Jean-Dominique Cassini

Giovanni Domenico Cassini, known in France as Jean-Dominique Cassini and called Cassini I (1625–1712), was an astronomer of Italian origin, founder of the Cassini family and first director of the Paris Observatory. Born in Perinaldo, in the County of Nice, then under Italian rule, he studied in Genoa before becoming professor of astronomy at the University of Bologna, where he taught Ptolemaic cosmology, in accordance with the official doctrine of the Church, despite the advances of Copernicus, Galileo and Kepler. His reputation as an observer led the pope to entrust him with missions before Louis XIV and his minister Colbert called him to France to help build and run the Paris Observatory. Naturalized as a French citizen in 1673, he became a member of the Académie des sciences and settled permanently in Paris, where he directed the Observatory until the end of his life. His observational work was considerable: he discovered Jupiter's Great Red Spot in 1665, determined the rotation periods of Jupiter, Mars and Venus, discovered four satellites of Saturn—Iapetus in 1671, Rhea in 1672, Tethys and Dione in 1684—and identified the Cassini Division in Saturn's rings in 1675. In 1672, he made the first accurate measurement of the Earth–Sun distance, and in 1683, he determined the Sun's parallax. Around 1690, he was the first to observe differential rotation in Jupiter's atmosphere. He took part in measuring the Observatory's meridian and contributed to determining the variation of gravity with latitude during observations conducted in Cayenne. Furthermore, he maintained that the Sun describes an oval with the Earth as one of its foci, a geometric curve that has remained associated with his name as the Cassini oval. From 1668 to 1693, he published the Éphémérides des satellites de Jupiter, produced a detailed map of the lunar surface and wrote numerous papers. Having become blind in 1710, he died on 17 September 1712 at the age of 87. The asteroid (24101) Cassini and the Cassini-Huygens space probe were named in his honour.