Giovanni Cassini

Giovanni Domenico Cassini was an Italian-born French astronomer who became the first director of the Paris Observatory. Born near Nice, he earned fame in Italy for his precise observations of Jupiter and determination of the rotation periods of Jupiter and Mars. In 1669, King Louis XIV invited him to Paris, where he spent the rest of his career. At the Paris Observatory, Cassini made his most celebrated discoveries: four moons of Saturn (Iapetus, Rhea, Tethys, and Dione) and the dark gap in Saturn's rings now known as the Cassini Division. He was the first to observe the differential rotation of Jupiter's atmosphere and produced improved tables of Jupiter's satellites. The Cassini spacecraft mission to Saturn (1997–2017) was named in his honor.

Discovered four moons of Saturn: Iapetus (1671), Rhea (1672), Tethys and Dione (1684). Discovered the Cassini Division — the prominent gap in Saturn's rings. Determined the rotation periods of Jupiter and Mars. First to observe differential rotation in Jupiter's atmosphere. Led the first accurate measurement of the distance to Mars (and thus the scale of the solar system).