Italian-French astronomer. Born 1625, died 1712. From 1673 using the name Jean Dominique Cassini.
Cassini is most famous for his important observations of Jupiter and Saturn, and for determining the distance to Mars (a feat that made it possible to determine distances within the Solar System quite precisely).
In 1665, while Cassini was a professor at Bologna, he measured Jupiter's period of rotation. Subsequently, Cassini's tables of the motions of the four major moons of Jupiter were used by Danish astronomer Ole Rømer to determine the speed of light.
Cassini became director of the new-built Paris Observatory in 1671, and it was here that he discovered four of Saturn's moons and noted the large gap in the rings of Saturn, now known as the Cassini division. He also set forth the first valid explanation of the nature of the rings. Finally, he described and named the zodiacal light.
See also:
Jacques Cassini (his son),
César François Cassini de Thury (his grandson),
Jacques-Dominique Cassini (his great-grandson), and
Cassini (space probe).