Apennines, Alps, and Caucasus. Paris Observatory.
5. The Systems of Bright Rays, radiating from certain craters, remain the most enigmatic of the features of lunar scenery. Many of these systems have been traced and mapped, but we need only mention the three principal—those connected with Tycho, Copernicus, and Kepler, all shown on Plate [XII.] The Tycho system is by far the most noteworthy, and at once attracts the eye when even the smallest telescope is directed towards the full moon. The rays, which are of great brilliancy, appear to start, not exactly from the crater itself, but from a greyish area surrounding it, and they radiate in all directions over the surface, passing over, and almost completely masking in their course some of the largest of the lunar craters. Clavius, for example, and Maginus (Plate [XIV.]), become at full almost unidentifiable from this cause, though Neison's statement that 'not the slightest trace of these great walled-plains, with their extremely lofty and massive walls, can be detected in full,' is certainly exaggerated. The rays are not well seen save under a high sun—i.e., at or near full, though some of them can still be faintly traced under oblique illumination.
In ordinary telescopes, and to most eyes, the Tycho rays appear to run on uninterruptedly for enormous distances, one of them traversing almost the whole breadth of the moon in a north-westerly direction, and crossing the Mare Serenitatis, on whose dark background it is conspicuous. Professor W. H. Pickering, who has made a special study of the subject under very favourable conditions, maintains, however, that this appearance of great length is an illusion, and that the Tycho rays proper extend only for a short distance, being reinforced at intervals by fresh rays issuing from small craters on their track. The whole subject is one which requires careful study with the best optical means.
None of the other ray-systems are at all comparable with that of Tycho, though those in connection with Copernicus and Kepler are very striking. As to the origin and nature of these strange features, little is known. There are almost as many theories as there are systems; but it cannot be said that any particular view has commanded anything like general acceptance. Nasmyth's well-known theory was that they represented cracks in the lunar surface, caused by internal pressure, through which lava had welled forth and spread to a considerable distance on either side of the original chasm. Pickering suggests that they may be caused by a deposit of white powder, pumice, perhaps, emitted by the craters from which the rays originate. Both ideas are ingenious, but both present grave difficulties, and neither has commended itself to any very great extent to observers, a remark which applies to all other attempts at explanation.
Such are the main objects of interest upon the visible hemisphere of our satellite. In observing them, the beginner will do well, after the inevitable preliminary debauch of moon-gazing, during which he may be permitted to range over the whole surface and observe anything and everything, not to attempt an attack on too wide a field. Let him rather confine his energies to the detailed study of one or two particular formations, and to the delineation of all their features within reach of his instrument under all aspects and illuminations. By so doing he will learn more of the actual condition of the lunar surface than by any amount of general and haphazard observation; and may, indeed, render valuable service to the study of the moon.
Neither let him think that observations made with a small telescope are now of no account, in view of the number of large instruments employed, and of the great photographic atlases which are at present being constructed. It has to be remembered that the famous map of Beer and Mädler was the result of observations made with a 3¾-inch telescope, and that Lohrmann used an instrument of only 4⅘ inches, and sometimes one of 3¼. Anyone who has seen the maps of these observers will not fail to have a profound respect for the work that can be done with very moderate means. Nor have even the beautiful photographs of the Paris, Lick, and Yerkes Observatories superseded as yet the work of the human eye and hand. The best of the Yerkes photographs, taken with a 40-inch refractor, are said to show detail 'sufficiently minute to tax the powers of a 6-inch telescope.' But this can be said only of a very few photographs; and, generally speaking, a good 3-inch glass will show more detail than can be seen on any but a few exceptionally good negatives.
In conducting his observations, the student should be careful to outline his drawing on such a scale as will permit of the easy inclusion of all the details which he can see, otherwise the sketch will speedily become so crowded as to be indistinct and valueless. A scale of 1 inch to about 20 miles, corresponding roughly to 100 inches to the moon's diameter, will be found none too large in the case of formations where much detail has to be inserted—that is to say, in the case of the vast majority of lunar objects. Further, only such a moderate amount of surface should be selected for representation as can be carefully and accurately sketched in a period of not much over an hour at most; for, though the lunar day is so much longer than our own, yet the changes in aspect of the various formations due to the increasing or diminishing height of the sun become very apparent if observation be prolonged unduly; and thus different portions of the sketch represent different angles of illumination, and the finished drawing, though true in each separate detail, will be untrue as a whole.