CASSINI (Jolts Dommc,) an eminent astronomer, was born of noble parents, at a town in Piedmont in Italy, June 8, 1625. After laying 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 some of his poems were pub lished when he was but 11 years old. At length he met with books of astronomy, which he read with great eagerness. Pursuing the bent of his inclinations in this way, in a short time he made so amazing a progress, that in 1650 the se nate of Bologna invited him to be their public mathematical professor. Cassini was but 25 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 he discovered that comets were not bodies accidentally ge nerated in the atmosphere, as had been supposed, but of the same nature, and probably governed by the same law, as the planets. The same year he resolved an astronomical problem, which Kepler and Bulliald had given up as insolvable ; viz, to determine geometrically the apo gee and eccentricity of a planet, from its true and mean place. In 1653, when a church in Bologna was repaired and en larged, he obtained leave of the senate to correct and settle a meridian line, which had been drawn by an astronomer in 1575. In 1657 he attended as an assistant to 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 he shewed so much skill and judgment in the management of the affair, that in 1663 the Pope's brother appointed him inspector-general of the fortifications of the castle of Urbino ; and he had after ward committed to him the care of all the rivers in the ecclesiastical state.
In the mean time he did not neglect his astronomical studies, and made several' discoveries relating to the planets Mars and Venus, particularly the revolution of Mars upon his own axis ; but the point he had chiefly in view was, to settle an accurate theory of Jupiter's satellites ; which, after much labour and observa tion, he happily effected, and published it at Rome, among other astronomical pieces, in 1666.
Picard, the French astronomer, get ting Cassini's tables of Jupiter's satellites, found them so very exact, that he con ceived the highest opinion of his skill ; and from that time his fame increased so fast in France, that the government de sired to have him a member of the aca demy. Cassini however could not leave his station without leave of his superiors ; and therefore the king, Lewis the XIVth, requested of the Pope, and the senate of Bologna, that Cassini might be- permit ted to come into France.' Leave was granted for six years, and he came to Pa ris in the beginning of 1669, where he was immediately made the king's astro nomer. When this term of six years was near expiring, the Pope and the senate of Bologna insisted upon his return, on pain of forfeiting his revenues and emo luments, which had hitherto been remit ted to him : but the minister Colbert pre vailed on him to stay, and he was natura lized in 1673; the same year also in which he was married.
The Royal Observatory of Paris had been finished some time, and Cassini was appointed to be the first inhabiter ; which he took possession of in Septem ber, 1671, when he set himself with fresh alacrity to attend the duties of his pro fession. In 1672 he endeavoured to de termine the parallax of Mars and the Sun ; and in 1677 he proved that the di urnal rotation of Jupiter round his axis was performed in 9 hours 58 minutes, from the motion of a spot in one of his larger belts : also in 1684 lie discovered four satellites of Saturn, besides that which Huygens bad found out. In 1693 he published a new edition of his " Ta bles of Jupiter's Satellites," corrected by later observations. In 1695 he took a journey to Bologna, to examine the me ridian line which he had fixed there in 1655; and lie shewed, in the presence of eminent mathematicians, that it had not varied in the least during that 40 years. In 1700 he continued the meri dian line through France, which Picard had begun, to the very southern limits of that country.
After our author had resided at the Royal Observatory for more than 40 years, making many excellent and use ful discoveries, which he published from time to time, he died September the 14th, 1712, at 87 years of age ; and was suc ceeded by his son James Cassini.