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Giuseppe Piazzi
Giuseppe Piazzi
Born	July 7, 1746 Ponte in Valtellina
Died	July 22, 1826 Naples
Nationality	Sicilian (Italian)
Fields	Astronomy
Religious stance	Roman Catholic

Giuseppe Piazzi (July 7, 1746 - July 22, 1826) was an Italian Theatine monk, mathematician, and astronomer. He was born in Ponte in Valtellina, and died in Naples. He established an observatory at Palermo, now the Osservatorio Astronomico di Palermo – Giuseppe S. Vaiana¹.

Astronomy career

Star cataloguing

He supervised the compilation of the Palermo Catalogue of stars, containing 7,646 star entries with unprecedented precision², including the star names "Garnet Star" from Herschel, and the original Rotanev and Sualocin. The work on this catalogue was started in 1789, enabling Piazzi and collaborators to observe the sky methodically. The catalogue wasn't finished for first edition publication until 1803.

Spurred by the success discovering Ceres (see below), and in the line of his catalogue program, Piazzi studied the proper motions of stars in order 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 Bessel³. The star system 61 Cygni is sometimes still called variously Piazzi's Flying Star and Bessel's Star.

The Asteroid Ceres

Piazzi discovered the Ceres, today known as the largest member of the asteroid belt. On January 1, 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 renowned 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.

Posthumous honors

In 1923, the 1000th asteroid to be numbered was named 1000 Piazzia in his honor. More recently, a large albedo feature, probably a crater, imaged by the Hubble Space Telescope on Ceres, has been informally named 'Piazzi'.

References

• Cunningham, C. J. (2001). The First Asteroid. Star Lab Press. ISBN 0-9708162-2-7. 
• Foderà Serio, G.; Manara, A.; Sicoli, P. (2002). "Giuseppe Piazzi and the Discovery of Ceres", in W. F. Bottke Jr., A. Cellino, P. Paolicchi, and R. P. Binzel: Asteroids III (PDF), Tucson, Arizona: University of Arizona Press, pp. 17-24. 

External links

Wikimedia Commons has media related to:

Giuseppe Piazzi

• Catholic Encyclopedia entry for Giuseppe Piazzi
• Works by or about Giuseppe Piazzi in libraries (WorldCat catalog)