Cassini , Giovanni Domenico

(1625–1712) Italian–French astronomer

Born in Perinaldo, Italy, Cassini was educated in Genoa and at the early age of 25 became professor of astronomy at Bologna. He remained there until 1669 when he moved to France in order to take charge of Louis XIV's new Paris Observatory. He became a French citizen in 1673.

While still at Bologna he worked out, fairly accurately, the rotational periods of Jupiter and Mars. In 1668 he constructed a table of the movements of the Medici planets – the satellites of Jupiter discovered by Galileo. It was this table that allowed Ole Romer to calculate the speed of light. In Paris, using aerial telescopes up to 150 feet (45.7 m) long, he discovered four new satellites of Saturn – Iapetus in 1671, Rhea in 1672, and Dione and Tethys in 1684. In 1675 he discovered the gap that divides Saturn's rings into two parts and has since been called Cassini's division.

Cassini's most important work concerned the size of the solar system. Using data collected by Jean Richer in Cayenne, together with his own observations in Paris, he was able to work out the parallax of Mars and thus calculate the astronomical unit (AU) – the mean distance between the Earth and the Sun. His figure of 87 million miles (140 million km) may have been 7% too low, but compared with earlier figures of Tycho Brahe (5 million miles) and Johannes Kepler (15 million miles) his results gave mankind a realistic picture of the size of the universe for the first time. Cassini also made fundamental measurements on the size and shape of the Earth concluding, erroneously, that it was a prolate spheroid. He became blind in 1710 and was succeeded in the directorship of his observatory by both his son, Jacques Cassini, and his grandson, César François Cassini.