ASTROLABES

The so-called astrolabes constitute a fundamental corpus of early Babylonian astronomical texts. The earliest exemplar stems from the Middle Assyrian period (the reign of Tiglath-Pileser I, 1115-1077). It is probably already a copy from an earlier, perhaps Old Babylonian source, and the text was still being copied in the Seleucid period (third century or later). The term “astrolabe” is a misnomer insofar as the cuneiform exemplars are not planispheric. However, they do map out fixed stars, constellations, and even planets in various parts of the sky for the twelve months of an ideal year (360 days, i.e., twelve months of 30 days each), either by listing in groups, with the stars of the three roads set alongside one another, or by arranging them in rings in a concentric diagrammatic form, three stars per month for a total of thirty-six stars.

The groups of stars are defined by their locations with respect to the horizon, and in a rough sort of distribution into three “roads” (harrānu), differentiated by declination. The stars closest to the equator rise in their assigned month in the road of Anu, those to the north in that of Enlil, those to the south in that of Ea. These divisions of the sky, named for the three great cosmic deities, are also widely attested elsewhere, in the astronomical compendium MUL.APIN )discussed below), in prayers, scholia, and other star catalogs. The monthly rising of an astrolabe star represents the important reappearance of the star following its period of invisibility. The reemergence of the star in the sky is a seasonal event, with respect both to its date and its place of rising. The astrolabe texts selects thirty-fix heliacally rising stars (for fixed stars near to the ecliptic the heliacal rising is the first appearance in the morning following the seasonal period of invisibility when in conjunction with the sun), twelve in each of the “roads.”

The question of the purpose of the circular diagram of the astrolabe texts arises in the context of celestial mapping. The same elements are presented in the circular diagram as in an alternative list form of the astrolabe. In general, the mapping function of these texts derives from their attention to the association of fixed stars and their “roads,” and from the fact that the entire sky out the course of a year is taken into account. As Johannes Kock has pointed out (1989, 120), the circular astrolable was not an astronomical device, but rather an orientation guide and an aide to remembering which stars appear in which parts of the sky.