Giuseppe Piazzi was a Theatine monk, astronomer, and the first person to discover Ceres, the largest asteroid in our solar system. Piazzi was born in Lombardy, Italy in July 1746 and died in Naples in July 1826. He was also a mathematician of some note and held many scientific teaching positions in prominent European universities.

Ceres recently dubbed a “dwarf planet,” has a diameter of about 590 miles which makes it the largest object in the asteroid belt (a concentration of asteroids between Mars and Jupiter). It was difficult for early astronomers to observe Ceres since it only reflected approximately ten percent of the light striking its surface. Piazzi discovered Ceres by accident when he was looking for another star in the same area.

Piazzi studied at colleges of the Theatine order in Turin, Milan, Genoa, and Rome. Under various instructors he became interested in mathematics and astronomy early on. Then upon completion of his schooling, he moved to the University of Malta where he taught mathematics. Later he went to Genoa where he gave courses on philosophy. Then he was appointed to the chair of higher mathematics in Palermo shortly afterward. But Piazzi’s real passion was always astronomy and his desire was to build his own observatory. He made his wish known and soon the viceroy of Sicily, Prince Caramanico, gave him a large grant to build one and Piazzi set out to procure the necessary equipment. First, he travelled to England where he collected knowledge and advice from other experienced astronomers, and then he hired skilled people to build the necessary instruments. He installed the equipment in Palermo, atop a tower of the Royal Palace. He chose Palermo since it was in a more southern location than any other European observatory, and hence it would permit him to examine the previously inaccessible regions of the sky. And thus Piazzi went to work studying the solar system.

Because of his good location, his new high-powered equipment, and the favorable working conditions, Piazzi started making important discoveries and correcting other astronomer’s work as soon as he began. He measured ‘tropical years’ with more accuracy. He noted aberrations of light. And soon he started a project to catalogue the precise locations of stars. Over successive nights, Piazzi would make different observations of the same stars to hone in the accuracy of his measurements. His first star catalogue was published in 1803, which contained the positions of 6,748 stars. He received prizes for his work, mainly from the Institute of France; one in 1803 for his published list of stars, and another in 1814 for his other contributions.

Piazzi originally discovered Ceres on January 1, 1801, while searching for a small star in a catalogue he was correcting. He made 24 observations of Ceres between January and February, and then announced the discovery to his colleagues as being a comet – he decided to proclaim the object as only a comet to stay on the safe side. Here is an excerpt of what he wrote to the astronomer, Barnaba Oriani, who was located in Milan: “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.”

By mid February, Ceres had completely disappeared. Piazzi could not see it due to the powerful glare emanating from the sun. But the more Piazzi thought about it, the more he became convinced that his discovery was indeed a planet. And hence he became even more determined to locate it again. He tried calculating its orbit but failed since the known mathematical methods of the time were not yet sophisticated enough.

Enter the brilliant German mathematician, Carl Friederich Gauss. He stepped in and used the data from Piazzi’s initial observations (he had recorded nine degrees of Ceres’s orbit before it had vanished) to invent an entirely new method of calculating planetary orbits. He extrapolated upon Piazzi’s original data and combined it with his “least squares approximation method” to calculate where the dwarf planet should be. Then Zach, an astronomer in Germany, located Ceres in December of 1801, right where Gauss had predicted. When they found it again they were confident that Ceres was a planet, although smaller than others in our solar system.

Piazzi originally named his planet, “Ceres Ferdinandea,” after King Ferdinand IV of Sicily and the Roman goddess of grain. (But for political reasons the latter half was later removed.) When King Ferdinand learned what Piazzi had named Ceres, he wanted to make a gold medal with Piazzi’s image engraved in the center to honor the wonderful discovery. But instead, Giuseppe asked the king if the money could be used to buy a new equatorial telescope for his observatory.

Giuseppe Piazzi’s patience and detailed work coupled with his fine equipment and dedication raised the standards of accuracy in all aspects of astronomy. In his honor, several astronomical objects have been named after him; “1000 Piazzi” is the title given to the 1000th asteroid ever to be cataloged; and just a few years ago, a large crater on Ceres found by the Hubble Space Telescope was named simply, ‘Piazzi.’

Giuseppe Piazzi Image Source: Costanzo Angelini, Public domain, via Wikimedia Commons

Ceres Image Source : NASA/JPL-CalTech/UCLA/MPS/DLR/IDA