Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian-French mathematician, astronomer, engineer, and astrologer. 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 honor). He also discovered the Cassini Division in the rings of Saturn. 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 distance. He went blind in 1711, and died in 1712 at the age of 87.