Popular Science Monthly/Volume 3/October 1873/The Moon
OUR satellite holds a somewhat anomalous position in the literature of astronomy. The most beautiful object in the heavens, the orb which telescopists study under the most favorable conditions, and the planet—for a planet she is—which has afforded the most important information respecting the economy of the universe, she nevertheless has not received that attention from descriptive writers which she really merits. The cause is, perhaps, not far to seek. The beauty of the moon can scarcely be described in words, and cannot be
pictured by the most skilful artist; the information conveyed by the telescope is too definite to permit of speculation as with the other planets, yet not definite enough to solve the questions about which the students of astronomical works take most interest; and the information which astronomers have obtained from the moon's motions can only be appreciated when those motions are thoroughly analyzed, and it has not been found easy to simplify this analysis, that the general reader might fairly be expected to take interest in the matter.
The work before us is intended to remove this long-recognized want in the literature of astronomy. The time has come when this is practicable. The splendid photographs of Rutherford, of New York, and De La Rue, in England, supply the means of exhibiting truthfully the real nature of our satellite's surface. Mr. Proctor has been fortunate in obtaining from Mr. Rutherford permission to use three of his most effective photographs of the moon to illustrate the present work. Recent researches, again, into the processes which are going on within the solar system (so long mistakenly supposed to be unchanging in condition), suggest considerations respecting the past condition of the moon, at once bringing her within the range of speculation and theory. Telescopic observations, also more scrutinizing than those made of yore, and applied more persistently, begin to indicate the possibility at least of recognizing the signs of change, and perhaps of showing that our moon is not the dead and arid waste which astronomers have hitherto supposed her to be. The heat measurements of Lord Rosse also throw important light on the question of her present condition. And then, as respects those points which constitute the main scientific interest of our satellite, her motions under the varying influences to which she is subjected, Mr. Proctor has devoted here his full energies and the results of a long experience, to the endeavor to make clear, even to those who are not mathematicians, the considerations which, weighed and analyzed in the wonderful brain of Newton, supplied the means of demonstrating the theory of the universe.
On this important department of his subject, Mr. Proctor makes the following remarks in his preface: "In Chapter II. I have given a very full account of the peculiarities of the moon's motions; and, notwithstanding the acknowledged difficulty of the subject, I think my account is sufficiently clear and simple to be understood by any one, even though not acquainted with the elements of mathematics, who will be at the pains to read it attentively through. I have sought to make the subject clear to a far wider range of readers than the class for which Sir G. Airy's treatise on 'Gravitation' was written, while yet not omitting any essential points in the argument. In order to combine independence of treatment with exactness and completeness, I first wrote the chapter without consulting any other work. Then I went through it afresh, carefully comparing each section with the corresponding part of Sir G. Airy's 'Gravitation,' and Sir J. Herschel's chapters on the lunar motions in his 'Outlines of Astronomy.' I was thus able to correct any errors in my own work, while in turn I detected a few (mentioned in the notes) in the works referred to. I have adopted a much more complete and exact system of illustration in dealing with the moon's motions than either of my predecessors in the explanation of this subject. I attach great importance to this feature of my explanation, experience having satisfied me not only that such matters should be very freely illustrated, but that the illustrations should aim at correctness of detail, and (wherever practicable) of scale also. Some features, as the advance of the perigee and the retreat of the nodes, have, I believe, never before been illustrated at all."
In Chapter III. Mr. Proctor gives, among other matters, a full explanation of the effects due to the strange balancing motion called the lunar librations. He says: "I have been surprised to find how imperfectly this interesting and important subject has been dealt with hitherto. In fact, I have sought in vain for any discussion of the subject with which to compare my own results. I have, however, in various ways sufficiently tested these results."
But probably, to the greater number of readers, the main interest of the book will be found in the chapters relating to the condition of the moon's surface—the mountains, craters, hills, valleys, which diversify its strange varieties of brightness, color, and tone, and the changes of appearance which are noted as the illumination varies, and as the lunar librations change the position of different regions. It is, by-the-way,to be noted that the moon, which we regard as of silvery whiteness, is in reality more nearly black than white, a fact which will recall to many of our readers a remark of Prof. Tyndall's in the first lecture of the course recently delivered here.
"The moon appears to us," he said, "as if
'Clothed in white samite, mystic, beautiful,'
but, were she covered with the blackest velvet, she would still hang in the heavens as a white orb, shining upon the world substantially as she does now."
Mr. Proctor discusses also the phenomena presented to lunarians, if such there be. The extreme rarity of the lunar atmosphere renders the idea of existence on the moon rather strange to our conceptions, but, as Sir J. Herschel has said in a similar case, "we should do wrong to judge of the fitness or unfitness of" the condition of lunarians "from what we see around us, when perhaps the very combinations which convey to our minds only images of horror may be, in reality, theatres of the most striking and glorious displays of beneficent contrivance." Speaking of the appearances presented by lunar landscapes, two of which we borrow from his work, Mr. Proctor remarks that "we know far too little respecting the real details of lunar scenery to form any satisfactory opinion on the subject. If a landscape-painter were invited to draw a picture presenting his conceptions of the scenery of a region which he had only viewed from a distance of a hundred miles, he would be under no greater difficulties than the astronomer who undertakes to draw a lunar landscape, as it would actually appear to any one placed on the surface of the moon. "We know certain facts—we know that there are striking forms of irregularity, that the shadows must be much darker as well during the lunar day as during an earth-lit lunar light, than on our own earth in sunlight or moonlight, and we know that, whatever features of our own landscapes are certainly due to the action of water in river, rain, or flood, to the action of wind and weather, or to the growth of forms of vegetation with which we are familiar, ought assuredly not to be shown in any lunar landscape. But a multitude of details absolutely necessary for the due presentation of lunar scenery are absolutely unknown to us. Nor is it so easy as many imagine to draw a landscape which shall be correct even as respects the circumstances known to us. For instance, though I have seen many pictures called lunar landscapes, I have never seen one in which there have not been features manifestly due to weathering and to the action of running water. The shadows, again, are never shown as they would be actually seen if regions of the indicated configuration were illuminated by a sun, but not by a sky of light. Again, aerial perspective is never totally abandoned, as it ought to be in any delineation of lunar scenery. I do not profess to have done better myself in the accompanying lunar landscapes. I have, in fact, cared rather to indicate the celestial than the lunarian features shown in these drawings. Still, I have selected a class of lunar objects which may be regarded as, on the whole, more characteristic than the mountain-scenery usually exhibited. And, by picturing the greater part of the landscape as at a considerable distance, I have been freer to reproduce what the telescope actually reveals. In looking at one of these views, the observer must suppose himself stationed at the summit of some very lofty peak, and that the view shows only a very small portion of what would really be seen under such circumstances in any particular direction. The portion of the sky shown in either picture extends only a few degrees from the horizon, as is manifest from the dimensions of the earth's disk; and thus it is shown that only a few degrees of the horizon are included in the landscape."
Our author then pictures the aspect of the lunar heavens by night and by day. "We have space but for a few passages from this description: "To an observer stationed upon a summit of the lunar Apennines on the evening of November 1, 1872, a scene was presented unlike any known to the inhabitants of earth. It was near the middle of the long lunar night. On a sky of inky blackness stars innumerable were spread, among which the orbs forming our constellations could be recognized by their superior lustre, but yet were almost lost amid myriads of stars unseen by the inhabitants of earth. Nearly overhead shone the Pleiades, closely girt round by hundreds of lesser lights. From them toward Aldebaran and the clustering Hyades, and onward to the belted Orion, streams and convolutions of stars, interwoven as in fantastic garlands, marked the presence of that mysterious branch-like extension of the Milky-Way which the observer on earth can, with unaided vision, trace no farther than the winged foot of Perseus. High overhead, and toward the north, the Milky-Way shone resplendent, like a vast inclined arch, full 'thick inlaid with patines of bright gold.' Instead of that faint, cloud-like zone known to terrestrial astronomers, the galaxy presented itself as an infinitely complicated star-region—
'With isles of light and silvery streams,
And gloomy griefs of mystic shade.'
"On all sides, this mighty star-belt spread its outlying bands of stars, far away on the one hand toward Lyra and Boötes, where on earth we see no traces of milky lustre, and on the other toward the Twins and the clustering glories of Cancer the—'dark constellation' of the ancients, but full of telescopic splendors. Most marvellous, too, appeared the great dark gap which lies between the Milky-Way and Taurus; here, in the very heart of the richest region of the heavens—with Orion and the Hyades and Pleiades blazing on one side, and on the other the splendid stream laving the feet of the Twins—there lay a deep, black gulf which seemed like an opening through our star-system into starless depths beyond.
Yet, though the sky was thus aglow with starlight, though stars far fainter than the least we see on the clearest and darkest night were shining in countless myriads, an orb was above the horizon whose light would pale the lustre of our brightest stars. This orb occupied a space on the heavens more than twelve times larger than is occupied by the full moon as we see her. Its light, unlike the moon's, was tinted with beautiful and well-marked colors. . . .
"The globe which thus adorned the lunar sky, and illuminated the lunar lands with a light far exceeding that of the full moon, was our earth, The scene was not unlike that shown to Satan when Uriel—
'One of the seven
Who in God's presence, nearest to the throne,
pointing earthward from his station amid the splendor of the sun, said to the arch-fiend:
'Look down-ward on that glohe whose hither side
"With light from hence, though hut reflected, shines:
"In all other respects the scene presented to the spectator on the moon was similar; but, as seen from the lunar Apennines, the glorious orb of earth shone high in the heavens; and the sun, source of the light then bathing her oceans and continents, lay far down below the level of the lunar horizon. . . .
"Infinitely more wonderful, however, and transcending in sublimity all that the heavens display to the contemplation of the inhabitants of earth, was the scene presented when the sun himself had risen. I shall venture here to borrow some passages from an essay entitled 'A Voyage to the Sun,' in which a friend of mine has described the aspect of the sun as seen from a station outside that atmosphere of ours which veils the chief glories of the luminary of day: 'The sun's orb was more brilliantly white than when seen through the air, but close scrutiny revealed a diminution of brilliancy toward the edge of the disk, which, when fully recognized, presented him at once as the globe he really is. On this globe could be distinguished the spots and the bright streaks called faculæ. This globe was surrounded with the most amazingly complex halo of glory. Close around the bright whiteness of the disk, and shining far more beautiful by contrast with that whiteness than as seen against the black disk of the moon in total eclipses, stood the colored region called the chromatosphere, not red, as it appears during eclipses, but gleaming with a mixed lustre of pink and green, through which, from time to time, passed the most startlingly brilliant coruscations of orange and golden yellow light. Above this delicate circle of color towered tall prominences and multitudes of smaller ones. These, like the chromatosphere, were not red, but beautifully variegated. . . .'
"Much more might be said on this inviting subject, only that the requirements of space forbid, obliging me to remember that the moon and not the sun is the subject of this treatise. The reader, therefore, must picture to himself the advance of the sun with his splendid and complicated surroundings toward the earth, suspended almost unchangingly in the heavens, but assuming gradually the crescent form as the sun drew slowly near. He must imagine also how, in the mean time, the star-sphere was slowly moving westward, the constellations of the ecliptic in orderly succession passing behind the earth at a rate slightly exceeding that of the sun's approach, so that he, like the earth, only more slowly, was moving eastward, so far as the star-sphere was concerned, even while the moon's slow diurnal rotation was carrying him westward toward the earth."
In the last chapter the physical condition of the moon's surface is treated, and the processes by which she probably reached her present condition are discussed at considerable length.