Page:The Moon (Pickering).djvu/120

84 discovered the libration in latitude, and measured trigonometrically the heights of some of the principal mountains.

The first satisfactory map of the Moon was constructed by Hevelius, the celebrated astronomer of Dantzic, in the year 1647. He named some 250 points on the Moon's surface, in general after terrestrial formations that they were supposed to resemble. These names, accepted at first, were later rejected, and only six of them are in use at the present time. He discovered the libration in longitude.

In 1651 Riccioli published his map of the Moon, based largely on drawings made by his pupil, Grimaldi. It is perhaps slightly inferior in accuracy but superior in detail to that of Hevelius (see Plate I). He proposed a new system of nomenclature in which the mares, or seas, were named after astrological influences that were supposed to be exerted by the Moon, while the craters were named for astronomers and men of science. Unfortxmately, his plan superseded that of Hevelius, about 200 of his names being still in use, many of them commemorating persons whom the modern astronomer would scarcely think of associating with the Moon.

In 1680 Dominic Cassini published his map of the Moon on a scale of twenty inches in diameter. It was more complete than those of Hevelius and Riccioli, but rather deficient in accuracy.

In 1687 Newton published his great work, the "Principia," stating the law of gravitation and applying it to the case of the Moon. He showed not only why the Moon revolved about the Earth, but he also explained the causes, and computed the amount of the chief inequalities in its motion.

During the first half of the eighteenth century but little of moment was discovered with regard to the Moon, but during the last half the six great mathematicians, Euler, D'Alembert, Clairault, Lalande, Lagrange and Laplace, contributed profound researches on the theory of the Moon's motions, and published tables from which at any time its position could be computed.

In 1775 appeared Tobias Mayer's map of the Moon, based on accurate measurements with the meridian circle (see Plate J). Although only eight inches in diameter and lacking in detail, it was the first and only accurate map of the Moon until about 1805.

In 1791 and 1802 appeared the two volumes of Schröter's " Selenotopographische Fragmente," the first great work giving detailed drawings and descriptions of the various lunar formations. Schroter was practically, if not the founder, at least the first great