A Study of Babylonian Goal-Year Planetary Astronomy

Abstract

Throughout the Late Babylonian Period, Mesopotamian astronomers made nightly observations of the planets, Moon and stars. Based on these observations, they developed several different techniques for predicting future astronomical events. The present study aims to improve our understanding of a particular empirical method of prediction, which made use of planetary periods – a period of time over which a planet‘s motion recurs very closely – to predict that planet‘s future motion.

Various planetary periods are referred to in many Late Babylonian astronomical texts. By collecting together these periods and analysing their effectiveness, it was found that, generally, the most effective of the planetary periods were those which were used in the production of a particular type of text known as a Goal-Year Text. The Goal-Year Texts contain excerpts of astronomical observational records, with the planetary records having been taken from particular observation years with these planetary periods in mind – such that each planet‘s motion will recur during the same, specific, future year. It has been suggested that they form an intermediate step towards the compilation of the non-mathematical predictive texts known as Almanacs and Normal Star Almanacs.

An analysis of theoretically calculated dates of planetary events showed that, if the Goal-Year Texts were to be used as a source for making empirical predictions, particular corrections (specific to each planet) would need to be applied to the dates of the planetary records found in the Goal-Year Texts. These corrections take the form of regular corrections to the day of an event (a ―date correction‖), and more irregular corrections of ±1 month (a ―month shift‖). An extensive investigation of the Babylonian non-mathematical texts demonstrated that the observed differences in the dates of events, when comparing equivalent records in all known extant Almanacs and Normal Star Almanacs with those in the Goal-Year Texts, were extremely consistent with theoretical expectations. This lends considerable support to the theory that the Goal-Year Texts‘ records formed the ―raw data‖ used in the compilation of the Almanacs and Normal Star Almanacs.

It was also possible to analyse several other aspects of Late Babylonian non-mathematical astronomy during the course of this study. These topics include the usage of particular stars in the predictive texts, the meaning of certain terminology found in records of the Babylonian zodiacal signs, and the specific issues related to the planet Mercury‘s periods of visibility and invisibility. Therefore, this investigation enhances many aspects of our knowledge of Late Babylonian astronomical practices.