A study of Babylonian records of planetary stations (Q2040446)

A planetary station is the point when and where a planet appears to be stationary in the sky. This happens at the beginning and end of retrograde motion with the start of retrograde motion happening in the morning and its ending taking place in the evening. Planetary stations are relatively easy phenomena to observe since the planet does not move significantly for several days/nights and the event often happens when the sky is dark, as opposed to first and last visibilities of planets, which tend to take place near the horizon and present more challenges to observation. Planetary stations are believed to be a cornerstone of Late Babylonian astronomy. Planetary stations are represented both in observational records, as for example in the astronomical diaries, and predictions in almanacs and normal star almanacs. Together these form a collection of several hundred records and the authors have constructed a comprehensive database to probe this corpus more deeply. The authors make some interesting determinations. Most of the results concern three planets: Mars, Jupiter, and Saturn. Results are broadly consistent across the planets, with one glaring exception. For five of the six stations (east and west for each planet) both predictions and observations have early dates compared to modern calculations. Saturn's eastern stations are consistently dated later than modern computations give. No explanation for this anomaly is offered. Predicted and observed dates are in good agreement. Babylonian astronomers considered a planetary station not as a point, as it is viewed in modern astronomy, but as an interval, and were interested in measuring the beginning of that interval. They were more concerned about position of the event than date, and observations used predicted dates as a guide for when to view the phenomena. Planetary station data were recorded in three different formats. Type Z merely located the station in a sign of the zodiac; Type S gave the position of the station with reference to a Normal Star, also noting whether the event was in the east or the west, and the less common intermediate Type I also used Normal Stars, but did not mention whether it was in the east or west. Predicted stations were always given in Type Z format; observational records varied across the types. There does not seem to be much chronological change in the choice of types.