Cassini, John Dominic

, an eminent astronomer, was born of noble parents, at a town in Piedmont in Italy, June 8, 1635. After he had laid a proper foundation in his studies at home, he was sent to continue them in a college of Jesuits at Genoa. He had an uncommon turn for Latin poetry, which he exercised so very early, that poems of his were published when he was but eleven years old. At length he fell in with books of astronomy, which he read with great eagerness; and feeling a strong propensity to proceed farther in that science, in a short time he made so amazing a progress, that, in 1650, the senate of Bologna invited him to be their public methematical professor. He was not more than fifteen years of age when he went to Bologna, where he taught mathematics, and made observations upon the heavens with great care and assiduity. In 1652 a comet appeared, which he observed with great accuracy; and discovered, that comets were not bodies accidentally generated in the atmosphere, as had usually been supposed, but of the same nature, and probably governed by the same laws, as the planets. The same year he solved an astronomical problem, which Kepler and Bullialdus had given up as insolvable; viz. to determine geometrically the apogee and eccentricity of a planet from its true and mean place. In 1653, when a church of Bologna was repaired and enlarged, he obtained leave of the senate to correct and settle a meridian line, which had been drawn by an astronomer in 1575. These were circumstances very remarkable in one who had not yet attained his twentieth year. In 1657 he attended, as an assistant, a nobleman, who was sent to Rome to compose some differences which had arisen between Bologna and Ferrara, from the inundations of the Po; and shewed so much skill and judgment in the management of that affair, that in 1663, Marius Chigi, brother of pope Alexander VII. appointed him inspector-general of the fortifications of the castle of Urbino; and he had afterwards committed to him the care of all the rivers in the ecclesiastical state.

Meanwhile he did not neglect his astronomical studies, | but cultivated them with great care. He made many discoveries relative to the planets Mars and Venus, especially the revolution of Mars upon his own axis; but his principal object was to settle an accurate theory of Jupiter’s satellites, which after much labour and watching he happily effected, and published it at Rome, among other astronomical pieces, in 1666. Picard, the French astronomer, getting Cassini’s tables of Jupiter’s satellites, found them so very exact, that he conceived the highest opinion of his skill; and from that time his fame increased so fast in France, that Lewis XIV. desired to have him a member of the academy. Cassini, however, could not leave his station, without leave of his superiors; and therefore Lewis requested of pope Clement IX. and of the senate of Bologna, that Cassini might be permitted to come into France. Leave was granted for six years; and he came to Paris in the beginning of 1669, where he was immediately made the king’s astronomer. When this term was near expiring, the pope and the senate of Bologna insisted upon his return, on pain of forfeiting his revenues and emoluments, which had hitherto been remitted to him; but the minister Colbert prevailed on him to stay, and he was naturalized in the latter end of 1673, in which same year also he married.

The royal observatory of Paris had been finished some time. The occasion of its being built was this: In 1638, the famous minim Mersenne was the author and institutor of a society, where several ingenious and learned men met together to talk upon physical and astronomical subjects; among whom were Gassendi, Descartes, Monmort, Thevenot, Bulliald, our countryman Hobbes, &c. and this society was kept up by a succession of such men for many years. At length Lewis XIV. considering that a number of learned men acting in a body would succeed abundantly better in the promotion of science, than if they acted separately, each in his particular art or province, established under the direction of Colbert, in 1666, the royal academy of sciences: and for the advancement of astronomy in particular, erected the royal observatory at Paris, and furnished it with all kinds of instruments that were necessary to make observations. The foundation of this noble pile was laid in 1667, and the building completed in 1670. Cassini was appointed to be the first inhabiter of the observatory; and he took possession of it Sept. 1671, when he | applied himself with fresh alacrity to the business of his profession. In 1672 he endeavoured to determine the parallax of Mars and the sun, by comparing some observations which he made at Paris, with some which were made at the same time in America* In 1677 he demonstrated the diurnal revolution of Jupiter round his axis, to be performed in nine hours and fifty-eight minutes, from the motion of a spot in one of his larger belts. In 1684 he discovered four satellites of Saturn, besides that which Huygens had found out. In 1693 he published a new edition of his “Tables of Jupiter’s Satellites,” corrected by later observations. In 1695 he took a journey to Bologna, to examine the meridian line, which he had fixed there in 1655; and he shewed, in the presence of eminent mathematicians, that it had not varied in the least during those forty years. In 1700 he continued the meridian line through France, which Picard had begun, to the extremest southern part of that country.

After Cassini had inhabited the royal observatory for more than forty years, and done great honour to himself and his royal master by many excellent and useful discoveries which he published from time to time, he died Sept. 14, 1712. He had been deprived of his sight for a few years before his death, but had no other complaint, and preserved the amiable simplicity and tranquillity of his mind and character to the last. His works are extremely numerous. Fabroni, who has written the best life of Cassini, has also given the most complete list of his works, the titles of which would occupy nearly a sheet of this work. ¹