In 1672 Cassini 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. Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian-French mathematician, astronomer, engineer, and astrologer. Attracted to the heavens in his youth, his first interest was in astrology. While young he read widely on the subject of astrology, and soon was very knowledgeable about it; this extensive knowledge of astrology led to his first appointment as an astronomer. 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). In addition he discovered the Cassini Division in the rings of Saturn (1675). He went blind in 1711, and died in 1712 at the age of 87.