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Portrait after an engraving of Giovanni Cassini (1625-1712), Italian-French astronomer. Although born in Italy, Cassini went to Paris in 1669, never to return. As director of the Observatory, he added greatly to our knowledge of the Solar System. He first worked out the rotational periods of Jupiter, Mars 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 (called the Cassini Division). Using simultaneous observation data of Mars from Paris and French Guiana, he produced the first accurate measurement of the distance between the Sun and Earth (only 7% low).
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