Studies on Babylonian goal-year astronomy II: the Babylonian calendar and goal-year methods of prediction

Abstract

This paper is the second part of an investigation into Babylonian non-mathematical astronomical texts and the relationships between Babylonian observational and predicted astronomical data. Part I (Gray and Steele 2008) showed that the predictions found in the Almanacs and Normal Star Almanacs were almost certainly made by applying Goal-Year periods to observations recorded in the Goal-Year Texts. The paper showed that the differences in dates of records between the Goal-Year Texts and the Almanacs or Normal Star Almanacs were consistent with the date corrections of a few days which, according to theoretical calculations, should be added to allow for the inexactness of Goal-Year periods. The current paper follows on from our earlier study to consider the effect of the Babylonian calendar on Goal-Year methods of prediction. Due to the fact that the Babylonian calendar year can contain either 12 or 13 months, a Goal-Year period can occasionally be month longer or shorter than usual. This suggests that there should in theory be certain points in the Metonic intercalation cycle where a predicted event occurs one Babylonian month earlier or later than the corresponding event a Goal-Year period later. By comparing dates of lunar and planetary records in the Astronomical Diaries, Goal-Year Texts, Almanacs and Normal Star Almanacs, we show that these month differences between the observational records and the predictions occur in the expected years. This lends further support to the theory that the Almanacs’ and Normal Star Almanacs’ predictions originated from records in the Goal-Year Texts, and clarifies how the Goal-Year periods were used in practice.