1. Overview
Giuseppe Piazzi was an Italian Catholic priest of the Theatine order, a distinguished mathematician, and an influential astronomer. Born on July 16, 1746, in Ponte in Valtellina, he passed away on July 22, 1826, in Naples. Piazzi is widely celebrated for his groundbreaking discovery of Ceres on January 1, 1801, which was initially identified as a planet and later reclassified as the first and largest asteroid, and subsequently a dwarf planet. He was instrumental in the establishment of the Palermo Astronomical Observatory, where he served as director, and later became the General Director of the Naples and Sicily Observatories. His significant contributions to astronomy also include the meticulous compilation of the Palermo Catalogue of stars and his research into stellar proper motions.
2. Life and Education
Giuseppe Piazzi's early life and academic journey laid the foundation for his eminent career in astronomy.
2.1. Early Life and Education
While detailed documented accounts of Giuseppe Piazzi's early scientific education are scarce in his biographies, it is confirmed that he pursued studies in Turin, where he likely attended lessons by the notable scholar Giovan Battista Beccaria. Between 1768 and 1770, Piazzi resided at the Theatines' Home in Sant'Andrea della Valle, Rome, dedicating his time to studying mathematics under the guidance of François Jacquier.
2.2. Academic Career and Appointments
Piazzi's academic career saw him hold various prestigious positions across Italy. In July 1770, he was appointed to the chair of Mathematics at the University of Malta. By December 1773, he had moved to Ravenna, where he served as "prefetto degli studenti" and lectured in Philosophy and Mathematics at the Collegio dei Nobili, a role he maintained until early 1779. After brief periods in Cremona and Rome, Piazzi relocated to Palermo in March 1781, taking up a position as a lecturer in Mathematics at the University of Palermo (then known as the "Accademia de' Regj Studi"). He held this lecturing position until January 19, 1787, when he was promoted to Professor of Astronomy at the same institution.
2.3. International Experience and Observatory Foundation
Almost concurrently with his appointment as Professor of Astronomy, Piazzi was granted permission to spend two years abroad in Paris and London. This period, from March 13, 1787, until the end of 1789, was crucial for his practical training in astronomy and for acquiring specialized instruments for the Palermo Observatory, whose foundation he was tasked with overseeing. During his time abroad, Piazzi became acquainted with many leading French and English astronomers of his era. He notably secured the famous altazimuthal circle crafted by Jesse Ramsden, one of the most skilled instrument-makers of the 18th century. This instrument became the most vital piece of equipment for the Palermo Astronomical Observatory - Giuseppe S. Vaiana, whose official foundation took place on July 1, 1790.
2.4. Directorship of Neapolitan Observatories
In 1817, King Ferdinand I appointed Giuseppe Piazzi as the General Director of the Naples and Sicily Observatories. In this significant role, Piazzi was specifically charged with overseeing the completion of the Capodimonte (Naples) Observatory.
3. Astronomical Achievements
Giuseppe Piazzi's career was marked by several pivotal contributions to the field of astronomy, most notably his extensive star cataloguing efforts and the discovery of Ceres.
3.1. Star Cataloguing
Piazzi meticulously supervised the compilation of the Palermo Catalogue of stars, a monumental work that included 7,646 star entries. This catalogue was distinguished by its unprecedented precision, setting a new standard for astronomical observation. Among the stars included were the "Garnet Star", a designation from William Herschel, and the original entries for Rotanev and Sualocin, which are components of Alpha Delphini. The systematic work involved in observing the sky for this catalogue was extensive. The first edition of the Palermo Catalogue was published in 1803, with a second, updated edition following in 1814.
3.2. Discovery of Ceres
Giuseppe Piazzi's most renowned achievement was the discovery of Ceres. On January 1, 1801, Piazzi discovered a "stellar object" that exhibited movement against the background of fixed stars. At first he thought it was a stationary star, but upon noticing its motion, he became convinced it was a planet, referring to it as "a new star."
In his journal, Piazzi recorded his observations: "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."
Despite his strong assumption that it was a planet, Piazzi adopted a conservative approach and publicly announced his discovery as a comet. However, in a letter to his colleague, astronomer Barnaba Oriani of Milan, Piazzi privately conveyed his true suspicions, 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."
Piazzi was unable to observe the object for an extended period as it soon became obscured by the glare of the Sun. Due to the limitations of existing methods, its orbit could not be computed. This challenge prompted the brilliant mathematician Carl Friedrich Gauss to develop a novel method for orbit calculation, which subsequently enabled astronomers to locate Ceres once again. In December 1801, Ceres was successfully rediscovered by Franz von Zach and Wilhelm Olbers. Once its orbit was more accurately determined, it became clear that Piazzi's initial hypothesis was correct: the object was not a comet but rather a small planet. Coincidentally, its position in the solar system aligned almost perfectly with the prediction made by the Titius-Bode law.
Piazzi named his discovery "Ceres Ferdinandea," honoring both Ceres, the Roman and Sicilian goddess of grain, and King Ferdinand IV of Naples and Sicily. The "Ferdinandea" portion of the name was later removed for political reasons. Ceres was recognized as the first and largest of the asteroids found within the asteroid belt. In 2006, Ceres was reclassified as a dwarf planet.
3.3. Other Astronomical Research
Beyond his monumental discovery of Ceres, Piazzi also made significant contributions to the study of stellar proper motions. He diligently investigated the movements of stars to identify promising candidates for parallax measurements, a technique used to determine the distance to stars. Among these, he specifically highlighted 61 Cygni as an excellent candidate for measuring parallax. This measurement was later successfully carried out by the renowned astronomer Friedrich Wilhelm Bessel. The star system 61 Cygni is sometimes still referred to as Piazzi's Flying Star or Bessel's Star, acknowledging the contributions of both astronomers.
4. Posthumous Honors and Commemoration
Giuseppe Piazzi's profound impact on astronomy has been recognized through various posthumous honors and commemorations, ensuring his legacy endures.
4.1. Naming Conventions
Several celestial bodies and individuals have been named in honor of Giuseppe Piazzi. The astronomer Charles Piazzi Smyth, son of the prominent astronomer William Henry Smyth, was named in Piazzi's honor. 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 honor. This asteroid was discovered by Karl Reinmuth in Heidelberg. The lunar crater Piazzi was named after him in 1935. More recently, a large albedo feature, believed to be an impact crater, observed on Ceres by the Hubble Space Telescope, has been informally designated Piazzi.
5. Works
Giuseppe Piazzi authored several significant works during his career, contributing to astronomical literature and academic discourse. His notable published works include:
- Discorso recitato nell'aprirsi la prima volta la Cattedra di astronomia nell'Accademia de' r. Studj di Palermo (1790), an inaugural address delivered upon the opening of the first Chair of Astronomy at the Royal Academy of Studies in Palermo.
6. Related Figures
Several key individuals were closely associated with Giuseppe Piazzi's life and work, contributing to or being influenced by his astronomical endeavors.
- Niccolò Cacciatore: Piazzi's dedicated assistant and his eventual successor as the director of the Palermo Observatory.
- Jesse Ramsden: A highly skilled 18th-century instrument-maker who crafted the crucial altazimuthal circle for the Palermo Observatory, under Piazzi's commission.
- Barnaba Oriani: A prominent astronomer from Milan with whom Piazzi corresponded, notably sharing his initial suspicions about the true nature of Ceres.
- Carl Friedrich Gauss: The brilliant mathematician who developed a revolutionary new method for orbit calculation, which was instrumental in the successful rediscovery of Ceres after it was lost in the Sun's glare.
- William Herschel: A renowned astronomer who named the "Garnet Star" (Mu Cephei), a star that Piazzi included in his precise Palermo Catalogue.
- Friedrich Wilhelm Bessel: An influential astronomer who later successfully measured the parallax of 61 Cygni, a star that Piazzi had identified as a prime candidate for such measurements.
- Franz von Zach and Wilhelm Olbers: Astronomers who, using Gauss's calculations, successfully rediscovered Ceres in December 1801.
- Costantino Corti: The sculptor responsible for creating Piazzi's memorial statue, dedicated in his birthplace.
- William Henry Smyth: An astronomer and the father of Charles Piazzi Smyth, who was named in honor of Giuseppe Piazzi.
- Karl Reinmuth: The astronomer who discovered the 1000th asteroid, which was subsequently named 1000 Piazzia in Piazzi's honor.