Gerberto e le fistulae: tubi acustici ed astronomici

Abstract

Gerbert of Aurillac wrote to Constantine of Fleury in 978 a letter to describe in detail the procedure to point the star nearest to the North celestial pole. This was made to align a sphere equipped with tubes to observe the celestial pole, the polar circles, the solstices and equinoxes. The use of tubes in astronomical observation is later reported by Alhazen in his treatise on optics (1011-1021). The description of pointing to the celestial pole indicates that the instrument must be accurately aligned with the true pole, materialized at that epoch by a star of fifth magnitude, at the limit of naked eye visibility, and then the instrument must remain fixed. Solstices and Equinoxes are points of the orbit of the Sun, so the sphere could be used as a tool for observing the Sun and probably determine the duration of the tropical year. This sphere was much more than a didactic tool, given the long procedure for the accurate alignement. Moreover "Rogatus a pluribus" (asked by his many students), Gerbert wrote a treatise on acoustic tubes (fistulae) in 980: Mensura Fistularum. He knew the difference in behavior of the fistulae compared with the acoustic strings, already studied by the Pythagoreans, and the treaty is intended to present the law that governs the length of the organ pipes in two octaves, compared to the corresponding acoustic strings. In terms of modern physics we know that acoustic tubes require an "end correction" to be tuned, which is proportional to the diameter of the tube. This proportionality is the same for every note. The mathematical law is simple, but Gerbert preferred to create a law in which the proportions of pipes and strings should be calculated through a series of fractions linked to the number 12 and its multiples.