Far coincidere mappa e realtà: Giovanni Domenico Cassini e Luigi Ferdinando Marsili e le misure di longitudine per il "Danubius Pannonico-mysicus"

This contribution reconstructs the scientific exchange between Giovanni Domenico Cassini and Luigi Ferdinando Marsili (1658 – 1730) during the latter's military and diplomatic mission in Hun-gary (1695 – 1699). Combining duty with a passion for science, Marsili collected materials and ob-servations that would later form his monumental work, Danubius Pannonico-mysicus (1726). This "anatomy of the Danube" fully reflects Marsili's concept that the unity of science should be sought by exploring its individual parts and their interrelationships. Therefore, the Danubian geography is explored through the physical sciences—hydrodynamics, chemistry, geology—the life sciences—biology, zoology, and botany—and the human sciences, such as history and anthropology. Hidden within the six volumes of this colossal work, a brief but significant section—Pars Astronomica—is dedicated to astronomy as a tool for reforming geography. To this end, Marsili combined his daytime surveys with a nighttime measurement campaign, de-termining the latitude and longitude of the locations he visited. The combination of the published source with original archival sources allows to reconstruct how Marsili acquired scientific instru-ments like compasses and movable quadrants, sought assistance with measurements, and corre-sponded with Cassini, asking for guidance on the method to follow and reporting the results of his measurements. Cassini—already a reference for a community that, through his famous tables for observing the eclipses of Jupiter's satellites, was correcting the longitude distortions found in an-cient maps—did not hesitate to respond, comparing Marsili's observations with his own. The reconstruction of their correspondence and a comparison with Marsili's published work reveal a wealth of details regarding the "practical application" of Cassini's method, also exposing the tribulations of a non-professional—and mostly itinerant—astronomer, who lacked the ideal condi-tions of an observatory, which he was in the process of founding in his own city.