Jacques Cassini

Jacques Cassini (1677–1756), known as Cassini II, was a French astronomer and the son of Giovanni Domenico Cassini. He worked alongside his father from an early age, succeeded him as director of the Paris Observatory in 1712, and continued the family's work in astronomy and geodesy. He took part in numerous observing expeditions and resumed the measurement of the Observatory's meridian arc, which he completed in 1718. From this measurement, he concluded that the Earth is elongated at the poles, a position consistent with Cartesian physics, of which he was an ardent defender, and which placed him in direct opposition to Newton's supporters, for whom the Earth is flattened at the poles. This major scientific controversy of the 18th century would not be settled until the measurements made by his son César-François. Realizing around 1740 that his arguments could no longer withstand the new data, he abandoned his scientific activities and handed over the directorship of the Observatory to his son. Besides his work as a geodesist, Jacques Cassini distinguished himself through his astronomical observations of planets and their satellites, which he used to determine orbital inclinations, and through his detection of the proper motion of stars in 1738. He wrote a Traité de la grandeur et de la figure de la Terre (Treatise on the Size and Shape of the Earth), published in 1720. He died in 1756 following a carriage accident. The asteroid (24102) Jacquescassini was named in his honour.