Giovanni Cassini

Giovanni Cassini was an Italian astronomer who made several major discoveries about Saturn and the solar system using the improved telescopes available in the second half of the seventeenth century. Between 1671 and 1684, working mainly at the Paris Observatory, which he became the first director of after being invited to France by King Louis XIV, Cassini discovered four of Saturn's moons, Iapetus, Rhea, Tethys and Dione, and in 1675 he identified a distinct gap within Saturn's ring system that is now known as the Cassini Division. He also made early measurements of the rotation periods of Jupiter and Mars, and he contributed to determining the astronomical unit, the average distance between the Earth and the Sun, with substantially improved accuracy for his time by combining simultaneous observations of Mars made from Paris and French Guiana. Cassini remained a somewhat traditional figure who never fully accepted the Copernican model that the planets orbit the Sun, even as his own observational work continued to supply data later generations used to support it, and the NASA-ESA Cassini spacecraft mission to Saturn, launched in 1997, was later named in his honor.