The fact that the moon shows phases signifies that she shines only by reflected light; and it is surprising to notice how little of the light that falls upon her is really reflected by her. On an ordinarily clear night most people would probably say that the moon is much brighter than any terrestrial object viewed in the daytime, when it also is lit by the sun, as the moon is. Yet a very simple comparison will show that this is not so. If the moon be compared during the daytime with the clouds floating around her, she will be seen to be certainly not brighter than they, generally much less bright; indeed, even an ordinary surface of sandstone will look as bright as her disc. In fact, the reason of her great apparent brightness at night is merely the contrast between her and the dark background against which she is seen; a fragment of our own world, put in her place, would shine quite as brightly, perhaps even more so. It is possibly rather difficult at first to realize that our earth is shining to the moon and to the other planets as they do to us, but anyone who watches the moon for a few days after new will find convincing evidence of the fact. Within the arms of the thin crescent can be seen the whole body of the lunar globe, shining with a dingy coppery kind of light—'the ashen light,' as it is called. People talk of this as 'the old moon in the young moon's arms,' and weather-wise (or foolish) individuals pronounce it to be a sign of bad weather. It is, of course, nothing of the sort, for it can be seen every month when the sky is reasonably clear; but it is the sign that our world shines to the other worlds of space as they do to her; for this dim light upon the part of the moon unlit by the sun is simply the light which our own world reflects from her surface to the moon. In amount it is thirteen times more than that which the moon gives to us, as the earth presents to her satellite a disc thirteen times as large as that exhibited by the latter.
The moon's function in causing eclipses of the sun has already been briefly alluded to. In turn she is herself eclipsed, by passing behind the earth and into the long cone of shadow which our world casts behind it into space (Fig. 19). It is obvious that such eclipses can only happen when the moon is full. A total eclipse of the moon, though by no means so important as a solar eclipse, is yet a very interesting and beautiful sight. The faint shadow or penumbra is often scarcely perceptible as the moon passes through it; but the passage of the dark umbra over the various lunar formations can be readily traced, and is most impressive. Cases of 'black eclipses' have been sometimes recorded, in which the moon at totality has seemed actually to disappear as though blotted out of the heavens; but in general this is not the case. The lunar disc still remains visible, shining with a dull coppery light, something like the ashen light, but of a redder tone. This is due to the fact that our earth is not, like its satellite, a next to airless globe, but is possessed of a pretty extensive atmosphere. By this atmosphere those rays of the sun which would otherwise have just passed the edge of the world are caught and refracted so that they are directed upon the face of the eclipsed moon, lighting it up feebly. The redness of the light is due to that same atmospheric absorption of the green and blue rays which causes the body of the setting sun to seem red when viewed through the dense layer of vapours near the horizon. When the moon appears totally eclipsed to us, the sun must appear totally eclipsed to an observer stationed on the moon. A total solar eclipse seen from the moon must present features of interest differing to some extent from those which the similar phenomenon exhibits to us. The duration of totality will be much longer, and, in addition to the usual display of prominences and corona, there will be the strange and weird effect of the black globe of our world becoming gradually bordered with a rim of ruddy light as our atmosphere catches and bends the solar rays inwards upon the lunar surface.
In nine cases out of ten the moon will be the first object to which the beginner turns his telescope, and he will find in our satellite a never-failing source of interest, and a sphere in which, by patient observation and the practice of steadily recording what is seen, he may not only amuse and instruct himself, but actually do work that may become genuinely useful in the furtherance of the science. The possession of powerful instrumental means is not an absolute essential here, for the comparative nearness of the object brings it well within the reach of moderate glasses. The writer well remembers the keen feeling of delight with which he first discovered that a very humble and commonplace telescope—nothing more, in fact, than a small ordinary spy-glass with an object-glass of about 1 inch in aperture—was able to reveal many of the more prominent features of lunar scenery; and the possessor of any telescope, no matter whether its powers be great or small, may be assured that there is enough work awaiting him on the moon to occupy the spare time of many years with one of the most enthralling of studies. The view that is given by even the smallest instrument is one of infinite variety and beauty; and its interest is accentuated by the fact that the moon is a sphere where practically every detail is new and strange.
If the moon be crescent, or near one or other of her quarters at the time of observation, the eye will at once be caught by a multitude of circular, or nearly circular depressions, more clearly marked the nearer they are to the line of division between the illuminated and unilluminated portions of the disc. (This line is known as the Terminator, the circular outline, fully illuminated, being called the Limb). The margins of some of these depressions will be seen actually to project like rings of light into the darkness, while their interiors are filled with black shadow (Plates [XI.], [XIII.], [XV.], and [XVI.]). At one or two points long bright ridges will be seen, extending for many miles across the surface, and marking the line of one or other of the prominent ranges of lunar mountains (Plates [XI.], [XIII.], [XVI.], [XVII.]); while the whole disc is mottled over with patches of varied colour, ranging from dark grey up to a brilliant yellow which, in some instances, nearly approaches to white.
If observation be conducted at or near the full, the conditions will be found to have entirely changed. There are now very few ruggednesses visible on the edge of the disc, which now presents an almost smooth circular outline, nor are there any shadows traceable on the surface. The circular depressions, formerly so conspicuous, have now almost entirely vanished, though the positions and outlines of a few of them may still be traced by their contrast in colour with the surrounding regions. The observer's attention is now claimed by the extraordinary brilliance and variety of the tones which diversify the sphere, and particularly by the curious systems of bright streaks radiating from certain well-marked centres, one of which, the system originating near Tycho, a prominent crater not very far from the South Pole, is so conspicuous as to give the full moon very much the appearance of a badly-peeled orange (Plate [XII.]).
The Moon, November 13, 1902. Paris Observatory.
As soon as the moon has passed the full, the ruggedness of its margin begins once more to become apparent, but this time on the opposite side; and the observer, if he have the patience to work late at night or early in the morning, has the opportunity of seeing again all the features which he saw on the waxing moon, but this time with the shadows thrown the reverse way—under evening instead of under morning illumination. In fact the character of any formation cannot be truly appreciated until it has been carefully studied under the setting as well as under the rising and meridian sun.
We must now turn our attention to the various types of formation which are to be found upon the moon. These may be roughly summarized as follows: (1) The great grey plains, commonly known as Maria, or seas; (2) the circular or approximately circular formations, known generally as the lunar craters, but divided by astronomers into a number of classes to which reference will be made later; (3) the mountain ranges, corresponding with more or less closeness to similar features on our own globe; (4) the clefts or rills; (5) the systems of bright rays, to which allusion has already been made.