Cassini, Giovanni Domenico (Jean Dominique) (1625–1712)

Italian-born French astronomer. He discovered four moons of Saturn and the gap in the rings of Saturn now called the Cassini division.

Cassini was born at Perinaldo near Nice (then in Italy), and studied at the Jesuits' College, Genoa. Having assisted two astronomers at an observatory near Bologna, he was made professor of astronomy at the University of Bologna 1650 at the age of 25.

Cassini refused to accept the Copernican cosmological model and rejected the concept of a finite speed of light, although its proof was demonstrated by Danish astronomer Ole Römer using Cassini's own data.

During 1664–67 Cassini determined the rotation periods of Mars, Jupiter, and Venus. In 1669 he departed for France at the invitation of King Louis XIV, to construct and run the Paris Observatory. In 1675 he distinguished two zones within what was thought to be the single ring around Saturn. Cassini correctly suggested that the rings were composed of myriads of tiny satellites.

In the 1670s Cassini made many observations of details on the lunar surface. He also took advantage of a good opposition of Mars 1672 to determine the distance between the Earth and that planet. He arranged for Jean Richer (1630–1696) to make measurements from his base in Cayenne, on the northeast coast of South America, while Cassini made simultaneous measurements in Paris, which permitted them to make a triangulation of Mars. From the result, Cassini was able to deduce many other astronomical distances. When he went blind 1710, his son Jacques Cassini succeeded him at the Paris Observatory.