Jean-Dominique Cassini de Thury

Jean-Dominique Cassini de Thury (1748–1845), known as Cassini IV or the Count of Cassini, was the great-grandson of Giovanni Domenico Cassini and the son of César-François Cassini. He was the last representative of the Cassini line of astronomers to head the Paris Observatory. In 1768, he was sent on a mission to the Atlantic as a commissioner responsible for testing the marine clocks designed by Pierre Le Roy to determine longitude at sea. He gradually took over the running of the Observatory from his ailing father and officially became its director in 1784. He completed publication of the map of France begun by his father and took part in the geodetic operations linking the Paris and Greenwich meridians. At the beginning of the Revolution, he briefly accepted several political responsibilities and collaborated for a few months on the work of the commission responsible for developing the metric system. Deeply attached to the monarchy, he resigned from his duties in September 1793, shortly after the plates for the map of France had been transferred to the War Depot. Imprisoned and then released, he retired to his château at Thury, resigned from the Bureau des longitudes and the new Institut national, but was elected to the Académie des sciences in 1799. He devoted his final years to defending his family's scientific prestige and published in 1810 the Mémoires pour servir à l'histoire des sciences et à celle de l'Observatoire royal de Paris. His son Alexandre-Henri (1781–1832), sometimes referred to as Cassini V, was a botanist, thereby definitively ending the family's astronomical tradition.