CASSINI, JOHANNES DOMINICUS, an excellent astronomer, was born at Piedmont in 1635. His early proficiency in astronomy procured him an invitation to become mathematical professor at Bologna when he was only fifteen years of age; and a comet having appeared in 1652, he discovered that bodies of this kind are not accidental meteors, but of the same nature, and probably governed by the same laws, as the planets. In the same year he solved a problem abandoned by Kepler and Bullialdus as insolvable; which was to determine geometrically the apogee and eccentricity of a planet from its true and mean place. In 1663 he was appointed inspector-general of the fortifications of the castle of Urbino, and had afterwards the care of all the rivers in the ecclesiastical states. He still, however, prosecuted his astronomical studies, in the course of which he discovered the revolution of Mars round his own axis; and in 1666 he published his theory of Jupiter's satellites. Cassini was invited by Louis XIV. in 1669 to visit France, where he settled as the first professor in the royal observatory. In 1677 he demonstrated the line of Jupiter's diurnal rotation; and in 1684 discovered four more satellites belonging to Saturn, Huygens having observed one. Cassini was principally instrumental in causing the French government undertake the expedition to Cayenne, which served to correct many errors relative to the figure of the earth, at the same time that it demonstrated the decrease of the intensity of terrestrial gravity from the equator to the poles; a phenomenon which exhibits a striking confirmation of the theory of gravitation. In 1693 he published new tables of the satellites of Jupiter, more exact than those which he had given to the world in 1668. In 1693 he went to revisit his meridian of St. Pétron, which he regarded with honest pride; but he was now occupied with another and much longer meridian, commenced in 1669 by Picard, continued to the north of Paris in 1683 by De Lahire, and lastly, in 1700, carried by Cassini himself to the extremity of Roussillon; being the same line which, forty years after, was measured anew by François Cassini and La Caille, and, at the same distance of a century, measured a third time by Méchain and Delambre, with a precision which left little more to be desired. In the last years of his life he lost his sight, a misfortune which befell him in common with Galileo, and which probably proceeded from the same cause, namely, excessive application to the delicate observations of astronomy. Cassini inhabited the royal observatory at Paris more than forty years, and died on the 14th September 1712, "sans maladie, sans douleur, uniquement par la nécessité de mourir." In Lalande's Bibliographie Astronomique will be found a complete enumeration of the various works of Cassini; we need only cite here, 1. Observationes Cometæ Ann. 1655 et 1653,
Modena, 1653, fol.; 2. Opera Astronomica, Rome, 1666; 3. Nuntii Syderei Interpres; 4. Cosmographie.