stars in the zodiacal belt. The relationship between the Normal Star system and the system of zodiacal signs can be determined from a fragment of a catalog of Normal Stars, belonging probably to the fourth century B.C., and also in conjunction with readings from cuneiform texts giving planetary or lunar positions in both systems simultaneously. For example, the catalog gives the Normal Star α Librae in terms of the system of zodiacal signs as 20˚ Libra, i.e. the star α Librae is located at 20 degrees in the Babylonian (sidereal) sign of Libra. Similarly, the longitude of β Librae is 25˚ Libra, or 25 degrees in the Babylonian (sidereal) sign of Libra.6 Peter Huber, using all available data at his disposal, determined the zero point (0˚ Aries) of the Babylonian zodiac,7 and the Babylonian sidereal zodiac has been reconstructed in its entirety by cataloging Ptolemy?s 1022 stars, listed in his star catalog from the Almagest, in terms of the Babylonian signs of the zodiac.8
This is not the place to enter into the complex early history of the zodiac in Mesopotamia, which is discussed by B.L. van der Waerden in his paper ?History of the Zodiac.?9 However, it is necessary to add a few brief remarks concerning the relationship of Normal Stars to the Babylonian signs of the zodiac, since this is crucial to our consideration of the origin of the Babylonian zodiac. How did the Normal Stars come to be assigned given degree positions in the various Babylonian zodiacal signs? Was it a purely arbitrary process, or is it possible to find some underlying structure in the relationship between Normal Stars and zodiacal signs?
Among the list of 31 Normal Stars there are five stars of 1st magnitude: Aldebaran, Pollux, Regulus, Spica, and Antares. It is reasonable to assume that for Babylonian astronomers, making their observations of the Moon and planets in relation to Normal Stars, the brightest of these stars would assume greater significance ? just as is the case in modern popular astronomy. A conjunction of the Moon with Spica is more noteworthy than a conjunction with its relatively faint neighbor, Porrima. Hence, the first magnitude Normal Stars ? Aldebaran, Pollux, Regulus, Spica, and Antares ? are of primary importance in considering the inherent structure of the Babylonian signs of the zodiac. From Peter Huber?s analysis of the location of the zero point (0˚ Aries) of the Babylonian zodiac, the longitudes of these first magnitude stars may be reconstructed. To the nearest degree, their positions in the Babylonian zodiac are: Aldebaran (α Tauri) = 15˚ Taurus, Pollux (β Geminorum) = 29˚ Gemini, Regulus (α Leonis) = 5˚ Leo, Spica (α Virginis) = 29˚ Virgo, Antares (α Scorpii) = 15˚ Scorpio.10 This reconstruction reveals the remarkable fact that the two 1st magnitude stars, Aldebaran and Antares, lie diametrically opposite one another in the zodiac, each in the center of their respective signs.
These two 1st magnitude Normal Stars thereby stand out from the other three 1st magnitude Normal Stars, and indeed from all the Normal Stars, since the rising of one coincides with the setting of the other. Thereby they divide the zodiacal belt exactly in two. This striking property, relating to two of the brightest stars in the zodiacal belt, evidently led to these two stars being chosen as the ?fiducial axis? for the Babylonian zodiac, i.e., an axis dividing the zodiac in half ? in relation to which the stellar longitudes of the other Normal Stars could be measured. With the increasing astronomical and mathematical prowess of the Babylonians, and the need for better, more accurate ephemerides for the prediction of eclipses and other planetary phenomena, the adoption of such a reference axis for measurement and computation was necessitated, and undoubtedly it evolved naturally from the more primitive system of Normal Stars. The two stars ? Aldebaran and Antares ? specifying this reference axis of the Babylonian zodiac were defined to be in the center of the zodiacal signs of Taurus and Scorpio rather than elsewhere (for example, at the beginning of their respective signs), as they both lie centrally in groups of stars which had long been recognized as distinct stellar configurations marking the constellations of Taurus and Scorpio. Confirmation of this line of reasoning is to be found in excerpts relating to two Greek astrological texts:
Cleomedes states (De motu I, 11, p. 106,25 to 108,5 Ziegler) that there exist two bright stars such that the rising of one coincides with the setting of the other: Aldebaran (α Tauri) and Antares (α Scorpii), both being located at the 15th degree of their respective sign.11
?the diametrically opposite positions of Aldebaran and Antares in Taurus 15 degrees and Scorpio 15 degrees, respectively?is also given in a Greek treatise which goes under the name of the ?Anonymous of the Year 379?.12
The Greek astrologer Hephaestion of Thebes (fourth century A.D.) also lists the longitude of Aldebaran as 15˚ Taurus.13
Why, however, should these statements drawn from Greek astrology have any bearing on the Babylonian zodiac? Greek astrology is relevant here, since it is know that Greek astrologers were the direct recipients of Babylonian star lore, as is evident in the case of the astrological school founded by Berossos on the Greek island of Cos early in the third century B.C.14 From sources such as Berossos, Babylonian star lore was transmitted to Greece and became incorporated into the corpus of Greek astrology. The statements of Greek astrologers may thus offer direct insight into the nature of Babylonian astronomy, and in this instance the singling out of Aldebaran and Antares from all other stars, by virtue of their special relationship to one another, could reflect the reasoning underlying the original definition of the signs of the Babylonian zodiac as the system to replace the system of Normal Stars. Hence, we may suppose that at some time in the sixth (or early fifth) century B.C. some Babylonian astronomer (or group of astronomers), while making observations of the Moon and planets in relation to Normal Stars, realized that two of the most prominent Normal Stars, Aldebaran and Antares, divide the zodiac in half, and that the axis between them could therefore serve as a reference axis for all the stars of the zodiacal belt. In this way, therefore, the Normal Stars came to be related to a new system, namely the system of zodiacal signs in which the zodiacal belt was divided into twelve 30-degree sectors or signs, with the two signs Taurus and Scorpio defined so that Aldebaran and Antares were located at the center of these signs, respectively ? Aldebaran at 15˚ Taurus and Antares at 15˚ Scorpio. With this as the basic, initial definition of the structure of the Babylonian zodiac, it was then simply a matter of measuring the distance in degrees of other Normal Stars from the Aldebaran-Antares axis in order to deduce the longitudes of Normal Stars in the various signs, with Regulus at 5˚ Leo, Spica at 29˚ Virgo, etc.15
With respect to the question as to why the zodiacal belt of Normal Stars was divided into twelve signs, each 30 degrees long, B.L. van der Waerden, in ?History of the Zodiac,? writes:
There are twelve signs, because there are twelve months in the schematic year of mul-APIN. The signs were made of equal length in order to get months of equal duration; they were divided in 30 degrees each because the schematical months were supposed to contain 30 days each.16
In other words, in the sixth (or early fifth) century B.C., when the division of the zodiacal belt of Normal Stars into twelve signs was originally formulated, there already existed a schematic calendar devised earlier by Babylonian astronomers, which we know of from the text mul-APIN. Mul-APIN consists of two tablets, dated around 700 B.C., in which are listed the rising of 18 bright stars and constellations in the zodiacal belt, in terms of a schematic year of twelve months each 30 days long.17 This is a ?schematic? year, because the actual civil calendar in Babylon operated with lunar months, which fluctuate in length, being either 29 or 30 days long, and in an intercalation year (roughly every third year) there were thirteen instead of twelve lunar months. The mul-APIN calendar scheme thus represented an idealized year: the ideal of the actual year of twelve (or thirteen) variable-length lunar months. With this scheme already in existence, the originator of the system of zodiacal signs was influenced by it in such a way as to specify a twelvefold division of the zodiacal belt into signs, each sign consisting of 30 degrees, analogous to the twelvefold division of the year into schematic months, with each month consisting of 30 days. Once the idea of this division of the zodiacal belt, analogous to the schematic division of the year, had been formulated, it was simply a matter of defining where the signs should lie in relation to the Normal Stars comprising the zodiacal belt.
In conclusion, then, the Babylonian zodiac originated in the sixth (or early fifth) century B.C. It was devised as an alternative system to that of the Normal Stars belonging to the zodiacal belt. The division of the zodiacal belt into twelve signs each 30 degrees long was analogous to the schematic division of the year into twelve months, each 30 days long, formulated in the text mul-APIN around 700 B.C. The relationship between the Normal Stars belonging to the zodiacal belt and the division into zodiacal signs was specified by the adoption of the Aldebaran-Antares axis as the fiducial axis for the Babylonian zodiac, with Aldebaran at the middle of the sign of Taurus and Antares at the middle of the sign of Scorpio. Once adopted, the longitudes of the remaining Normal Stars were defined in terms of sign and degree in the Babylonian zodiac, by determining their distances from the Aldebaran-Antares axis. The relationship between Normal Stars and the Babylonian zodiac was recorded in a star catalog, probably the world?s first star catalog, thus constituting the definition of the Babylonian zodiac. In this way the transition from the system of Normal Stars to the system of zodiacal signs was accomplished, and herein lies the origin of the Babylonian zodiac.
1 Ptolemy, Almagest III (ed. J.L. Heiberg, Leipzig, 1898, p. 254).
2 Cf. O. Neugebauer, A History of Ancient Mathematical Astronomy (3 vols; Berlin-Heidelberg-New York, 1975), vol. i, pp. 545-547; hereafter cited as HAMA.
3 The terminology ?Normal Stars? was introduced by J. Epping,Astronomisches aus Babylon (Freiburg i. Br., 1889), p. 115.
4 Cf. A. Sachs, ?Babylonian Observational Astronomy,? Philosophical Transactions of the Royal Society of London, Series A, 276 (1974), pp. 43-50, esp. p. 46.
5 Cf. A. Aaboe-A. Sachs, ?Two Lunar Texts of the Achaemenid Period from Babylon,? Centaurus 14 (1969), pp. 1-22, esp. p. 17.
6 Cf. A. Sachs, ?A Late Babylonian Star Catalogue,? Journal of Cuneiform Studies 6 (1952), pp. 146-150, esp. p. 146.
7 P. Huber, ?Über den Nullpunkt der babylonischen Eklipik,? Centaurus5 (1958), pp. 192-208.
8 R.A. Powell, History of the Zodiac (unsubmitted doctoral thesis).
9 B.L. van der Waerden, ?History of the Zodiac,? Archiv für Orientforschung 16 (1953), pp. 216-230.
10 Cf. R.A. Powell, op. cit., apppendix 1.
11 O. Neugebauer, HAMA ii, p. 960.
13 Cf. O. Neugebauer-H.P. van Hoesen, Greek Horoscopes (Philadelphia, 1959), p. 187.
14 Cf. P. Schnabel, Berossos und die babylonisch-hellenistische Literatur(Leipzig, 1923),pp. 250-275 for Greek fragments of Berosos? writings.
15 Abraham Sachs? reading (see note 6) for Spica from the late Babylonian star catalog is 28˚ Virgo, which does not really affect the line of reasoning presented here, since it may simply reflect a degree of inaccuracy inherent in the measurement of stellar longitudes by Babylonian astronomers.
16 B.L. van der Waerden, op. cit., p. 218.
17 Cf. E. Weidner, ?Ein babylonisches Kompedium der Himmelskunde,? American Journal of Semitic Languages and Literatures 40 (1924), pp. 186-208; cf. also, E. Weidner, ?Der Tierkrreis und die Wege am Himmel,? Archiv für Orientforschung 7 (1931-32), pp. 170-178, and W.K. Pritchett ? B.L. van der Warden, ?Thucydidean Time-Reckoning and Euctemon?s Seasonal Calendar,? Bulletin de Correspondance Hellénique85 (1961), pp. 17-51, esp. pp. 43ff.