The study of the surface of the moon was carried on with great care by _John Hevel_ of Danzig (1611-1687), who published in 1647 his _Selenographia_, or description of the moon, magnificently illustrated by plates engraved as well as drawn by himself. The chief features of the moon—mountains, craters, and the dark spaces then believed to be seas—were systematically described and named, for the most part after corresponding features of our own earth. Hevel’s names for the chief mountain ranges, _e.g._ the _Apennines_ and the _Alps_, and for the seas, _e.g._ _Mare Serenitatis_ or Pacific Ocean, have lasted till to-day; but similar names given by him to single mountains and craters have disappeared, and they are now called after various distinguished men of science and philosophers, _e.g._ _Plato_ and _Coppernicus_, in accordance with a system introduced by _John Baptist Riccioli_ (1598-1671) in his bulky treatise on astronomy called the _New Almagest_ (1651). Hevel, who was an indefatigable worker, published two large books on comets, _Prodromus Cometicus_ (1654) and _Cometographia_ (1668), containing the first systematic account of all recorded comets. He constructed also a catalogue of about 1,500 stars, observed on the whole with accuracy rather greater than Tycho’s, though still without the use of the telescope; he published in addition an improved set of tables of the sun, and a variety of other calculations and observations. 154. The planets were also watched with interest by a number of observers, who detected at different times bright or dark markings on Jupiter, Mars, and Venus. The two appendages of Saturn which Galilei had discovered in 1610 and had been unable to see two years later (chapter VI., § 123) were seen and described by a number of astronomers under a perplexing variety of appearances, and the mystery was only unravelled, nearly half a century after Galilei’s first observation, by the greatest astronomer of this period, _Christiaan Huygens_ (1629-1695), a native of the Hague. Huygens possessed remarkable ability, both practical and theoretical, in several different directions, and his contributions to astronomy were only a small part of his services to science. Having acquired the art of grinding lenses with unusual accuracy, he was able to construct telescopes of much greater power than his predecessors. By the help of one of these instruments he discovered in 1655 a satellite of Saturn (_Titan_). With one of those remnants of mediaeval mysticism from which even the soberest minds of the century freed themselves with the greatest difficulty, he asserted that, as the total number of planets and satellites now reached the perfect number 12, no more remained to be discovered—a prophecy which has been abundantly falsified since (§ 160; chapter XII., §§ 253, 255; chapter XIII., §§ 289, 294, 295).