Giovanni Cassini had been attracted to the heavens in his youth, his first interest being astrology.
Having read widely on the subject of astrology while young, had had soon become versed in its intricacies; this extensive knowledge of astrology had led to his first appointment as an astronomer at the Panzano Observatory in Bologna, from 1648 to 1669.
Later in life, as he becomes increasingly involved in the Scientific Revolution, he will focus almost exclusively on astronomy and all but denounce astrology.
Having in 1671 become director of the Paris Observatory that he has helped to set up, Cassini had in 1672 sent his colleague Jean Richer to Cayenne, French Guiana, while he himself stayed in Paris.
The two made simultaneous observations of Mars and, by computing the parallax, determined its distance, thus measuring for the first time the true dimensions of the solar system.
Cassini is the first to observe four of Saturn's moons, which he calls Sidera Lodoicea, including Iapetus, whose anomalous variations in brightness he correctly ascribes as being due to the presence of dark matter on one hemisphere (now called Cassini regio in his honor).
He discovers, in 1675, the Cassini Division, a forty-eight hundred kilometer- (two thousand nine hundred and eighty mile-) wide region between Saturn’s A Ring and B Ring: from Earth, the Division appears as a thin black gap in the rings.
