Astronomer
Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669. He was a professor of astronomy at the University of Bologna and became, in 1671, director of the Paris Observatory. He thoroughly adopted his new country, to the extent that he became interchangeably known as Jean-Dominique Cassini — although that is also the name of his great-grandson, Dominique, comte de Cassini.
Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea, including Iapetus, whose anomalous variations in brightness he correctly ascribed as being due to the presence of dark material on one hemisphere (now called Cassini regio in his honour). In addition he discovered the Cassini Division in the rings of Saturn (1675). He shares with Robert Hooke credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.
In 1672 he 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 from Earth. This allowed for the first time an estimation of the dimensions of the solar system: since the relative ratios of various sun-planet distances were already known from geometry, only a single absolute interplanetary distance was needed to calculate all of the distances.
Cassini initially held the Earth to be the centre of the solar system, though later observations compelled him to accept the model of the solar system proposed by Tycho Brahe, and eventually that of Nicolaus Copernicus. Cassini also rejected Newton's theory of universal gravitation, after an experiment he conducted which wrongly suggested that the Earth was elongated at its poles.
Cassini was also the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the satellites of Jupiter as a clock.
Read more about this topic: Giovanni Domenico Cassini