Giovanni Cassini, Italian-French astronomer (1625- 1712), coloured artwork. Although born in Italy, Cassini emigrated to Paris, France, in 1669. As Director of the Paris Observatory (seen at left), he added greatly to our knowledge of the solar system. He calculated rotational periods for Mars, Jupiter and Venus, and tabulated the motions of the Galilean satellites of Jupiter. He discovered four new moons of Saturn, and observed the gap in Saturn's ring system (named the Cassini Division). Using simultaneous observation data of Mars from Paris and French Guiana, he produced the first accurate measurement (7 percent less than the real value) of the distance between the Sun and Earth.