Galileo’s best telescopes magnified only some thirty times, and the views which he thus obtained, must have been similar to those exhibited by the smaller photographs of the moon produced in late years by Mr. De la Rue and now familiar to the scientific public. Of course there is in the natural moon as viewed with a small telescope a vivid brilliancy which no art can imitate, and in photographs especially there is a tendency to exaggeration of the depths of shade in a lunar picture. This arises from the circumstance that various regions of the moon do not impress a chemically sensitized plate as they impress the retina of the eye. Some portions, notably the so-called “seas” of the moon, which to the eye appear but slightly duller than the brighter parts, give off so little actinic light that they appear as nearly black patches upon a photograph, and thus give an undue impression of the relative brightness of various parts of the lunar surface. Doubtless by sufficient exposure of the plate in the camera-telescope the dark patches might be rendered lighter, but in that case the more strongly illuminated portions, which after all are those most desirable to be preserved, would be lost by the effect which photographers understand as “solarization.”
In speaking of a view of the moon with a magnifying power of thirty, it is necessary to bear in mind that the visible features will differ considerably with the diameter of the object-glass of the telescope to which this power is applied. The same details would not be seen alike with the same power upon an object-glass of 10 inches diameter and one of 2 inches. The superior illumination of the image in the former case would bring into view minute details that could not be perceived with the smaller aperture. He who would for curiosity wish to see the moon, or any other object, as Galileo saw it, must use a telescope of the same size and character in all respects as Galileo’s: it will not do to put his magnifying power upon a larger telescope. With large telescopes, and low powers used upon bright objects like the moon, there is a blinding flood of light which tends to contract the pupil of the eye and prevent the passage of the whole of the pencil of rays coming through the eye-piece. Although this last result may be productive of no inconvenience, it is clearly a waste of light, and it points to a rule that the lowest power that a telescope should bear is that which gives a pencil of light equal in diameter to the pupil of the eye under the circumstances of brightness attendant upon the object viewed. In observing faint objects this point assumes more importance, since it is then necessary that all available light should enter the pupil. The thought suggests itself that an artificial enlargement of the pupil, as by a dose of belladonna, might be of assistance in searching for faint objects, such as nebulæ and comets: but we prefer to leave the experiment for those to try who pursue that branch of astronomical observation.
A merely cursory examination of the moon with the low power to which we have alluded is sufficient to show us the more salient features. In the first place we cannot help being struck with the immense preponderance of circular or craterform asperities, and with the general tendency to circular shape which is apparent in nearly all the lunar surface markings; for even the larger regions known as the “seas” and the smaller patches of the same character seem to repeat in their outlines the round form of the craters. It is at the boundary of sunlight on the lunar globe that we see these craterform spots to the best advantage, as it is there that the rising or setting sun casts long shadows over the lunar landscape, and brings elevations and asperities into bold relief. They vary greatly in size, some are so large as to bear an estimable proportion to the moon’s diameter, and the smallest are so minute as to need the most powerful telescopes and the finest conditions of atmosphere to perceive them. It is doubtful whether the smallest of them have ever been seen, for there is no reason to doubt that there exist countless numbers that are beyond the revealing powers of our finest telescopes.
From the great number and persistent character of these circumvallations, Kepler was led to think that they were of artificial construction. He regarded them as pits excavated by the supposed habitants of the moon to shelter themselves from the long and intense action of the sun. Had he known their real dimensions, of which we shall have to speak when we come to describe them more in detail, he would have hesitated in propounding such a hypothesis; nevertheless it was, to a certain extent, justified by the regular and seemingly unnatural recurrence of one particular form of structure, the like of which is, too, so seldom met with as a structural feature of the surface of our own globe.
The next most striking features, revealed by a low telescopic power upon the moon, are the seemingly smooth plains that have the appearance of dusky spots, and that collectively cover a considerable portion—about two-thirds—of the entire disc. The larger of these spots retain the name of seas, the term having been given when they were supposed to be watery expanses, and having been retained, possibly to avoid the confusion inevitable from a change of name, after the existence of water upon the moon was disproved. Following the same order of nomenclature, the smaller spots have received the appellations of lakes, bays and fens. We see that many of these “seas” are partially surrounded by ramparts or bulwarks which, under closer examination, and having regard to their real magnitude, resolve themselves into immense mountain chains. The general resemblance in form which the bulwarked plains thus exhibit to the circular craters of large size, would lead us to suppose that the two classes of objects had the same formative origin, but when we take into account the immense size of the former, and the process by which we infer the latter to have been developed, the supposition becomes untenable.
Another of the prominent features which we notice as highly curious, and in some phases of the moon—at about the time of full—the most remarkable of all, are certain bright lines that appear to radiate from some of the more conspicuous craters, and extend for hundreds of miles around. No selenological formations have so sorely puzzled observers as these peculiar streaks, and a great deal of fanciful theorizing has been bestowed upon them. As we are now only glancing at the moon, we do not enter upon explanations concerning them or any other class of details; all such will receive due consideration in their proper order in succeeding chapters.
We thus see that the classes of features observable upon the moon are not great in number: they may be summed up as craters and their central cones, mountain chains, with occasional isolated peaks, smooth plains, with more or less of irregularity of surface, and bright radiating streaks. But when we come to study with higher powers the individual examples of each class we meet with considerable diversity. This is especially the case with the craters, which appear under very numerous variations of the one order of structure, viz., the ring-form. A higher telescopic power shows us that not only do these craters exist of all magnitudes within a limit of largeness, but seemingly with no limit of smallness, but that in their structure and arrangement they present a great variety of points of difference. Some are seen to be considerably elevated above the surrounding surface, others are basins hollowed out of that surface and with low surrounding ramparts; some are merely like walled plains or amphitheatres with flat plateaux, while the majority have their lowest point of hollowness considerably below the general level of the surrounding surface; some are isolated upon the plains, others are aggregated into a thick crowd, and overlapping and intruding upon each other; some have elevated peaks or cones in their centres, and some are without these central cones, while the plateaux of others again contain several minute craters instead; some have their ramparts whole and perfect, others have them breached or malformed, and many have them divided into terraces, especially on their inner sides.
In the plains, what with a low power appeared smooth as a water surface becomes, under greater magnification, a rough and furrowed area, here gently undulated and there broken into ridges and declivities, with now and then deep rents or cracks extending for miles and spreading like river-beds into numerous ramifications. Craters of all sizes and classes are scattered over the plains; these appear generally of a different tint to the surrounding surface, for the light reflected from the plains has been observed to be slightly tinged with colour, The tint is not the same in all cases: one large sea has a dingy greenish tinge, others are merely grey and some others present a pale reddish hue. The cause of this diversity of colour is mysterious; it has been supposed to indicate the existence of vegetation of some sort; but this involves conditions that we know do not exist.
The mountains, under higher magnification, do not present such diversity of formation as the craters, or at least the points of difference are not so apparent; but they exhibit a plentiful variety of combinations. There are a few perfectly isolated examples that cast long shadows over the plains on which they stand like those of a towering cathedral in the rising or setting sun. Sometimes they are collected into groups, but mostly they are connected into stupendous chains. In one of the grandest of these chains, it has been estimated that a good telescope will show 3000 mountains clustered together, without approach to symmetrical order. The scenery which they would present, could we get any other than the “bird’s eye view” to which we are confined, must be imposing in the extreme, far exceeding in sublime grandeur anything that the Alps or the Himalayas offer; for while on the one hand the lunar mountains equal those of the earth in altitude, the absence of an atmosphere, and consequently of the effects produced thereby, must give rise to alternations of dazzling light and black depths of shade combining to form panoramas of wild scenery that, for want of a parallel on earth, we may well call unearthly. But we are debarred the pleasure of actually contemplating such pictures by the circumstance that we look down upon the mountain tops and into the valleys, so that the great height and close aggregation of the peaks and hills are not so apparent. To compare the lunar and terrestrial mountain scenery would be “to compare the different views of a town seen from the car of a balloon, with the more interesting prospects by a progress through the streets.” Some of the peculiarities of the lunar scenery we have, however, endeavoured to realize in a subsequent Chapter.
A high power gives us little more evidence than a low one upon the nature of the long bright streaks that radiate from some of the more conspicuous craters, but it enables us to see that those streaks do not arise from any perceptible difference of level of the surface—that they have no very definite outline, and that they do not present any sloping sides to catch more sunlight, and thus shine brighter, than the general surface. Indeed, one great peculiarity of them is that they come out most forcibly where the sun is shining perpendicularly upon them; hence they are best seen where the moon is at full, and they are not visible at all at those regions upon which the sun is rising or setting. We also see that they are not diverted by elevations in their path, as they traverse in their course craters, mountains, and plains alike, giving a slight additional brightness to all objects over which they pass, but producing no other effect upon them. To employ a commonplace simile, they look as though, after the whole surface of the moon had assumed its final configuration, a vast brush charged with a whitish pigment had been drawn over the globe in straight lines radiating from a central point, leaving its trail upon everything it touched, but obscuring nothing.