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This observer was Father Riccioli, a contemporary of Hevelius. He drew up a rough map full of errors. But he gave to the lunar mountains the names of great men of antiquity and savants of his own epoch. A third map of the moon was executed in the seventeenth century by Dominique Cassini; superior to that of Riccioli in the execution, it is inexact in the measurements.
In the old days when the moon was supposed to be inhabited, those terrestrial godfathers, led by the astronomer Riccioli, who were busy bestowing names upon the "seas" and mountains of our patient satellite, may have pleased their imagination by picturing this arm of the "Serene Sea" as a peculiarly romantic sheet of water, amid whose magical influences the lunar gentlefolk, drifting softly in their silver galleons and barges, and enjoying the splendors of "full earth" poured upon their delightful little world, were accustomed to fall into charming reveries, as even we hard-headed sons of Adam occasionally do when the waters under the keel are calm and smooth and the balmy air of a moonlit night invokes the twin spirits of poetry and music.
Cluverius was a man of extensive and accurate erudition, which he applied to the illustration of ancient geography. Riccioli, an Italian Jesuit, devoted his abilities and leisure to the study of mathematics, and the sciences dependent upon it, particularly astronomy; and was thus enabled to render important service to the higher parts of geography.
"That's Plato;" said M'Nicholl; "I know it well; it's the darkest spot on the Moon: many a night I gazed at it from my little observatory in Broad Street, Philadelphia." "Right, Captain," said Barbican; "the crater Plato, is, indeed, generally considered the blackest spot on the Moon, but I am inclined to consider the spots Grimaldi and Riccioli on the extreme eastern edge to be somewhat darker.
But Beer and Madler measured the heights of lunar mountains by their shadows, and found four of them over 20,000 feet above the surrounding plains. Langrenus was the first to do serious work on selenography, and named the lunar features after eminent men. Riccioli also made lunar charts. In 1692 Cassini made a chart of the full moon.
It was then that Galileo explained the phenomena of light produced in certain phases by the existence of mountains, to which he gave an average height of 27,000 feet. After him, Hevelius, an astronomer of Dantzig, lowered the highest altitudes to 15,000 feet; but his contemporary, Riccioli, brought them up again to 21,000 feet.
The "secondary," or "ashen light," of Venus was first noticed by Riccioli in 1643; it was seen by Derham about 1715, by Kirch in 1721, by Schröter and Harding in 1806; and the reality of the appearance has since been authenticated by numerous and trustworthy observations.
The most brilliant object on the surface is the central peak of the ring-plain Aristarchus, the darkest the floor of Grimaldi, or perhaps a portion of that of the neighbouring Riccioli. Between these extremes, there is every gradation of tone.
They follow a parallel course, and terminate on the S. side of a crater-row, consisting of three bright craters ranging in a line parallel to the coarse valley. On the N. side of these objects, and tangential to them, is another cleft, which traverses the W. and E. walls of Lohrmann, and, crossing the region between it and Riccioli, terminates apparently at the W. wall of the latter formation.
Though a prominent and beautiful object under a low sun, its attenuated border and the tone of the floor, which scarcely differs from that of the surrounding surface, render it difficult to trace under a high angle of illumination, and perhaps accounts for the fact that it escaped the notice of Hevel and Riccioli; though it is certainly strange that a formation which is thrown into such strong relief at sunrise and sunset should have been overlooked, while others hardly more prominent at these times have been drawn and described.
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