THE MOON

Our attention is next engaged by the body which is our nearest neighbour in space and our most faithful attendant and useful servant. The moon is an orb of 2,163 miles in diameter, which revolves round our earth in a slightly elliptical orbit, at a mean distance of about 240,000 miles. The face which she turns to us is a trifle greater in area than the Russian Empire, while her total surface is almost exactly equal to the areas of North and South America, islands excluded. Her volume is about ²/99 of that of the earth; her materials are, however, much less dense than those of which our world is composed, so that it would take about eighty-one moons to balance the earth. One result of these relations is that the force of gravity at the lunar surface is only about one-sixth of that at the surface of the earth, so that a twelve-stone man, if transported to the moon, would weigh only two stone, and would be capable of gigantic feats in the way of leaping and lifting weights. The fact of the diminished force of gravity is of importance in the consideration of the question of lunar surfacing.

The most conspicuous service which our satellite performs for us is that of raising the tides. The complete statement of the manner in which she does this would be too long for our pages; but the general outline of it will be seen from the accompanying rough diagram (Fig. 21), which, it must be remembered, makes no attempt at representing the scale either of the bodies concerned or of their distances from one another, but simply pictures their relations to one another at the times of spring and neap tides. The moon (M in Fig. 21, A) attracts the whole earth towards it. Its attraction is greatest at the point nearest to it, and therefore the water on the moonward side is drawn up, as it were, into a heap, making high tide on that side of the earth. But there is also high tide at the opposite side, the reason being that the solid body of the earth, which is nearer to the moon than the water on the further side, is more strongly attracted, and so leaves the water behind it. Thus there are high tides at the two opposite sides of the earth which lie in a straight line with the moon, and corresponding low tides at the intermediate positions. Tides are also produced by the attraction of the sun, but his vastly greater distance causes his tide-producing power to be much less than that of the moon. His influence is seen in the difference between spring and neap tides. Spring tides occur at new or full moon (Fig. 21, A, case of new moon). At these two periods the sun, moon, and earth, are all in one straight line, and the pull of the sun is therefore added to that of the moon to produce a spring tide. At the first and third quarters the sun and moon are at right angles to one another; their respective pulls therefore, to some extent, neutralize each other, and in consequence we have neap tide at these seasons.

No one can fail to notice the beautiful set of phases through which the moon passes every month. A little after the almanac has announced 'new moon,' she begins to appear as a thin crescent low down in the West, and setting shortly after the sun. Night by night we can watch her moving eastward among the stars, and showing more and more of her illuminated surface, until at first quarter half of her disc is bright. The reader must distinguish this real eastward movement from the apparent east to west movement due to the daily rotation of the earth. Its reality can readily be seen by noting the position of the moon relatively to any bright star. It will be observed that if she is a little west of the star on one night, she will have moved to a position a little east of it by the next. Still moving farther East, she reaches full, and is opposite to the sun, rising when he sets, and setting when he rises. After full, her light begins to wane, till at third quarter the opposite half of her disc is bright, and she is seen high in the heavens in the early morning, a pale ghost of her evening glories. Gradually she draws nearer to the sun, thinning down to the crescent shape again until she is lost once more in his radiance, only to re-emerge and begin again the same cycle of change.

The time which the moon actually takes to complete her journey round the earth is twenty-seven days, seven hours, and forty-three minutes; and if the earth were fixed in space, this period, which is called the sidereal month, would be the actual time from new moon to new moon. While the moon has been making her revolution, however, the earth has also been moving onwards in its journey round the sun, so that the moon has a little further to travel in order to reach the 'new moon' position again, and the time between two new moons amounts to twenty-nine days, twelve hours, forty-four minutes. This period is called a lunar month, and is also the synodic period of our satellite, a term which signifies generally the period occupied by any planet or satellite in getting back to the same position with respect to the sun, as observed from the earth.

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.).

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.

1. The Great Grey Plains.—These are, of course, the most conspicuous features of the lunar surface. A number of them can be easily seen with the naked eye; and, so viewed, they unite with the brighter portions to form that resemblance to a human face—'the man in the moon'—with which everyone is familiar. A field-glass or small telescope brings out their boundaries with distinctness, and suggests a likeness to our own terrestrial oceans and seas. Hence the name Maria, which was applied to them by the earlier astronomers, whose telescopes were not of sufficient power to reveal more than their broader outlines. But a comparatively small aperture is sufficient to dispel the idea that these plains have any right to the title of 'seas.' The smoothness which at first suggests water proves to be only relative. They are smooth compared with the brighter regions of the moon, which are rugged beyond all terrestrial precedent; but they would probably be considered no smoother than the average of our own non-mountainous land surfaces. A 2 or 2½-inch telescope will reveal the fact that they are dotted over with numerous irregularities, some of them very considerable. It is indeed not common to find a crater of the largest size associated with them; but, at the same time, craters which on our earth would be considered huge are by no means uncommon upon their surface, and every increase of telescopic power reveals a corresponding increase in the number of these objects (Plates XIII., XV., XVII.).

Further, the grey plains are characterized by features of which instances may be seen with a very small instrument, though the more delicate specimens require considerable power—namely, the long winding ridges which either run concentrically with the margins of the plains, or cross their surface from side to side. Of these the most notable is the great serpentine ridge which traverses the Mare Serenitatis in the north-west quadrant of the moon. As it runs, approximately, in a north and south direction, it is well placed for observation, and even a low power will bring out a good deal of remarkable detail in connection with it. It rises in some places to a height of 700 or 800 feet (Neison), and is well shown on many of the fine lunar photographs now so common. Another point of interest in connection with the Maria is the existence on their borders of a number of large crater formations which present the appearance of having had their walls breached and ruined on the side next the mare by the action of some obscure agency. From consideration of these ruined craters, and of the 'ghost craters,' not uncommon on the plains, which present merely a faint outline, as though almost entirely submerged, it has been suggested, by Elger and others, that the Maria, as we see them represent, not the beds of ancient seas, but the consolidated crust of some fluid or viscous substance such as lava, which has welled forth from vents connected with the interior of the moon, overflowing many of the smaller formations, and partially destroying the walls of these larger craters. Notable instances of these half-ruined formations will be found in Fracastorius (Plate XIX., No. 78, and Plate XI.), and Pitatus (Plate XIX., No. 63, and Plate XV.). The grey plains vary in size from the vast Oceanus Procellarum, nearly 2,000,000 square miles in area, down to the Mare Humboldtianum, whose area of 42,000 square miles is less than that of England.

2. The Circular, or Approximately Circular Formations.—These, the great distinguishing feature of lunar scenery, have been classified according to the characteristics, more or less marked, which distinguish them from one another, as walled-plains, mountain-rings, ring-plains, craters, crater-cones, craterlets, crater-pits, and depressions. For general purposes we may content ourselves with the single title craters, using the more specific titles in outstanding instances.

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 2?-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.

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.

3. The Mountain Ranges.—These are comparatively few in number, and are never of such magnitude as to put them, like the craters, beyond terrestrial standards of comparison. The most conspicuous range is that known as the Lunar Apennines, which runs in a north-west and south-east direction for a distance of upwards of 400 miles along the border of the Mare Imbrium, from which its mass rises in a steep escarpment, towering in one instance (Mount Huygens) to a height of more than 18,000 feet. On the western side the range slopes gradually away in a gentle declivity. The spectacle presented by the Apennines about first quarter is one of indescribable grandeur. The shadows of the great peaks are cast for many miles over the surface of the Mare Imbrium, magnificently contrasting with the wild tract of hill-country behind, in which rugged summits and winding valleys are mingled in a scene of confusion which baffles all attempt at delineation. Two other important ranges—the Caucasus and the Alps—lie in close proximity to the Apennines; the latter of the two notable for the curious Alpine Valley which runs through it in a straight line for upwards of eighty miles. This wonderful chasm varies in breadth from about two miles, at its narrowest neck, to about six at its widest point. It is closely bordered, for a considerable portion of its length, by almost vertical cliffs thousands of feet in height, and under low magnifying powers appears so regular as to suggest nothing so much as the mark of a gigantic chisel, driven by main force through the midst of the mountain mass. The Alpine Valley is an easy object, and a power of 50 on a 2-inch telescope will show its main outlines quite clearly. Indeed, the whole neighbourhood is one which will well repay the student, some of the finest of the lunar craters, such as Plato, Archimedes, Autolycus, and Aristillus, lying in the immediate vicinity (Plates XIII. and XVII.).

Among the other mountain-ranges may be mentioned the Altai Mountains, in the south-west quadrant (Plate XVI.), the Carpathians, close to the great crater Copernicus, and the beautiful semicircle of hills which borders the Sinus Iridum, or Bay of Rainbows, to the east of the Alpine range. This bay forms one of the loveliest of lunar landscapes, and under certain conditions of illumination its eastern cape, the Heraclides Promontory, presents a curious resemblance, which I have only seen once or twice, to the head of a girl with long floating hair—'the moon-maiden.' The Leibnitz and Doerfel Mountains, with other ranges whose summits appear on the edge of the moon, are seldom to be seen to great advantage, though they are sometimes very noticeably projected upon the bright disc of the sun during the progress of an eclipse.* They embrace some of the loftiest lunar peaks reaching 26,000 feet in one of or two instances, according to SchrÖter and MÄdler.

4. The Clefts or Rills.—In these, and in the ray-systems, we again meet with features to which a terrestrial parallel is absolutely lacking. SchrÖter of Lilienthal was the first observer to detect the existence of these strange chasms, and since his time the number known has been constantly increasing, till at present it runs to upwards of a thousand. These objects range from comparatively coarse features, such as the Herodotus Valley (Fig. 23), and the well-known AriadÆus and Hyginus clefts, down to the most delicate threads, only to be seen under very favourable conditions, and taxing the powers of the finest instruments. They present all the appearance of cracks in a shrinking surface, and this is the explanation of their existence which at present seems to find most favour. In some cases, such as that of the great Sirsalis cleft, they extend to a length of 300 miles; their breadth varies from half a mile, or less, to two miles; their depth is very variously estimated, Nasmyth putting it at ten miles, while Elger only allows 100 to 400 yards. In a number of instances they appear either to originate from a small crater, or to pass through one or more craters in their course. The student will quickly find out for himself that they frequently affect the neighbourhood of one or other of the mountain ranges (as, for example, under the eastern face of the Apennines, Plate XVII.), or of some great crater, such as Archimedes. They are also frequently found traversing the floor of a great walled-plain, and at least forty have been detected in the interior of Gassendi (Plate XIX., No. 90). Smaller instruments are, of course, incompetent to reveal more than a few of the larger and coarser of these strange features. The Serpentine Valley of Herodotus, the cleft crossing the floor of Petavius, and the AriadÆus and Hyginus rills are among the most conspicuous, and may all be seen with a 2½-inch telescope and a power of 100.

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 4? 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.

Above all, care must be taken to set down only what is seen with certainty, and nothing more. The drawing may be good or bad, but it must be true. A coarse or clumsy sketch which is truthful to the facts seen is worth fifty beautiful works of art where the artist has employed imagination or recollection to eke out the meagre results of observation. The astronomer's primary object is to record facts, not to make pictures. If he is skilful in recording what he sees, his sketch will be so much the more truthful; but the facts must come first. Such practical falsehoods as the insertion of uncertain details, or the practice of drawing upon one's recollection of the work of other observers, or of altering portions of a sketch which do not please the eye, are to be studiously avoided. The observer's record of what he has seen should be above suspicion. It may be imperfect; it should never be false. Such cautions may seem superfluous, but a small acquaintance with the subject of astronomical drawing will show that they are not.

The want of a good lunar chart will speedily make itself felt. Fortunately in these days it can be easily supplied. The great photographic atlases now appearing are, of course, for the luxurious; and the elaborate maps of Beer and MÄdler or Schmidt are equally out of the question for beginners. The smaller chart of the former observers is, however, inexpensive and good, though a little crowded. For a start there is still nothing much better than Webb's reduction of Beer and MÄdler's large chart, published in 'Celestial Objects for Common Telescopes.' It can also be obtained separately; but requires to be backed before use. Mellor's chart is also useful, and is published in a handy form, mounted on mill-board. Those who wish charts between these and the more elaborate ones will find their wants met by such books as those of Neison or Elger. Neison's volume contains a chart in twenty-two sections on a scale of 2 feet to the moon's diameter. It includes a great amount of detail, and is accompanied by an elaborate description of all the features delineated. Its chief drawbacks are the fact that it was published thirty years ago, and that it is an extremely awkward and clumsy volume to handle, especially in the dim light of an observatory. Elger's volume is, perhaps, for English students, the handiest general guide to the moon. Its chart is on a scale of 18 inches to the moon's diameter, and is accompanied by a full description. With either this or Webb's chart, the beginner will find himself amply provided with material for many a long and delightful evenings work.

The small chart which accompanies this chapter, and which, with its key-map, I owe to the courtesy of Mr. John Murray, the publisher of Messrs. Nasmyth and Carpenter's volume on the moon, is not in any sense meant as a substitute for those already mentioned, but merely as an introduction to some of the more prominent features of lunar scenery. The list of 229 named and numbered formations will be sufficient to occupy the student for some time; and the essential particulars with regard to a few of the more important formations are added in as brief a form as possible (Appendix I.).

Before we leave our satellite, something must be said as to the conditions prevailing on her surface. The early astronomers who devoted attention to lunar study were drawn on in their labours largely by the hope of detecting resemblances to our own earth, or even traces of human habitation. SchrÖter and Gruithuisen imagined that they had discovered not only indications of a lunar atmosphere, but also evidence of change upon the surface, and traces of the handiwork of lunarian inhabitants. Gruithuisen, in particular, was confident that in due time it would become possible to trace the cities and the works of the Lunarians. Gradually these hopes have receded into the distance. The existence of a lunar atmosphere is, indeed, no longer positively denied now, as it was a few years ago; but it is certain that such atmosphere as may exist is of extreme rarity, quite inadequate to support animal life as we understand such a thing. Certain delicate changes of colour which take place within some of the craters—Plato for instance—have been referred to vegetation; and Professor Pickering has intimated his observation of something which he considers to be the forming and melting of hoar-frost within certain areas, Messier and a small crater near Herodotus among others. But the observations at best are very delicate and the inferences uncertain. It cannot be denied that the moon may have an atmosphere; but positive traces of its existence are so faint that, even if their reality be admitted, very little can be built upon them.

At the same time when the affirmation is made that the moon is 'a world where there is no weather, and where nothing ever happens,' the most careful modern students of lunar matters would be the first to question such a statement. Even supposing it to be true that no concrete evidence of change upon the lunar surface can be had, this would not necessarily mean that no change takes place. The moon has certainly never been studied to advantage with any power exceeding 1,000, and the average powers employed have been much less. Nasmyth puts 300 as about the profitable limit, and 500 would be almost an outside estimate for anything like regular work. But even assuming the use of a power of 1,000, that means that the moon is seen as large as though she were only 240 miles distant from us. The reader can judge how entirely all but the very largest features of our world would be lost to sight at such a distance, and how changes involving the destruction of large areas might take place and the observer be none the wiser. When it is remembered that even at this long range we are viewing our object through a sea of troubled air of which every tremor is magnified in proportion to the telescopic power employed, until the finer details are necessarily blurred and indistinct, it will be seen that the case has been understated. Indeed it may be questioned if the moon has ever been as well seen as though it had been situated at a distance of 500 miles from the earth. At such a distance nothing short of the vastest cataclysms would be visible; and it is therefore going quite beyond the mark to assume that nothing ever happens on the moon simply because we do not see it happening. Moreover, the balance of evidence does appear to be inclining, slightly perhaps, but still almost unquestionably, towards the view that change does occur upon the moon. Some of the observations which seem to imply change may be explained on other grounds; but there is a certain residuum which appears to defy explanation, and it is very noteworthy that while those who at once dismiss the idea of lunar change are, generally speaking, those who have made no special study of the moon's surface, the contrary opinion is most strongly maintained by eminent observers who have devoted much time to our satellite with the best modern instruments to aid them in their work.

The admission of the possibility of change does not, however, imply anything like fitness for human habitation. The moon, to use Beer and MÄdler's oft-quoted phrase, is 'no copy of the earth'; and the conditions of her surface differ widely from anything that we are acquainted with. The extreme rarity of her atmosphere must render her, were other conditions equally favourable, an ideal situation for an observatory. From her surface the stars, which are hidden from us in the daytime by the diffused light in our air, would be visible at broad noonday; while multitudes of the smaller magnitudes which here require telescopic power would there be plain to the unaided eye. The lunar night would be lit by our own earth, a gigantic moon, presenting a surface more than thirteen times as large as that which the full moon offers us, and hanging almost stationary in the heavens, while exhibiting all the effects of rapid rotation upon its own axis. Those appendages of the sun, which only the spectroscope or the fleeting total eclipse can reveal to us, the corona, the chromosphere, and the prominences, would there be constantly visible.

Our astronomers who are painfully wrestling with atmospheric disturbance, and are gradually being driven from the plains to the summits of higher and higher hills in search of suitable sites for the giant telescopes of to-day, may well long for a world where atmospheric disturbance must be unknown, or at least a negligible quantity.

* See drawings by Colonel Markwick with 2¾-inch refractor, of the eclipse of August 30, 1905, 'The Total Solar Eclipse, 1905,' British Astronomical Association, pp. 59, 60.