ESHS/HSS Annual Meeting

A Step in the Right Direction? Debates over Simplifying Calculations on the Celestial Sphere with the Directorium (ca. 1330)

• In Event: Contested Success and Celebrated Failure: The Practice of Mathematical Instruments in Varied Settings II

English Abstract

In the 1320s, the astronomer and master of arts, John of Lignères, composed several treatises on astronomical instruments, which appear to have served, in part, to augment instruction in the astral sciences for students at the University of Paris. One such text ascribed to him gives instructions in the composition and use of an instrument called the directorium (BnF lat. 7295, Erfurt 4o 349, Oxford Digby 48, etc.). Primarily used for astrological calculations, the directorium was constructed in much the same way as the astrolabe. One remarkable difference, however, is that the local horizon and meridian of the directorium was made to rotate about a fixed celestial equator and ecliptic, thereby simulating a rotating Earth and stationary heavenly vault. John of Lignères’ student, John of Saxony, referred to the instrument in lectures he gave to students of the arts faculty in 1331. He recommended working with a directorium as a supplement to the lengthy and complex operations needed to compute planetary directions with tables. By invoking the directorium, however, John of Saxony also expressed misgivings over the limitations of using instruments to perform precise computations. His excursus serves to highlight several tensions at play among astronomers in fourteenth-century Paris, which manifested themselves as debates over the relative importance of precision, representation, and computation in astronomical instrument design.

Author

• Nicholas A. Jacobson, University of Chicago