Cassini käs-sē´nē [key], name of a family of Italian-French astronomers, four generations of whom were directors of the Paris Observatory.
Gian Domenico Cassini, 1625–1712, was born in Italy and distinguished himself while at Bologna by his studies of the sun and planets, particularly Jupiter; he determined rotational periods for Jupiter, Mars, and Venus. He was called to Paris in 1669 to supervise the building of the Royal Observatory and remained there to direct it. While at Paris he discovered four of Saturn's satellites, studied the division in the planet's ring system that now bears his name, and began the mapping of the meridian passing through Paris in order to verify the Cartesian hypothesis of the elongation of the earth. His son Jacques Cassini, 1677–1756, took over the observatory after 1700 and continued the mapping of the Paris meridian, adding to it a measurement of the perpendicular to the arc in 1733–34. The triumph of the opposing Newtonian hypothesis of the flattening of the earth caused him to retire in 1740, and he was replaced by his son, Cesar-François Cassini de Thury, 1714–84, who continued his father's geodesic work and planned the first modern map of France. On his death, his son Jean-Dominique Cassini, 1748–1845, undertook the reorganization and restoration of the observatory. He completed his father's map of France and participated in the geodesic operations joining the Paris and Greenwich meridians. He lost his post in 1793 because of his monarchial views and was briefly imprisoned by the revolutionary government in 1794. He abandoned scientific work in 1800, becoming president of the General Council of Oise. He was decorated by Napoleon I and Louis XVIII and retired in 1818.
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