Orateur
Description
In mediaeval mathematical astronomy, the positions of the planets and luminaries, as well as eclipse predictions, were calculated using sets of astronomical tables. The defining principle of this approach is the use of tables of mean motions. This requires the tacit assumption of a mean solar day, which is always of the same length, regardless of the season or epoch. A solar day is defined as the period of time between two subsequent meridian passages of the Sun. However, due to the Sun’s irregular motion throughout the year, the length of solar days varies. This inequality is described by apparent solar days. The difference between mean and apparent solar days is captured by the equation of time, which is relatively small on a daily basis, but can amount to 30 minutes over the course of a year. As daily cultural life was organized around apparent days and the time that could be read on a sundial, whereas astronomical prediction was based on mean days, the equation of time usually had to be considered. However, as the equation of time itself depends on certain changing astronomical quantities, it had to be recalculated after a certain period. In this talk, I will present a survey of different tables for the equation of time from Latin, Hebrew, Greek, and Arabic sources from the 9th to the 16th century. I will discuss how different users, compilers, and editors of sets of astronomical tables copied, corrected, recalculated, or modified tables for the equation of time. Particular emphasis will be given to the historical-mathematical practices employed to derive these different tables. As the equation of time is one of the few tables that required updating, it provides valuable insight into the evolution of mathematical practices in astronomy over time.