Searching for precision: Lorenz Eichstadt's Tabulae harmonicae coelestium motuum (Stetin 1644) and astronomical...

In the century between the creation of the first large, European astronomical observatory by Tycho Brahe in the 1580s and the national observatories of France and England in the 1660-1670s, astronomers constructed ever more sets of tables, derived from various geometrical and physical models, to compute planetary positions. But how were these tables to be evaluated? What level of precision or accuracy should be expected from mathematical astronomy? In 1644, the Stetin astronomer and calendar-maker Lorenz Eichstadt published a new set of tables, mostly cobbled together from earlier tables, which include a running commentary on how his tables might be expected to match 'observed' planetary positions. His earlier works also often display a rhetoric of 'exactitude' and 'error'. Eichstadt thus offers a case study of explicit discussions of 'precision' in mid-seventeenth astronomy. Although some tables could generate positions to arcseconds, Eichstadt argued that a regime of five arcminutes should be enough for most table users who were, presumably, computing horoscopes.