CASSINI AND HIS DISCOVERIES
Giovanni Domenico Cassini (1625-1717) is famous for discovering the gap between the A and B rings of Saturn. He also proposed that the rings were made up of individual particles rather than being a solid body as many thought at the time.
Born in Italy, Cassini taught at Bologna and determined the rotation period of Mars to within two minutes of its correct value. He later did the same with Jupiter. In 1669, Louis XIV of France invited him to take over the Paris Observatory. Cassini stayed in France for the rest of his life and became a French citizen.
His most important work came in I 672 when he used observations of Mars to determine the size of the planets' orbits. He derived a value for the Earth-Sun distance that was only about 7 percent too small—the most accurate estimate at that time.
In 1671, Cassini discovered the first of four new moons of Saturn. This moon now bears the. name lapetus. The following year he found another, Rhea, and on one night in 1684 he discovered two more moons, Dione and Tethys.
In 1675, he observed that the "ring" of Saturn contained a dark gap. To this day, it is known as the Cassini Division.