who moved to Paris in 1669; also known as Jean Dominique Cassini. He measured the rotation periods of Mars and Jupiter, discovered four of Saturn’s satellites, and in 1675 observed the division in Saturn’s rings now named after him. In 1672, with the help of observations by his countryman Jean Richer (1630–96), he measured the distance to Mars by triangulation, and was thus able to refine the dimensions of the Solar System, obtaining a value for the astronomical unit that was short by only 7%. Cassini drew up improved tables for Jupiter’s satellites, which played a part in O. C. Römer’s determination of the velocity of light.