(Johannes Dominicus), a most excellent astronomer, born at Piccadilly in 1635. His early proficiency in astronomy procured him an invitation to be mathematical professor at Bologna when he was no more than 15 years of age: and a comet appearing in 1652, he discovered that comets were 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 given up 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 state: he still however prosecuted his astronomical studies, by discovering the revolution of Mars round his own axis; and, in 1666, published his theory of Jupiter's satellites. Cassini was invited into France by Louis XIV. in 1669, 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 found one before. He inhabited the royal observatory at Paris more than forty years; and when he died in 1712, was succeeded by his only son James Cassini.