Giovanni Domenico Cassini -French Astronomer

Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669. He was aprofessor of astronomy at the University of Bologna and became, in 1671, director of theParis Observatory. He thoroughly adopted his new country, to the extent that he became interchangeably known as Jean-Dominique Cassini —although that is also the name of hisgreat-grandson.

Along with Robert Hooke, Cassini is given credit for the discovery of the Great Red Spot onJupiter (ca. 1665). Cassini was the first to observe four of Saturn's moons, which he calledSidera Lodoicea; he also discovered the Cassini Division (1675). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.

Cassini was the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the satellites of Jupiter as a clock.

In 1669 Cassini moved to France and through a grant from Louis XIV of France helped to set up theParis Observatory which opened in 1671; Cassini would remain the director of the observatory for the rest of his career until his death in 1712. In 1673 he became a French citizen. For the remaining forty-one years of his life Cassini served asastronomer/astrologer to Louis XIV ("The Sun King"); serving the expected dual role yet focusing the overwhelming majority of his time on astronomy rather than the astrology he had studied so much of in his youth.

During this time, Cassini's method of determining longitude was used to measure accurately the size of France for the first time. The country turned out to be considerably smaller than expected, and the king quipped that Cassini had taken more of his kingdom from him than he had won in all his wars.