Giuseppe Piazzi was born inPonte in Valtellina, around 100 km north east ofMilan, 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 byGiovanni Battista Beccaria. Between 1768 and 1770, he resided at the Theatines' Home in Sant'Andrea della Valle, Rome, where he studied mathematics underFrançois 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 byJesse 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.
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" fromHerschel, and theoriginalRotanev andSualocin. 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 theInstitut de France awarded it theLalande 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 byFriedrich Wilhelm Bessel.[6] The star system61 Cygni is sometimes still called variouslyPiazzi's Flying Star andBessel's Star.
Piazzi discoveredCeres. On 1 January 1801 Piazzi discovered a "stellar object" that moved against the background ofstars. 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 ofJupiter, but similar to many others which generally are reckoned of the eighthmagnitude. 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 acomet. In a letter to astronomerBarnaba Oriani ofMilan 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 theSun. Unable to compute itsorbit with existing methods, the mathematicianCarl 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 smallplanet. Coincidentally, it was also almost exactly where theTitius–Bode law predicted a planet would be.
Born in Italy and named in his honour was the astronomerCharles Piazzi Smyth, son of the astronomerWilliam Henry Smyth. In 1871, a memorial statue of Piazzi sculpted byCostantino Corti was dedicated in the main plaza of his birthplace, Ponte. In 1923, the 1000th asteroid to be numbered was named1000 Piazzia in his honour.[7] The lunar craterPiazzi was named after him in 1935. More recently, a largealbedo feature, probably acrater, imaged by theHubble Space Telescope on Ceres, has been informally namedPiazzi.
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.
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.ISBN978-3-031-48388-2.
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 fromthe original(PDF) on 24 January 2017.
