PIAZZI, GIUSEPPE. - A religious of the Order of the Theatines, one of the most illustrious Italian astronomers of the early 19th century, b. at Ponte (Valtellina) on 16 July 1746 and d. at Naples on 22 July 1826.
He entered the Order of the Theatines and, after completing advanced studies in mathematics, philosophy and theology in Rome and in Milan, at the Jesuits' College of Brera, he was ordained a priest and sent first to Rome (1778) as professor of philosophy, then (1780) to Palermo as professor of sublime calculus. A few years later, once the fame of his learning had spread, Abbot P. was charged with founding an astronomical observatory in Palermo, which the city still lacked. For this purpose, after a journey abroad to visit the principal astronomical observatories then existing, he chose the great tower of S. Ninfa, which dominates the whole city, and there founded an observatory which he made famous through his important works. Among these, mention must first be made of the great star catalogue (the largest of all the catalogues of Italian astronomers), which cost P. twenty years of continuous toil and was published in 1814 under the title Praeciparum stellarum positiones mediae, ineunte saeculo XIX, ex observationibus habitis in specula Panormitana ab anno 1792 ad annum 1812 (which is the 2nd ed. of the work published in 1803, completed and emended with the new observations carried out from 1792 to 1813). The work of 1803 published the catalogue of 6,784 stars; that of 1814 contains 7,646.
This grandiose work, of observations and reduction calculations, immediately won the prize of the Academy of Sciences of Paris; and it remains of fundamental importance even now, since it makes it possible to obtain the "proper motions" of the stars by comparing their present positions with those determined by P. This explains why P.'s catalogue has been the subject of numerous studies on the part of the astronomers who followed him; the latest memoir on the subject is the one published in 1933 by F. Porro in the Atti della Reale Accademia d'Italia under the title Fondamenti delle riduzioni per un nuovo catalogo di stelle dedotto dalle osservazioni di G. P. a Palermo.
Other very important researches of P. concern the determination of the obliquity of the ecliptic and the problem of the precession of the equinoxes. But the discovery that has made him truly immortal in the history of astronomy is that of the first of the small planets comprised between Mars and Jupiter, which were then completely unknown. The discovery occurred on 1 January 1801, that is, exactly at the beginning of the 19th century; and the new body received from P. the name of Ceres Ferdinandea, to recall the mythological goddess protectress of the harvests of Sicily and at the same time to honour King Ferdinand I, his patron. In 1815, after the fall of Murat and the return of the Bourbon dynasty, P. was charged with introducing the decimal metric system in the then Kingdom of the Two Sicilies, and two years later (1817) he was called to Naples with the task of founding and directing the new astronomical observatory on the hill of Capodimonte that overlooks the great city. And there death overtook him.
Bibliography2 works
Giuseppe Armellini