Giuseppe Piazzi

Biography

Giuseppe Piazzi was born on July 16, 1746, in Ponte in Valtellina, a small town in northern Italy's Lombardy region. He joined the Theatine order, a Catholic religious group, and became a priest. His interests went beyond theology to include mathematics and the natural sciences. He studied in Rome, Turin, and Milan, gaining recognition for his mathematical skills. In 1780, he became a professor of mathematics at the Academy of Palermo.

While in Palermo, Piazzi saw the need for an astronomical observatory in Sicily. He traveled to France and Britain to learn about observational methods and to get instruments, including a precision five-foot vertical circle made by Jesse Ramsden in London. In 1790, he set up the Palermo Observatory, which became one of the most advanced facilities of its time. Piazzi was its director for many years, and the observatory is now called the Osservatorio Astronomico di Palermo – Giuseppe S. Vaiana in his honor.

Piazzi’s most famous discovery occurred on the night of January 1, 1801, when he spotted a moving object in the sky while working on a star catalogue. He first thought it was a comet, but later observations indicated it was a small body orbiting the Sun between Mars and Jupiter. He named it Ceres after the Roman goddess of grain and the patron deity of Sicily. After Piazzi's initial observations, Ceres was lost because of his illness and its proximity to the Sun. Mathematician Carl Friedrich Gauss used Piazzi’s data to develop a method to calculate its orbit, allowing astronomers to find Ceres again later that year. Ceres is now classified as a dwarf planet and is the largest object in the asteroid belt.

Besides discovering Ceres, Piazzi made important contributions to positional astronomy. He published two major star catalogues: the Catalogue of Palermo Stars with 7,646 stars in 1803, and an updated edition with the same number of entries in 1814. These catalogues were known for their precision and were widely used by astronomers across Europe. He also accurately measured the proper motion of the star 61 Cygni, noting its rapid movement across the sky, which helped Friedrich Wilhelm Bessel make the first measurement of stellar parallax in 1838.

Piazzi was elected a Fellow of the Royal Society in 1804 and won the Lalande Prize from the French Academy of Sciences in 1804 and 1814, the latter for his second star catalogue. Later, he became the director of the Naples Observatory and served as inspector general of observatories for the Kingdom of the Two Sicilies. He died in Naples on July 22, 1826, just six days after turning eighty.