Giovanni Domenico Cassini was a 17th century astronomer and the first of the Cassini family of astronomers. He discovered four of the moons of Saturn and the makeup of the planet's rings.
Cassini was an Itallian. His early interests were in astrology, math and poetry. While knowledgeable in astrology, his knowledge led him to the belief that it held no truth. His knowledge of the subject was responsible for his appointment at the Panzano Observatory and later as a professor at the University of Bologna.
In 1669, Cassini moved to Paris and set up the Paris Observatory. He became a citizen of France in 1673 and adopted the French version of his name, Jean.
Cassini used triangulation to make the first accurate measurement of the size of France. He used parallax to determine the speed of the rotation of Mars. He, along with Robert Hook, receives credit for the discovery of the flat spots at the poles of Jupiter.
Cassini was a traditionalist. He based his work on an Earth-centered view of the universe. When he discovered that light had a finite speed, he rejected the discovery and looked to find other explanations for the discrepancies in data, but he is a central figure in the Scientific Revolution.