Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669. He was a professor, of astronomy, at the University of Bologna and became, in 1671, director of the Paris Observatory.
Along with Hooke, Cassini is given credit for the discovery of the Great Red Spot (~1665). Cassini was the first to observe four of Saturn's moonss; he also discovered the Cassini Division (1675). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.