Jean-Dominique Cassini (Cassini I, as four Cassinis succeeded from father to son at the Paris Observatory) arrived from Italy in 1669, invited by Colbert to take responsibility for the newly created observatory. An exceptional observer, he already had a great career, focusing on Jupiter, the period of rotation of which he had measured, and on its satellites. He also obtained the rotation period of Mars. In Paris, he continued his observations of Jupiter’s satellites, which lead to the discovery of the finite speed of light by Rømer and himself, and that allowed for the first time accurate determinations of longitudes. He discovered four satellites of Saturn, and was interested in comets. He thought that some of them might be periodic. And, like most astronomers of his time, he took a very active part in geodetic measurements.