Cassini, Giovanni Domenico (1625-1712), was an Italian-born French astronomer who discovered four moons of Saturn and a large gap in Saturn’s ring system. The gap is now known as the Cassini division. Cassini is also known by the French name Jean Dominique Cassini. Because of his discoveries pertaining to Saturn, a space probe that the United States launched in 1997 to investigate that planet was named after him.
Cassini was born on June 8, 1625, in Perinaldo, in what is now northern Italy. In 1650, he became a professor of astronomy at the University of Bologna. He went to Paris in 1669 and soon became the first director of the Paris Observatory. He became a French citizen in 1673.
Cassini’s tables of the sun, published in 1662, established his reputation as an astronomer. He had precisely measured the sun’s apparent motion through the sky. Later, he closely approximated the distance from Earth to the sun.
Cassini’s observations of Jupiter were so precise that he could distinguish between shadows cast by moons of Jupiter and fixed shadows on Jupiter’s surface. Cassini used the moon shadows to create tables of the motions of the moons. He used the fixed shadows to determine the length of Jupiter’s day. Cassini died in Paris on Sept. 14, 1712.
