Cassini

Cassini, the name of an Italian family of astronomers, four generations of whom succeeded each other in official charge of the observatory at Paris.

Giovanni Domenico Cassini (1625-1712), the first of these, was born at Perinaldo near Nice on the 8th of June 1625. Educated by the Jesuits at Genoa, he was nominated in 1650 professor of astronomy in the university of Bologna; he observed and wrote a treatise on the comet of 1652; was employed by the senate of Bologna as hydraulic engineer; and appointed by Pope Alexander VII. inspector of fortifications in 1657, and subsequently director of waterways in the papal states. His determinations of the rotation-periods of Jupiter, Mars and Venus in 1665-1667 enhanced his fame; and Louis XIV. applied for his services in 1669 at the stately observatory then in course of erection at Paris. The pope (Clement IX.) reluctantly assented, on the understanding that the appointment was to be temporary; but it proved to be irrevocable. Cassini was naturalized as a French subject in 1673, having begun work at the observatory in September 1671. Between 1671 and 1684 he discovered four Saturnian satellites, and in 1675 the division in Saturn’s ring (see Saturn); made the earliest sustained observations of the zodiacal light, and published, in Les Éléments de l’astronomie vérifiés (1684), an account of Jean Richer’s (1630-1696) geodetical operations in Cayenne. Certain oval curves which he proposed to substitute for Kepler’s ellipses as the paths of the planets were named after him “Cassinians.” He died at the Paris observatory on the 11th of September 1712.

A partial autobiography left by Giovanni Domenico Cassini was published by his great-grandson, Count Cassini, in his Mémoires pour servir à l’histoire des sciences (1810). See also C. Wolf, Histoire de l’observatoire de Paris (1902); Max. Marie, Histoire des sciences, t. iv. p. 234; R. Wolf, Geschichte der Astronomie, p. 450, &c.

Jacques Cassini (1677-1756), son of Domenico Cassini, was born at the Paris observatory on the 8th of February 1677. Admitted at the age of seventeen to membership of the French Academy of Sciences, he was elected in 1696 a fellow of the Royal Society of London, and became maitre des comptes in 1706\. Having succeeded to his father’s position at the observatory in 1712, he measured in 1713 the arc of the meridian from Dunkirk to Perpignan, and published the results in a volume entitled De la grandeur et de la figure de la terre (1720) (see Geodesy). He wrote besides Élémens d’astronomie (1740), and died on the 18th of April 1756 at Thury, near Clermont. The first tables of the satellites of Saturn were supplied by him in 1716.

See C. Wolf, Histoire de l’observatoire de Paris; Max. Marie, Histoire des sciences, vii. 214; R. Wolf, Geschichte der Astronomie, p. 451; J.C. Houzeau, Bibl. astronomique; J. Delambre, Histoire de l’astronomie au XVIII’e siècle, pp. 250-275 (unfairly depreciatory); J.F. Montucla, Hist. des mathématiques, iv. 145, 248.

César François Cassini, or Cassini de Thury (1714-1784), son of Jacques Cassini, was born at the observatory of Paris on the 17th of June 1714. He succeeded to his father’s official employments, continued the hereditary surveying operations, and began in 1744 the construction of a great topographical map of France. The post of director of the Paris observatory was created for his benefit in 1771, when the establishment ceased to be a dependency of the Academy of Sciences. Cassini de Thury died at Thury on the 4th of September 1784. His chief works are:—Méridienne de l’observatoire de Paris (1744), Description géométrique de la terre (1775), and Description géométrique de la France (1784).

See C. Wolf, Histoire de l’observatoire de Paris, p. 287; Max. Marie, Histoire des sciences, viii. 158; J. Delambre, Histoire de I’astronomie au XVIII’e siècle, pp. 275-309; R. Wolf, Geschichte der Astronomie, p. 451; J.J. de Lalande, Bibliographic astronomique.

Jacques Dominique Cassini, Count (1748-1845), son of César François Cassini, was born at the observatory of Paris on the 30th of June 1748. He succeeded in 1784 to the directorate of the observatory; but his plans for its restoration and re-equipment were wrecked in 1793 by the animosity of the National Assembly. His position having become intolerable, he resigned on the 6th of September, and was thrown into prison in 1794, but released after seven months. He then withdrew to Thury, where he died, aged ninety-seven, on the 18th of October 1845\. He published in 1770 an account of a voyage to America in 1768, undertaken as the commissary of the Academy of Sciences with a view to testing Pierre Leroy’s watches at sea. A memoir in which he described the operations superintended by him in 1787 for connecting the observatories of Paris and Greenwich by longitude-determinations appeared in 1791. He visited England for the purposes of the work, and saw William Herschel at Slough. He completed his father’s map of France, which was published by the Academy of Sciences in 1793. It served as the basis for the Atlas National (1791), showing France in departments. Count Cassini’s Mémoires pour servir à l’histoire de l’observatoire de Paris (1810) embodied portions of an extensive work, the prospectus of which he had submitted to the Academy of Sciences in 1774. The volume included his Éloges of several academicians, and the autobiography of his great-grandfather, the first Cassini.

See J.F.S. Devic, Histoire de la vie et des travaux de J.D. Cassini (1851); J. Delambre, Histoire de l’astronomie au XVIII’e siècle, pp. 309-313; Phil. Mag. 3rd series, vol. xxviii. p. 412; C. Wolf, Histoire de l’observatoire de Paris (1902), p. 234 et passim.

(A. M. C.)

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