Babylonian Observations of Venus: Arcus Visionis, Atmospheric Extinction and Observational Practice

Abstract

1. INTRODUCTIONIn this paper analyse Babylonian observations of Venus from the second half of the first millennium b.c. to derive physical parameters determine its first and appearance as morning and/or evening star. The new feature in my approach is the use of a physical model for the visibility of Venus in a twilight atmosphere. Moreover, in this study make use of the most recent editions of the Babylonian observational material. The results provide a major advance in understanding the visibility phases of Venus compared to earlier attempts based on the use of the concept of the arcus visionis. ' The method used here has been applied before to analyse Babylonian observations of Saturn in the sixth and the seventh centuries b.c., of stars in the thirteenth century b.c., and of Venus in the seventeenth century b.c.22. THE OBSERVATIONSThe observational material analysed in this paper consists of all Babylonian records of first appearance and appearance of Venus in the Astronomical diaries and related texts from Babylonia, vols i-iii, ? and vi.3 This body of observational texts (henceforth referred to as Diaries and related texts) contains a large - although quite fragmentary - body of naked-eye astronomical observations of the Moon and the planets covering a period of many centuries (650-50 b.c.), and thereby form a unique database for the study of the astronomical practice in Babylon during the second half of the first millennium b.c.Among the observational material in the Diaries and related texts, the first appearances and appearances of the planets take a prominent place. In total the Diaries and related texts contain about 180 records of the first and appearance of Venus. These observations are scattered over the six centuries b.c., and due to the arbitrary nature of the preservation process they form a random selection of all first and appearances of Venus over period. Since one of the purposes of this paper is to study the effects of the variable atmosphere made an effort to select only 'bona fide' observations. This is not a trivial matter because the texts are often damaged and some dates are apparently estimated or computed rather than observed. The observations have discarded fall in several categories:* those with uncertain dates due to omissions or damaged passages in the text;* those with qualifications around, possible, expected;* those where the observer indicated no observations were carried out by recording clouds (DIR) or stating I did not watch (NU PAP); and* those given in the monthly summaries at the end of the month, because in cases where it can be verified these often turn out to be the expected 'ideal' dates rather than actually observed dates.The total number of observational records retained amounts to 92 observations (about 50% of all preserved records) over the period 525-80 b.c., and may be considered as a randomly selected sample comprising 8% of all first and appearances of Venus over period.Throughout the Diaries and related texts the Babylonian term SU (setting) has been translated as last Based on an analysis of Venus observations available at the time, Huber et al.4 pointed out the data make more sense if SU is translated as disappearance, consistent with its actual meaning. This is in agreement with the fact in the earliest set of Venus observations from the time of Ammisaduqa (seventeenth century b.c.) the verb it-bal is used which means disappeared.5 In a footnote on page X of the Diaries and related texts, vol. vi, the editors confess to having become convinced that by SU the texts refer to the first day when a planet could not be seen anymore but in the translation they would stick to last appearance for reasons of consistency. In the analysis below take SU to refer to the disappearance of Venus, which falls one day later than the date of appearance. …