Giovanni Domenico Cassini was an Italian mathematician, astronomer, engineer, and astrologer. Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669, professor of astronomy at the University of Bologna and became, in 1671, director of the Paris Observatory. Along with Robert Hooke, Cassini is given credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea. Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.