(b. Ponte in Valtellina, Italy [now Switzerland], 16 July 1746; d. Naples, Italy, 22 July 1826)
As a young man, Piazzi entered the Theatine Order in Milan. He completed his studies there and in Rome, obtaining the doctorate in philosophy and mathematics. From 1769 until 1779 Piazzi taught mathematics in a number of Italian cities; in 1780 he was summoned by the prince of Caramanico, Bourbon viceroy of Sicily, to fill the chair of higher mathematics at the Academy of Palermo. The viceroy encouraged Piazzi in his wish to establish an astronomical observatory in Palermo, and toward the end of the 1780’s Piazzi went to England in order to obtain the best possible equipment.
In England, Piazzi met Maskelyne, William Herschel, and Ramsden. He investigated Herschel’s large telescopes (indeed, he fell and broke his arm while examining one of them that was mounted outdoors) and, with Maskelyne, observed at Greenwich the solar eclipse of 3 June 1788. His first astronomical work, a study of the difference in longitude between various observatories, based on that of Greenwich, was published in the Philosophical Transcations of the Royal Society in 1789. The most important result of Piazzi’s English visit, however, was the great five-foot vertical circle, a masterpiece of eighteenth-century technology, that he commissioned from Ramsden. It was installed in the new observatory in the Santa Ninfa tower of the royal palace of Palermo in 1789, and is still preserved there. The Palermo observatory opened in 1790, and Piazzi was appointed its director, a post that he retained for most of the rest of his life.
Having returned to Palermo, Piazzi took up the problem of the precise determination of the astronomical coordinates (direct ascension and declination) of the principal stars. Palermo was then the southernmost European observatory, and the favorable climatic conditions that it provided allowed him to study more stars than had been previously cataloged, with a greater degree of accuracy. In the course of his regular observations Piazzi, on the night of 1 January 1801, was searching a region in Taurus in which he hoped to see a star of the seventh magnitude, listed in Lacaille’s catalog, which he had previously observed. Before that star appeared, however, he noticed the passage of a somewhat fainter body that Lacaille had not listed. Piazzi continued to observe the new body on the following evenings and ascertained from its movement that it must be a planet or comet. He watched it regularly until 11 February 1801, during which period its retrograde motion ceased and it began to advance, until it had moved near enough to the sun that it could not be seen at its passage to the meridian. It was therefore necessary to calculate its orbit around the sun to find it again.
Piazzi communicated his discovery almost immediately to his friend Barnaba Oriani, director of the Brera observatory in Milan, and to J. E. Bode, director of the Berlin observatory. As early as 24 January 1801, he wrote to Oriani:
I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public. I will try to calculate its elements when I have made more observations.
In his reply to Piazzi, Oriani wrote, “I congratulate you on your splendid discovery of this new star. I do not think that others have noticed it, and because of its smallness, it is unlikely that many astronomers will see it.”
Piazzi had made observations for a period of forty-one days, over a geocentric arc of only 30 He published his observations in 1801 as Risultati delle osservazioni della nuova stella scoperta il 10 gennaio 1801 nell’ Osservatorio di Palermo Other astronomers were eager to rediscover the new body; if it were a planet, it should be possible, on the example of Uranus, to compute from Piazzi’s observations a circular orbit, even if the arc of the presumably elliptical orbit were to prove short. In December 1801 Gauss calculated both such an orbit and an ephemeris for the new body. He communicated his calculations to F. X. von Zach, director of the Gotha observatory, who employed them to rediscover the body in almost exactly the position that Gauss had predicted. It was thus apparent that it was a planet, and in a publication of 1802, Della scoperta del nuovo pianeta“Cerere Ferdinandea,” Piazzi named it for Ceres, the patron goddess of Sicily.
Piazzi’s discovery involved him in a genteel polemic with William Herschel; on 22 May 1802, following Olbers’ discovery of Pallas, Herschel wrote to Piazzi from Slough to argue that these new planets could not be so called in the same sense as the planets within the solar system. He proposed that Ceres and Pallas be called“asteroids,” since they are intermingled with, and similar to, the small fixed stars. He further suggested that these bodies were not worthy of the name of planets, since they did not occupy the space between Mars and Jupiter“with sufficient dignity.” Hereschel went on to advocate three forms of celestial bodies—planets, asteroids, and comets—a hierarchy that led Piazzi to gloss his letter with the remark, “Soon we shall also be seeing counts, dukes, and marquesses in the sky!” (In the course of time, the question has been resolved in Piazzi’s favor, since“asteroid” has fallen into disuse, while the term“small planet” has become standard.)
In 1792 Piazzi returned to making precise determinations of the coordinates of the fixed stars. After ten years of intense and fatiguing work, he published at Palermo, in 1803, a catalog of the medial positions of 6,748 stars, under the title Praecipuarum stellarum inerrantium positiones mediae ineunte saeculo decimonono ex observationibus habitis in specula panoromitana ab anno 1792 ad annum 1802. This catalog was more accurate than any of its predecessors; Zach pronounced it epochal, and the Institut de France awarded it the Lalande prize for the best astronomical work published in 1803
Piazzi then carried his work a step further. Doubting the value of the precession of the equinoxes used at that time, Piazzi undertook to determine the right ascension of a number of basic stars, relating them directly to the sun, in order to improve on earlier observations (including those made at Greenwich). Since he was at that time in poor health, he enlisted the aid of Niccolo Cacciatore as his collaborator. Piazzi’s Praecipuarum stellarum inerrantium positiones mediae ineunte saeculo decimonono ex observationibus habits in specula panormitana ab anno 1792 ad annum 1813 published in Palermo in 1813, cataloged the mean position of 7,646 stars. It was widely esteemed among astronomers, and the Institut de France again awarded Piazzi a prize
In 1817 Piazzi published (again at Palermo) the two-volume Lezioni elementari di astronomia, which he sent to Oriani, who discussed it appreciatively. Oriani was also anxious to persuade Piazzi to republish his earlier observations, but he did not do so. (In 1845 L. von Littrow incorporated Piazzi’s observations of 1792 to 1795 in Annalen der K. K. Sternwarte in Wien.)
In March 1817 Piazzi was summoned to Naples by King Ferdinand I, who wished him to supervise the completion of the observatory already under construction on the hill at Capodimonte. The building had been begun under the direction of Joachim Murat, but had remained unfinished because of the kingdom. Although he received an enthusiastic reception, Piazzi was reluctant to leave Palermo. He stated his feelings in a letter to Oriani:“I shall never yield to the invitations and kindnesses that are showered upon me so that I might remain in Naples. I would thus stain the last years of my life with the vilest ingratitude.” The king nevertheless appointed him director general of the observatories of both Sicily and Naples, with the freedom to remain in whichever of the two kingdoms that he preferred, and Piazzi subsequently divided his time between the two. He took considerable pains in the building and equipping of the Naples observatory, and, on Oriani’s recommendation, secured Carlo Brioschi, of the Istituto Geografico Militare Lombardo, as its director. Brioschi gratified Piazzi by publishing, in 1824, Comentari astronomici della specola reale di Napoli.
Piazzi returned to settle in Naples in 1824, his health weakened. The king had commissioned him to reform the system of weights and measures, and he had been elected president of the Neapolitan Academy of Sciences. He died in Naples of an acute disease.
I. Original Works. Piazzi’s major works are cited in the text. In addition, a partial bibliography—beginning with his first publication, “Results of Calculations of the Observations Made at Various Places of the Eclipse of the Sun...on June 3, 1788,” in Philosophical Transactions of the Royal Society79 (1789), 55–61—is in the Royal Society Catalogue of Scientific Papers, IV , 897.
II. Secondary Literature. See G. Abetti, Storia dell’ astronomia (Florence, 1963); F. Angelitti, “Per il centenario della morte dell’ astronomo Giuseppe piazzi,” in Memorie della Sociata astronomica italiana3 (1925), 369–395; A Bemporad, “Giuseppe Piazzi—commemorazione tenuta nella R. Università di Napoli,” ibid 396–413; and Rudolf Wolf, Biographien zur kulturgeschichte der Schweiz, IV (Zurich, 1862), 275–292