To these strange formations we have scarcely the faintest analogy on earth. Their multitude will at once strike even the most casual observer. Galileo compared them to the 'eyes' in a peacock's tail, and the comparison is not inapt, especially when the moon is viewed with a small telescope and low powers. In the Southern Hemisphere particularly, they simply swarm to such an extent that the district near the terminator presents much the appearance of a honeycomb with very irregular cells, or a piece of very porous pumice (Plate [XIV.]). Their vast size is not less remarkable than their number. One of the most conspicuous, for example, is the great walled-plain Ptolemäus, which is well-placed for observation near the centre of the visible hemisphere. It measures 115 miles from side to side of its great rampart, which, in at least one peak, towers more than 9,000 feet above the floor of the plain within. The area of this enormous enclosure is about equal to the combined areas of Yorkshire, Lancashire, and Westmorland—an extent so vast that an observer stationed at its centre would see no trace of the mountain-wall which bounds it, save at one point towards the West, where the upper part of the great 9,000-feet peak already referred to would break the line of the horizon (Plate [XIX.], No. 111; Plate [XIII.]).
Nor is Ptolemäus by any means the largest of these objects. Clavius, lying towards the South Pole, measures no less than 142 miles from wall to wall, and includes within its tremendous rampart an area of at least 16,000 square miles. The great wall which encloses this space, itself no mean range of mountains, stands some 12,000 feet above the surface of the plain within, while in one peak it rises to a height of 17,000 feet. Clavius is remarkable also for the number of smaller craters associated with it. There are two conspicuous ones, one on the north, one on the south side of its wall, each about twenty-five miles in diameter, while the floor is broken by a chain of four large craters and a considerable number of smaller ones.
Though unfavourably placed for observation, there is no lunar feature which can compare in grandeur with Clavius when viewed either at sunrise or sunset. At sunrise the great plain appears first as a huge bay of black shadow, so large as distinctly to blunt the southern horn of the moon to the naked eye. As the sun climbs higher, a few bright points appear within this bay of darkness—the summits of the walls of the larger craters—these bright islands gradually forming fine rings of light in the shadow which still covers the floor of the great plain. In the East some star-like points mark where the peaks of the eastern wall are beginning to catch the dawn. Then delicate streaks of light begin to stream across the floor, and the dark mass of shadow divides itself into long pointed shafts, which stretch across the plain like the spires of some great cathedral. The whole spectacle is so magnificent and strange that no words can do justice to it; and once seen it will not readily be forgotten. Even a small telescope will enable the student to detect and draw the more important features of this great formation; and for those whose instruments are more powerful there is practically no limit to the work that may be done on Clavius, which has never been studied with the minuteness that so great and interesting an object deserves. (Clavius is No. 13, Plate [XIX.] See also Plates [XIII.] and [XV.], and Fig. 22, the latter a rough sketch with a 2⅝-inch refractor.)
From such gigantic forms as these, the craters range downwards in an unbroken sequence through striking objects such as Tycho and the grand Copernicus, both distinguished for their systems of bright rays, as well as for their massive and regular ramparts, to tiny pits of black shadow, a few hundred feet across, and with no visible walls, which tax the powers of the very finest instruments. Schmidt's great map lays down nearly 33,000 craters, and it is quite certain that these are not nearly all which can be seen even with a moderate-sized telescope.
Clavius, Tycho, and Mare Nubium. Yerkes Observatory.
As to the cause which has resulted in this multitude of circular forms, there is no definite consensus of opinion. Volcanic action is the agency generally invoked; but, even allowing for the diminished force of gravity upon the moon, it is difficult to conceive of volcanic action of such intensity as to have produced some of the great walled-plains. Indeed, Neison remarks that such formations are much more akin to the smaller Maria, and bear but little resemblance to true products of volcanic action. But it seems difficult to tell where a division is to be made, with any pretence to accuracy, between such forms as might certainly be thus produced and those next above them in size. The various classes of formation shade one into the other by almost imperceptible degrees.
FIG. 22.