Guiseppe Piazzi

1746-1826

Italian Astronomer

Guiseppe Piazzi was born in Ponte di Valtellina, Italy, in 1746. At the age of 18 he became a Theatine monk. While studying at colleges of that order throughout Italy, he developed a fascination with mathematics and astronomy. After serving as a professor of theology in Rome and, later, as professor of higher mathematics at the Academy of Palermo, Piazzi founded the government observatory of Palermo in 1791.

As the observatory's first director, he launched a lengthy project dedicated to updating existing star catalogues. Piazzi wanted to document precisely the exact number and astronomical position of several thousand stars. He soon demonstrated that most stars were in motion relative to the sun. His first compilation, published in 1803, included 6,784 stars. Eleven years later, he updated his list, publishing a second catalogue containing 7,646 stars. Both lists were awarded prizes by the Institute of France.

While working on his star maps, Piazzi made his greatest discovery. On January 1, 1801, he spotted a small object 470 miles in diameter in the constellation Taurus, between the orbits of Mars and Jupiter. The unusual object shone like a faint star, but behaved like a planet. Piazzi named it Ceres, after the Roman goddess of the harvest.

Piazzi's discovery caused great excitement in the astronomical community. For years scientists had been searching that same area of sky for a mystery planet. According to Bode's Law, a scientific formula for calculating distances between planets, there was a "missing" planet between Mars and Jupiter. But Piazzi's object was smaller in comparison to the planets, and its shifting motion ruled out the possibility of a new star. Unsure whether the strange heavenly object was a comet or a planet, Piazzi continued regular observations until mid-February 1801, when he fell ill. By the time he recovered a few days later, he had time to make just one more observation before the mystery object disappeared in the sun's glare.

What Piazzi had discovered was the first and largest asteroid (or "minor planet") with a mass 1/100,000 that of the earth. Alone, this single asteroid is equivalent to half the total weight of all known asteroids today.

Using data from Piazzi's three distinct observations, mathematician Carl Gauss (1777-1855) calculated Ceres' orbit with such accuracy that it was found one year later within 0.5 degrees of the predicted position. Ceres orbits the Sun every 4.6 years at an average distance of 257 million miles.

Soon afterwards, three other similar objects were discovered at the same distance. Those asteroids were named Pallas, Juno, and Vesta. Today many thousands of asteroids are known and catalogued. They are numbered in order of their discovery, for example 1 Ceres.

King Ferdinand wished to present Piazzi with a gold medal in commemoration of his discovery, but the astronomer requested that they money be used to purchase a much needed equatorial telescope. A decade before his death, Piazzi established a second observatory at Naples. Piazzi died in Naples on July 22, 1826. The one thousandth asteroid discovered was named Piazzia in his honor.

KELLI A. MILLER