Giuseppe Piazzi

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The Reverend Giuseppe Piazzi CR
Portrait by Costanzo Angelini, c. 1825
Born	Gioacchino Giuseppe Maria Ubaldo Nicolò Piazzi (1746-07-16)16 July 1746 Ponte in Valtellina, Duchy of Milan
Died	22 July 1826(1826-07-22) (aged 80) Naples, Kingdom of Naples
Nationality	Italian
Known for	Discovery of the first dwarf planet, Ceres
Awards	Lalande Prize (1803)
Scientific career
Fields	Astronomy
Institutions	University of Malta University of Palermo Palermo Astronomical Observatory

Giuseppe Piazzi CR (US: /ˈpjɑːtsi/ PYAHT-see,^[1] Italian: [dʒuˈzɛppe ˈpjattsi]; 16 July 1746 – 22 July 1826) was an Italian Catholic priest of the Theatine order, mathematician, and astronomer. He established an observatory at Palermo, now the Osservatorio Astronomico di Palermo – Giuseppe S. Vaiana.^[2] He is perhaps most famous for his discovery of the first dwarf planet, Ceres.

Giuseppe Piazzi was born in Ponte in Valtellina, around 100 km north east of Milan, to a well-to-do family. No documented account of his scientific education exists in any of the astronomer’s biographies, even the earliest ones. However, it is certain that Piazzi pursued studies in Turin, likely attending lessons by Giovanni Battista Beccaria. Between 1768 and 1770, he resided at the Theatines' Home in Sant'Andrea della Valle, Rome, where he studied mathematics under François Jacquier.

In July 1770, Piazzi was appointed to the chair of Mathematics at the University of Malta. In December 1773, he moved to Ravenna, where he served as "prefetto degli studenti" and as a lecturer in Philosophy and Mathematics at the Collegio dei Nobili, a position he held until early 1779. After brief periods in Cremona and Rome, Piazzi relocated to Palermo in March 1781, taking up a role as lecturer in Mathematics at the University of Palermo (then known as the "Accademia de' Regj Studi")

He kept this position until 19 January 1787, when he became Professor of Astronomy. Almost at the same time, he was granted permission to spend two years in Paris and London, to undergo some practical training in astronomy and also to get some instruments to be specially built for the Palermo Observatory, whose foundation he was in charge of.

In the period spent abroad, from 13 March 1787 until the end of 1789, Piazzi became acquainted with the major French and English astronomers of his time and was able to have the famous altazimuthal circle made by Jesse Ramsden, one of the most skilled instrument-makers of the 18th century. The circle was the most important instrument of the Palermo Observatory, whose official foundation took place on 1 July 1790.

In 1817, King Ferdinand put Piazzi in charge of the completion of the Capodimonte (Naples) Observatory, naming him General Director of the Naples and Sicily Observatories.

He supervised the compilation of the Palermo Catalogue of stars, containing 7,646 star entries with unprecedented precision,^[3] including the star names "Garnet Star" from Herschel, and the original Rotanev and Sualocin. The catalogue wasn't finished for first edition publication until 1803, with a second edition in 1814.^[4] Piazzi's catalogue was more accurate than any of its predecessors; Franz Xaver von Zach pronounced it epochal, and the Institut de France awarded it the Lalande Prize for the best astronomical work published in 1803.^[5]

Spurred by the success discovering Ceres (see below), and in the line of his catalogue program, Piazzi studied the proper motions of stars to find parallax measurement candidates. One of them, 61 Cygni, was specially appointed as a good candidate for measuring a parallax, which was later performed by Friedrich Wilhelm Bessel.^[6] The star system 61 Cygni is sometimes still called variously Piazzi's Flying Star and Bessel's Star.

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Piazzi discovered Ceres. On 1 January 1801 Piazzi discovered a "stellar object" that moved against the background of stars. At first he thought it was a fixed star, but once he noticed that it moved, he became convinced it was a planet, or as he called it, "a new star".

In his journal, he wrote:

The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. In the evening of the second I repeated my observations, and having found that it did not correspond either in time or in distance from the zenith with the former observation, I began to entertain some doubts of its accuracy. I conceived afterwards a great suspicion that it might be a new star. The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.

In spite of his assumption that it was a planet, he took the conservative route and announced it as a comet. In a letter to astronomer Barnaba Oriani of Milan he made his suspicions known in writing:

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.

He was not able to observe it long enough as it was soon lost in the glare of the Sun. Unable to compute its orbit with existing methods, the mathematician Carl Friedrich Gauss developed a new method of orbit calculation that allowed astronomers to locate it again. After its orbit was better determined, it was clear that Piazzi's assumption was correct and this object was not a comet but more like a small planet. Coincidentally, it was also almost exactly where the Titius–Bode law predicted a planet would be.

Piazzi named it "Ceres Ferdinandea," after the Roman and Sicilian goddess of grain and King Ferdinand IV of Naples and Sicily. The Ferdinandea part was later dropped for political reasons. Ceres turned out to be the first, and largest, of the asteroids existing within the asteroid belt. Ceres is today called a dwarf planet.

Born in Italy and named in his honour was the astronomer Charles Piazzi Smyth, son of the astronomer William Henry Smyth. In 1871, a memorial statue of Piazzi sculpted by Costantino Corti was dedicated in the main plaza of his birthplace, Ponte. In 1923, the 1000th asteroid to be numbered was named 1000 Piazzia in his honour.^[7] The lunar crater Piazzi was named after him in 1935. More recently, a large albedo feature, probably a crater, imaged by the Hubble Space Telescope on Ceres, has been informally named Piazzi.

• Discorso recitato nell'aprirsi la prima volta la Cattedra di astronomia nell'Accademia de' r. Studj di Palermo (in Italian). Palermo: Stamperia reale. 1790.
• Della specola astronomica de' regj studj di Palermo libri quattro (in Italian). Palermo: Stamperia reale. 1792.
• Sull'orologio Italiano e l'Europeo (in Italian). Palermo: Stamperia reale. 1798.
• Risultati delle osservazioni della nuova stella, scoperta il dì 1. Gennaio all'Osservatorio Reale di Palermo, Palermo: nella Reale Stamperia, 1801
• Della scoperta del nuovo pianeta Cerere Ferdinandea (Palermo, 1802)
• Praecipuarum stellarum inerrantium positiones mediae ineunte seculo 19. ex observationibus habitis in specula Panormitana ab anno 1792 ad annum 1802 (in Latin). Panormi: ex Regia Typographia Militari. 1803.
• Præcipuarum stellarum inerrantium positiones mediæ ineunte seculo XIX ex obsrvationibus habitis in specula Panormitana ab anno 1792 ad annum 1813 (in Latin). Panormi: ex Regia Typographia Militari. 1814.
• Codice metrico siculo (in Italian). 2 vols. Catania: Stamperia Dell'Università Degli Studi. 1812.{{cite book}}: CS1 maint: others (link)
• Lezioni di astronomia (in Italian). 2 vols. Palermo: Stamperia reale. 1817.{{cite book}}: CS1 maint: others (link)
• Ragguaglio del Reale Osservatorio di Napoli eretto sulla collina di Capodimonte (in Italian). Naples: Dalla Tipografia Francese. 1821.

• Niccolò Cacciatore, his assistant and successor in the post as director
• List of Roman Catholic scientist-clerics

• Cunningham, Clifford J.; Marsden, Brian; Orchiston, Wayne (2011). "Giuseppe Piazzi: the controversial discovery and loss of Ceres in 1801". Journal for the History of Astronomy. 42 (3): 283–306. Bibcode:2011JHA....42..283C. doi:10.1177/002182861104200301.
• Cunningham, C.J. (2016). Discovery of the First Asteroid, Ceres. Historical Studies in Asteroid Research. New York: Springer. ISBN 978-3319372877.
• Foderà Serio, G.; Manara, A.; Sicoli, P. (2002). "Giuseppe Piazzi and the Discovery of Ceres" (PDF). In W. F. Bottke Jr.; A. Cellino; P. Paolicchi; R. P. Binzel (eds.). Asteroids III. Tucson, Arizona: University of Arizona Press. pp. 17–24.
• Fox, William (1913). "Giuseppe Piazzi" . In Herbermann, Charles (ed.). Catholic Encyclopedia. New York: Robert Appleton Company.
• Chinnici, Ileana (2024). "The Discovery of Ceres: A "Scientific Comedy"". In Ileana Chinnici (ed.). Italian Contributions to Planetary Astronomy. From the Discovery of Ceres to Pluto's Orbit. Historical & Cultural Astronomy. Cham, Switzerland: Springer. pp. 77–102. doi:10.1007/978-3-031-48389-9_5. ISBN 978-3-031-48388-2.

Wikimedia Commons has media related to Giuseppe Piazzi.

• Abetti, Giorgio (1974). "Piazzi, Giuseppe". In Charles Coulston Gillispie (ed.). Dictionary of Scientific Biography. Vol. 10. New York: Charles Scribner's Sons. pp. 591–593. Retrieved 10 July 2025.
• Santaniello, Francesco (2015). "PIAZZI, Giuseppe". Dizionario Biografico degli Italiani (in Italian). Vol. 83: Piacentini–Pio V. Rome: Istituto dell'Enciclopedia Italiana. ISBN 978-88-12-00032-6.
• O'Connor, John J.; Robertson, Edmund F., "Giuseppe Piazzi", MacTutor History of Mathematics Archive, University of St Andrews
• Foderà, Serio G.; Manara, A.; Sicoli, P. (2002). "Giuseppe Piazzi and the Discovery of Ceres" (PDF). In W., Bottke; P., Paolicchi; R., Binzel; A., Cellino (eds.). Asteroids III. Tucson: University of Arizona Press. pp. 17–24. Archived from the original (PDF) on 24 January 2017.
• Portrait of Giuseppe Piazzi from the Lick Observatory Records Digital Archive, UC Santa Cruz Library's Digital Collections Archived 4 March 2016 at the Wayback Machine