A FULL DESCRIPTION AND MAP OF ITS PRINCIPAL PHYSICAL FEATURES
THOMAS GWYN ELGER, F.R.A.S.
DIRECTOR OF THE LUNAR SECTION OF THE BRITISH ASTRONOMICAL ASSOCIATION EX-PRESIDENT LIVERPOOL ASTRONOMICAL SOCIETY
"Altri fiumi, altri laghi, altre campagne Sono la su che non son qui tra noi, Altri piani, altre valli, altre montagne." ORLANDO FURIOSO, Canto xxxii.
LONDON GEORGE PHILIP & SON, 32 FLEET STREET, E.C. LIVERPOOL: 45 TO 51 SOUTH CASTLE STREET 1895
This book and the accompanying map is chiefly intended for the use of lunar observers, but it is hoped it may be acceptable to many who, though they cannot strictly be thus described, take a general interest in astronomy.
The increasing number of those who possess astronomical telescopes, and devote more or less of their leisure in following some particular line of research, is shown by the great success in recent years of societies, such as the British Astronomical Association with its several branches, the Astronomical Society of the Pacific, and similar institutions in various parts of the world. These societies are not only doing much in popularising the sublimest of the sciences, but are the means of developing and organising the capabilities of their members by discouraging aimless and desultory observations, and by pointing out how individual effort may be utilised and made of permanent value in almost every department of astronomy.
The work of the astronomer, like that of the votary of almost every other science, is becoming every year more and more specialised; and among its manifold subdivisions, the study of the physical features of the moon is undoubtedly increasing in popularity and importance. To those who are pursuing such observations, it is believed that this book will be a useful companion to the telescope, and convenient for reference.
Great care has been taken in the preparation of the map, which, so far as the positions of the various objects represented are concerned, is based on the last edition of Beer and Madler's chart, and on the more recent and much larger and elaborate map of Schmidt; while as regards the shape and details of most of the formations, the author's drawings and a large number of photographs have been utilised. Even on so small a scale as eighteen inches to the moon's diameter, more detail might have been inserted, but this, at the expense of distinctness, would have detracted from the value of the map for handy reference in the usually dim light of the observatory, without adding to its utility in other ways. Every named formation is prominently shown; and most other features of interest, including the principal rill-systems, are represented, though, as regards these, no attempt is made to indicate all their manifold details and ramifications, which, to do effectually, would in very many instances require a map on a much larger scale than any that has yet appeared.
The insertion of meridian lines and parallels of latitude at every ten degrees, and the substitution of names for reference numbers, will add to the usefulness of the map.
With respect to the text, a large proportion of the objects in the Catalogue and in the Appendix have been observed and drawn by the author many times during the last thirty years, and described in The Observatory and other publications. He has had, besides, the advantage of consulting excellent sketches by Mr W.H. MAW, F.R.A.S., Dr. SHELDON, F.R.A.S., Mr. A. MEE, F.R.A.S., Mr. G.P. HALLOWES, F.R.A.S., Dr. SMART, F.R.A.S., Mr. T. GORDON, F.R.A.S., Mr. G.T. DAVIS, Herr BRENNER, Herr KRIEGER, Mr. H. CORDER, and other members of the British Astronomical Association. Through the courtesy of Professor HOLDEN, Director of the Lick Observatory, and M. PRINZ, of the Royal Observatory of Brussels, many beautiful photographs and direct photographic enlargements have been available, as have also the exquisite heliogravures received by the author from Dr. L. WEINEK, Director of the Imperial Observatory of Prague, and the admirable examples of the photographic work of MM. PAUL and PROSPER HENRY of the Paris Observatory, which are occasionally published in Knowledge. The numerous representations of lunar objects which have appeared from time to time in that storehouse of astronomical information, The English Mechanic, and the invaluable notes in "Celestial Objects for Common Telescopes," and in various periodicals, by the late REV. PREBENDARY WEBB, to whom Selenography and Astronomy generally owe so much, have also been consulted.
As a rule, all the more prominent and important features are described, though very frequently interesting details are referred to which, from their minuteness, could not be shown in the map. The measurements (given in round numbers) are derived in most instances from NEISON'S (Nevill) "Moon," though occasionally those in the introduction to Schmidt's chart are adopted.
THOMAS GYWN ELGER. BEDFORD, 1895.
INTRODUCTION MARIA, OR PLAINS, TERMED "SEAS" RIDGES RING-MOUNTAINS, CRATERS, &C. Walled Plains Mountain Rings Ring-Plains Craters Crater Cones Craterlets, Crater Pits MOUNTAIN RANGES, ISOLATED MOUNTAINS, &c. CLEFTS, OR RILLS FAULTS VALLEYS BRIGHT RAY-SYSTEMS THE MOON'S ALBEDO, SURFACE BRIGHTNESS, &c. TEMPERATURE OF THE MOON'S SURFACE LUNAR OBSERVATION PROGRESS OF SELENOGRAPHY, LUNAR PHOTOGRAPHY
CATALOGUE OF LUNAR FORMATIONS FIRST QUADRANT— West Longitude 90 deg. to 60 deg. West Longitude 60 deg. to 40 deg. West Longitude 40 deg. to 20 deg. West Longitude 20 deg. to 0 deg. SECOND QUADRANT— East Longitude 0 deg. to 20 deg. East Longitude 20 deg. to 40 deg. East Longitude 40 deg. to 60 deg. East Longitude 60 deg. to 90 deg. THIRD QUADRANT— East Longitude 0 deg. to 20 deg. East Longitude 20 deg. to 40 deg. East Longitude 40 deg. to 60 deg. East Longitude 60 deg. to 90 deg. FOURTH QUADRANT— West Longitude 90 deg. to 60 deg. West Longitude 60 deg. to 40 deg. West Longitude 40 deg. to 20 deg. West Longitude 20 deg. to 0 deg.
MAP OF THE MOON First Quadrant Second Quadrant Third Quadrant Fourth Quadrant
APPENDIX Description of Map List of the Maria, or Grey Plains, termed "Seas," &c. List of some of the most Prominent Mountain Ranges, Promontories, Isolated Mountains, and Remarkable Hills List of the Principal Ray-Systems, Light-Surrounded Craters, and Light Spots Position of the Lunar Terminator Lunar Elements Alphabetical List of Formations
We know, both by tradition and published records, that from the earliest times the faint grey and light spots which diversify the face of our satellite excited the wonder and stimulated the curiosity of mankind, giving rise to suppositions more or less crude and erroneous as to their actual nature and significance. It is true that Anaxagoras, five centuries before our era, and probably other philosophers preceding him, —certainly Plutarch at a much later date—taught that these delicate markings and differences of tint, obvious to every one with normal vision, point to the existence of hills and valleys on her surface; the latter maintaining that the irregularities of outline presented by the "terminator," or line of demarcation between the illumined and unillumined portion of her spherical superficies, are due to mountains and their shadows; but more than fifteen centuries elapsed before the truth of this sagacious conjecture was unquestionably demonstrated. Selenography, as a branch of observational astronomy, dates from the spring of 1609, when Galileo directed his "optic tube" to the moon, and in the following year, in the Sidereus Nuncius, or "the Intelligencer of the Stars," gave to an astonished and incredulous world an account of the unsuspected marvels it revealed. In this remarkable little book we have the first attempt to represent the telescopic aspect of the moon's visible surface in the five rude woodcuts representing the curious features he perceived thereon, whose form and arrangement, he tells us, reminded him of the "ocelli" on the feathers of a peacock's tail,—a quaint but not altogether inappropriate simile to describe the appearance of groups of the larger ring-mountains partially illuminated by the sun, when seen in a small telescope.
The bright and dusky areas, so obvious to the unaided sight, were found by Galileo to be due to a very manifest difference in the character of the lunar surface, a large portion of the northern hemisphere, and no inconsiderable part of the south-eastern quadrant, being seen to consist of large grey monotonous tracts, often bordered by lofty mountains, while the remainder of the superficies was much more conspicuously brilliant, and, moreover, included by far the greater number of those curious ring- mountains and other extraordinary features whose remarkable aspect and peculiar arrangement first attracted his attention. Struck by the analogy which these contrasted regions present to the land and water surfaces of our globe, he suspected that the former are represented on the moon by the brighter and more rugged, and the latter by the smoother and more level areas; a view, however, which Kepler more distinctly formulated in the dictum, "Do maculas esse Maria, do lucidas esse terras." Besides making a rude lunar chart, he estimated the heights of some of the ring- mountains by measuring the distance from the terminator of their bright summit peaks, when they were either coming into or passing out of sunlight; and though his method was incapable of accuracy, and his results consequently untrustworthy, it served to demonstrate the immense altitude of these circumvallations, and to show how greatly they exceed any mountains on the earth if the relative dimensions of the two globes are taken into consideration.
Before the close of the century when selenography first became possible, Hevel of Dantzig, Scheiner, Langrenus (cosmographer to the King of Spain), Riccioli, the Jesuit astronomer of Bologna, and Dominic Cassini, the celebrated French astronomer, greatly extended the knowledge of the moon's surface, and published drawings of various phases, and charts, which, though very rude and incomplete, were a clear advance upon what Galileo, with his inferior optical means, had been able to accomplish. Langrenus, and after him Hevel, gave distinctive names to the various formations, mainly derived from terrestrial physical features, for which Riccioli subsequently substituted those of philosophers, mathematicians, and other celebrities; and Cassini determined by actual measurement the relative position of many of the principal objects on the disc, thus laying the foundation of an accurate system of lunar topography; while the labours of T. Mayer and Schroter in the last century, and of Lohrmann, Madler, Neison (Nevill), Schmidt, and other observers in the present, have been mainly devoted to the study of the minuter detail of the moon and its physical characteristics.
As was manifest to the earliest telescopic observers, its visible surface is clearly divisible into strongly contrasted areas, differing both in colour and structural character. Somewhat less than half of what we see of it consists of comparatively level dark tracts, some of them very many thousands of square miles in extent, the monotony of whose dusky superficies is often unrelieved for great distances by any prominent object; while the remainder, everywhere manifestly brighter, is not only more rugged and uneven, but is covered to a much greater extent with numbers of quasi-circular formations, differing widely in size, classed as walled-plains, ring-plains, craters, craterlets, crater-cones, &c. (the latter bearing a great outward resemblance to some terrestrial volcanoes), and mountain ranges of vast proportions, isolated hills, and other features.
Though nothing resembling sheets of water, either of small or large extent, have ever been detected on the surface, the superficial resemblance, in small telescopes, of the large grey tracts to the appearance which we may suppose our terrestrial lakes and oceans would present to an observer on the moon, naturally induced the early selenographers to term them Maria, or "seas"—a convenient name, which is still maintained, without, however, implying that these areas, as we now see them, are, or ever were, covered with water. Some, however, regard them as old sea-beds, from which every trace of fluid, owing to some unknown cause, has vanished, and that the folds and wrinkles, the ridges, swellings, and other peculiarities of structure observed upon them, represent some of the results of alluvial action. It is, of course, possible, and even probable, that at a remote epoch in the evolution of our satellite these lower regions were occupied by water, but that their surface, as it now appears, is actually this old sea-bottom, seems to be less likely than that it represents the consolidated crust of some semi- fluid or viscous material (possibly of a basaltic type) which has welled forth from orifices or rents communicating with the interior, and overspread and partially filled up these immense hollows, more or less overwhelming and destroying many formations which stood upon them before this catastrophe took place. Though this, like many other speculations of a similar character relating to lunar "geology," must remain, at least for the present, as a mere hypothesis; indications of this partial destruction by some agency or other is almost everywhere apparent in those formations which border the so-called seas, as, for example, Fracastorius in the Mare Nectaris; Le Monnier in the Mare Serenitatis; Pitatus and Hesiodus, on the south side of the Mare Nubium; Doppelmayer in the Mare Humorum, and in many other situations; while no observer can fail to notice innumerable instances of more or less complete obliteration and ruin among objects within these areas, in the form of obscure rings (mere scars on the surface), dusky craters, circular arrangements of isolated hills, reminding one of the monoliths of a Druidical temple; all of which we are justified in concluding were at one time formations of a normal type. It has been held by some selenologists —and Schmidt appears to be of the number,—that, seeing the comparative scarcity of large ring-plains and other massive formations on the Maria, these grey plains represent, as it were, a picture of the primitive surface of the moon before it was disturbed by the operations of interior forces; but this view affords no explanation of the undoubted existence of the relics of an earlier lunar world beneath their smooth superficies.
MARIA.—Leaving, however, these considerations for a more particular description of the Maria, it is clearly impossible, in referring to their level relatively to the higher and brighter land surface of the moon, to appeal to any hypsometrical standard. All that is known in this respect is, that they are invariably lower than the latter, and that some sink to a greater depth than others, or, in other words, that they do not all form a part of the same sphere. Though they are more or less of a greyish-slaty hue—some of them approximating very closely to that of the pigment known as "Payne's grey"—the tone, of course, depends upon the angle at which the solar rays impinge on that particular portion of the surface under observation. Speaking generally, they are, as would follow from optical considerations, conspicuously darker when viewed near the terminator, or when the sun is either rising or setting upon them, than under a more vertical angle of illumination. But even when it is possible to compare their colour by eye-estimation under similar solar altitudes, it is found that not only are some of the Maria, as a whole, notably darker than others, but nearly all of them exhibit local inequalities of hue, which, under good atmospheric and instrumental conditions, are especially remarkable. Under such circumstances I have frequently seen the surface, in many places covered with minute glittering points of light, shining with a silvery lustre, intermingled with darker spots and a network of streaks far too delicate and ethereal to represent in a drawing. In addition to these contrasts and differences in the sombre tone of these extended plains, many observers have remarked traces of a yellow or green tint on the surface of some of them. For example, the Mare Imbrium and the Mare Frigoris appear under certain conditions to be of a dirty yellow-green hue, the central parts of the Mare Humorum dusky green, and part of the Mare Serenitatis and the Mare Crisium light green, while the Palus Somnii has been noted a golden-brown yellow. To these may be added the district round Taruntius in the Mare Foecunditatis, and portions of other regions referred to in the catalogue, where I have remarked a very decided sepia colour under a low sun. It has been attempted to account for these phenomena by supposing the existence of some kind of vegetation; but as this involves the presence of an atmosphere, the idea hardly finds favour at the present time, though perhaps the possibility of plant growth in the low-lying districts, where a gaseous medium may prevail, is not altogether so chimerical a notion as to be unworthy of consideration. Nasmyth and others suggest that these tints may be due to broad expanses of coloured volcanic material, an hypothesis which, if we believe the Maria to be overspread with such matter, and knowing how it varies in colour in terrestrial volcanic regions, is more probable than the first. Anyway, whether we consider these appearances to be objective, or, after all, only due to purely physiological causes, they undoubtedly merit closer study and investigation than they have hitherto received.
There are twenty-three of these dusky areas which have received distinctive names; seventeen of them are wholly, or in great part, confined to the northern, and to the south-eastern quarter of the southern hemisphere—the south-western quadrant being to a great extent devoid of them. By far the largest is the vast Oceanus Procellarum, extending from a high northern latitude to beyond latitude 10 deg. in the south-eastern quadrant, and, according to Schmidt, with its bays and inflections, occupying an area of nearly two million square miles, or more than that of all the remaining Maria put together. Next in order of size come the Mare Nubium, of about one-fifth the superficies, covering a large portion of the south-eastern quadrant, and extending considerably north of the equator, and the Mare Imbrium, wholly confined to the northeastern quadrant, and including an area of about 340,000 square miles. These are by far the largest lunar "seas." The Mare Foecunditatis, in the western hemisphere, the greater part of it lying in the south- western quadrant, is scarcely half so big as the Mare Imbrium; while the Maria Serenitatis and Tranquilitatis, about equal in area (the former situated wholly north of the equator, and the latter only partially extending south of it), are still smaller. The arctic Mare Frigoris, some 100,000 square miles in extent, is the only remaining large sea,—the rest, such as the Mare Vaporum, the Sinus Medii, the Mare Crisium, the Mare Humorum, and the Mare Humboldtianum, are of comparatively small dimensions, the Mare Crisium not greatly exceeding 70,000 square miles, the Mare Humorum (about the size of England) 50,000 square miles, while the Mare Humboldtianum, according to Schmidt, includes only about 42,000 square miles, an area which is approached by some formations not classed with the Maria. This distinction, speaking generally, prevails among the Maria,—those of larger size, such as the Oceanus Procellarum, the Mare Nubium, and the Mare Foecunditatis, are less definitely enclosed, and, like terrestrial oceans, communicate with one another; while their borders, or, if the term may be allowed, their coast-line, is often comparatively low and ill-defined, exhibiting many inlets and irregularities in outline. Others, again, of considerable area, as, for example, the Mare Serenitatis and the Mare Imbrium, are bounded more or less completely by curved borders, consisting of towering mountain ranges, descending with a very steep escarpment to their surface: thus in form and other characteristics they resemble immense wall-surrounded plains. Among the best examples of enclosed Maria is the Mare Crisium, which is considered by Neison to be the deepest of all, and the Mare Humboldtianum.
Though these great plains are described as level, this term must only be taken in a comparative sense. No one who observes them when their surface is thrown into relief by the oblique rays of the rising or setting sun can fail to remark many low bubble-shaped swellings with gently rounded outlines, shallow trough-like hollows, and, in the majority of them, long sinuous ridges, either running concentrically with their borders or traversing them from side to side. Though none of these features are of any great altitude or depth, some of the ridges are as much as 700 feet in height, and probably in many instances the other elevations often rise to 150 feet or more above the low-lying parts of the plains on which they stand. Hence we may say that the Maria are only level in the sense that many districts in the English Midland counties are level, and not that their surface is absolutely flat. The same may be said as to their apparent smoothness, which, as is evident when they are viewed close to the terminator, is an expression needing qualification, for under these conditions they often appear to be covered with wrinkles, flexures, and little asperities, which, to be visible at all, must be of considerable size. In fact, were it possible to examine them from a distance of a few miles, instead of from a standpoint which, under the most favourable circumstances, cannot be reckoned at less than 300, and this through an interposed aerial medium always more or less perturbed, they would probably be described as rugged and uneven, as some modern lava sheets.
RIDGES.—Among the Maria which exhibit the most remarkable arrangement of ridges is the Mare Humorum, in the south-eastern quadrant. Here, if it be observed under a rising sun, a number of these objects will be seen extending from the region north of the ring-mountain Vitello in long undulating lines, roughly concentric with the western border of the "sea," and gradually diminishing in altitude as they spread out, with many ramifications, to a distance of 200 miles or more towards the north. At this stage of illumination they are strikingly beautiful in a good telescope, reminding one of the ripple-marks left by the tide on a soft sandy beach. Like most other objects of their class, they are very evanescent, gradually disappearing as the sun rises higher in the lunar firmament, and ultimately leaving nothing to indicate their presence beyond here and there a ghostly streak or vein of a somewhat lighter hue than that of the neighbouring surface. The Mare Nectaris, again, in the south-western quadrant, presents some fine examples of concentric ridges, which are seen to the best advantage when the morning sun is rising on Rosse, a prominent crater north of Fracastorius. This "sea" is evidently concave in cross-section, the central portion being considerably lower than the margin, and these ridges appear to mark the successive stages of the change of level from the coast-line to the centre. They suggest the "caving in" of the surface, similar to that observed on a frozen pond or river, where the "cat's ice" at the edge, through the sinking of the water beneath, is rent and tilted to a greater or less degree. The Mare Serenitatis and the Mare Imbrium, in the northern hemisphere, are also remarkable for the number of these peculiar features. They are very plentifully distributed round the margin and in other parts of the former, which includes besides one of the longest and loftiest on the moon's visible surface—the great serpentine ridge, first drawn and described nearly a hundred years ago by the famous selenographer, Schroter of Lilienthal. Originating at a little crater under the north- east wall of great ring-plain Posidonius, it follows a winding course across the Mare toward the south, throwing out many minor branches, and ultimately dies out under a great rocky promontory—the Promontory Acherusia, at the western termination of the Haemus range. A comparatively low power serves to show the curious structural character of this immense ridge, which appears to consist of a number of corrugations and folds massed together, rising in places, according to Neison, to a height of 700 feet and more. The Mare Imbrium also affords an example of a ridge, which, though shorter, is nearly as prominent, in that which runs from the bright little ring-plain Piazzi Smyth towards the west side of Plato. The region round Timocharis and other quarters of the Mare are likewise traversed by very noteworthy features of a similar class. The Oceanus Procellarum also presents good instances of ridges in the marvellous ramifications round Encke, Kepler, and Marius, and in the region north of Aristarchus and Herodotus. Perhaps the most perfect examples of surface swellings are those in the Mare Tranquilitatis, a little east of the ring-plain Arago, where there are two nearly equal circular mounds, at least ten miles in diameter, resembling tumuli seen from above. Similar, but more irregular, objects of a like kind are very plentiful in many other quarters.
It is a suggestive peculiarity of many of the lunar ridges, both on the Maria and elsewhere, that they are very generally found in association with craters of every size. Illustrations of this fact occur almost everywhere. Frequently small craters are found on the summits of these elevations, but more often on their flanks and near their base. Where a ridge suddenly changes its direction, a crater of some prominence generally marks the point, often forming a node, or crossing-place of other ridges, which thus appear to radiate from it as a centre. Sometimes they intrude within the smaller ring-mountains, passing through gaps in their walls as, for example, in the cases of Madler, Lassell, &c. Various hypotheses have been advanced to account for them. The late Professor Phillips, the geologist, who devoted much attention to the telescopic examination of the physical features of the moon, compared the lunar ridges to long, low, undulating mounds, of somewhat doubtful origin, called "kames" in Scotland, and "eskers" in Ireland, where on the low central plain they are commonly found in the form of extended banks (mainly of gravel), with more or less steep sides, rising to heights of from 20 to 70 feet. They are sometimes only a few yards wide at the top, while in other places they spread out into large humps, having circular or oval cavities on their summits, 50 or 60 yards across, and as much as 40 feet deep. Like the lunar ridges, they throw out branches and exhibit many breaches of continuity. By some geologists they are supposed to represent old submarine banks formed by tidal currents, like harbour bars, and by others to be glacial deposits; in either case, to be either directly or indirectly due to alluvial action. Their outward resemblance to some of the ridges on the moon is unquestionable; and if we could believe that the Maria, as we now see them, are dried-up sea-beds, it might be concluded that these ridges had a similar origin; but their close connection with centres of volcanic disturbance, and the numbers of little craters on or near their track, point to the supposition that they consist rather of material exuded from long-extending fissures in the crust of the "seas," and in other surfaces where they are superimposed. This conjecture is rendered still more probable by the fact that we sometimes find the direction of clefts (which are undoubted surface cracks) prolonged in the form of long narrow ridges or of rows of little hillocks. We are, however, not bound to assume that all the manifold corrugations observed on the lunar plains are due to one and the same cause; indeed, it is clear that some are merely the outward indications of sudden drops in the surface, as in the case of the ridges round the western margin of the Mare Nectaris, and in other situations, where subsidence is manifested by features assuming the outward aspect of ordinary ridges, but which are in reality of a very different structural character.
The Maria, like almost every other part of the visible surface, abound in craters of a minute type, which are scattered here and there without any apparent law or ascertained principle of arrangement. Seeing how imperfect is our acquaintance with even the larger objects of this class, it is rash to insist on the antiquity or permanence of such diminutive objects, or to dogmatise about the cessation of lunar activity in connection with features where the volcanic history of our globe, if it is of any value as an analogue, teaches us it is most likely to prevail.
Most observers will agree with Schmidt, that observations and drawings of objects on the sombre depressed plains of the moon are easier and pleasanter to make than on the dazzling highlands, and that the lunar "sea" is to the working selenographer like an oasis in the desert to the traveller—a relief in this case, however, not to an exhausted body, but to a weary eye.
RING-MOUNTAINS, CRATERS, &C.—It is these objects, in their almost endless variety and bewildering number, which, more than any others, give to our satellite that marvellous appearance in the telescope which since the days of Galileo has never failed to evoke the astonishment of the beholder. However familiar we may be with the lunar surface, we can never gaze on these extraordinary formations, whether massed together apparently in inextricable confusion, or standing in isolated grandeur, like Copernicus, on the grey surface of the plains, without experiencing, in a scarcely diminished degree, the same sensation of wonder and admiration with which they were beheld for the first time. Although the attempt to bring all these bizarre forms under a rigid scheme of classification has not been wholly successful, their structural peculiarities, the hypsometrical relation between their interior and the surrounding district, their size, and the character of their circumvallation, the dimensions of their cavernous opening as compared with that of the more or less truncated conical mass of matter surrounding it, all afford a basis for grouping them under distinctive titles, that are not only convenient to the selenographer, but which undoubtedly represent, as a rule, actual diversities in their origin and physical character.
These distinguishing titles, as adopted by Schroter, Lohrmann, and Madler, and accepted by subsequent observers, are WALLED-PLAINS, MOUNTAIN RINGS, RING-PLAINS, CRATERS, CRATER-CONES, CRATERLETS, CRATER-PITS, DEPRESSIONS.
WALLED-PLAINS.—These formations, approximating more or less to the circular form, though frequently deviating considerably from it, are among the largest enclosures on the moon. They vary from upwards of 150 to 60 miles or under in diameter, and are often encircled by a complex rampart of considerable breadth, rising in some instances to a height of 12,000 feet or more above the enclosed plain. This rampart is rarely continuous, but is generally interrupted by gaps, crossed by transverse valleys and passes, and broken by more recent craters and depressions. As a rule, the area within the circumvallation (usually termed "the floor") is only slightly, if at all, lower than the region outside: it is very generally of a dusky hue, similar to that of the grey plains or Maria, and, like them, is usually variegated by the presence of hills, ridges, and craters, and is sometimes traversed by delicate furrows, termed clefts or rills.
Ptolemaeus, in the third quadrant, and not far removed from the centre of the disc, may be taken as a typical example of the class. Here we have a vast plain, 115 miles from side to side, encircled by a massive but much broken wall, which at one peak towers more than 9000 feet above a level floor, which includes details of a very remarkable character. The adjoining Alphonsus is another, but somewhat smaller, object of the same type, as are also Albategnius, and Arzachel; and Plato, in a high northern latitude, with its noble many-peaked rampart and its variable steel-grey interior. Grimaldi, near the eastern limb (perhaps the darkest area on the moon), Schickard, nearly as big, on the south- eastern limb, and Bailly, larger than either (still farther south in the same quadrant), although they approach some of the smaller "seas" in size, are placed in the same category. The conspicuous central mountain, so frequently associated with other types of ringed enclosures, is by no means invariably found within the walled-plains; though, as in the case of Petavius, Langrenus, Gassendi, and several other noteworthy examples, it is very prominently displayed. The progress of sunrise on all these objects affords a magnificent spectacle. Very often when the rays impinge on their apparently level floor at an angle of from 1 deg. to 2 deg., it is seen to be coarse, rough grained, and covered with minute elevations, although an hour or so afterwards it appears as smooth as glass.
Although it is a distinguishing characteristic that there is no great difference in level between the outside and the inside of a walled-plain, there are some very interesting exceptions to this rule, which are termed by Schmidt "Transitional forms." Among these he places some of the most colossal formations, such as Clavius, Maurolycus, Stofler, Janssen, and Longomontanus. The first, which may be taken as representative of the class (well known to observers as one of the grandest of lunar objects), has a deeply sunken floor, fringed with mountains rising some 12,000 feet above it, though they scarcely stand a fourth of this height above the plain on the west, which ascends with a very gentle gradient to the summit of the wall. Hence the contrast between the shadows of the peaks of the western wall on the floor at sunrise, and of the same peaks on the region west of the border at sunset is very marked. In Gassendi, Phocylides, and Wargentin we have similar notable departures from the normal type. The floor of the former on the north stands 2000 feet above the Mare Humorum. In Phocylides, probably through "faulting," one portion of the interior suddenly sinks to a considerable depth below the remainder; while the very abnormal Wargentin has such an elevated floor, that, when viewed under favourable conditions, it reminds one of a shallow oval tray or dish filled with fluid to the point of overflowing. These examples, very far from being exhaustive, will be sufficient to show that the walled-plains exhibit noteworthy differences in other respects than size, height of rampart, or included detail. Still another peculiarity, confined, it is believed, to a very few, may be mentioned, viz., convexity of floor, prominently displayed in Petavius, Mersenius, and Hevel.
MOUNTAIN RINGS.—These objects, usually encircled by a low and broken border, seldom more than a few hundred feet in height, are closely allied to the walled-plains. They are more frequently found on the Maria than elsewhere. In some cases the ring consists of isolated dark sections, with here and there a bright mass of rock interposed; in others, of low curvilinear ridges, forming a more or less complete circumvallation. They vary in size from 60 or 70 miles to 15 miles and less. The great ring north of Flamsteed, 60 miles across, is a notable example; another lies west of it on the north of Wichmann; while a third will be found south- east of Encke;—indeed, the Mare Procellarum abounds in objects of this type. The curious formation on the Mare Imbrium immediately south of Plato (called "Newton" by Schroter), may be placed in this category, as may also many of the low dusky rings of much smaller dimensions found in many quarters of the Maria. As has been stated elsewhere, these features have the appearance of having once been formations of a much more prominent and important character, which have suffered destruction, more or less complete, through being partially overwhelmed by the material of the "seas."
RING-PLAINS.—These are by far the most numerous of the ramparted enclosures of the moon, and though it is occasionally difficult to decide in which class, walled-plain or ring-plain, some objects should be placed, yet, as a rule, the difference between the structural character of the two is abundantly obvious. The ring-plains vary in diameter from sixty to less than ten miles, and are far more regular in outline than the walled-plains. Their ramparts, often very massive, are more continuous, and fall with a steep declivity to a floor almost always greatly depressed below the outside region. The inner slopes generally display subordinate heights, called terraces, arranged more or less concentrically, and often extending in successive stages nearly down to the interior foot of the wall. With the intervening valleys, these features are very striking objects when viewed under good conditions with high powers. In some cases they may possibly represent the effects of the slipping of the upper portions of the wall, from a want of cohesiveness in the material of which it is composed; but this hardly explains why the highest terrace often stands nearly as high as the rampart. Nasmyth, in his eruption hypothesis, suggests that in such a case there may have been two eruptions from the same vent; one powerful, which formed the exterior circle, and a second, rather less powerful, which has formed the interior circle. Ultimately, however, coming to the conclusion that terraces, as a rule, are not due to any such freaks of the eruption, he ascribes them to landslips. In any case, we can hardly imagine that material standing at such a high angle of inclination as that forming the summit ridge of many of the ring-plains would not frequently slide down in great masses, and thus form irregular plateaus on the lower and flatter portions of the slope; but this fails to explain the symmetrical arrangement of the concentric terraces and intermediate valleys. The inner declivity of the north-eastern wall of Plato exhibits what to all appearance is an undoubted landslip, as does also that of Hercules on the northern side, and numerous other cases might be adduced; but in all of them the appearance is very different from that of the true terrace.
The glacis, or outer slope of a ring-plain, is invariably of a much gentler inclination than that which characterises the inner declivity: while the latter very frequently descends at an angle varying from 60 deg. to 50 deg. at the crest of the wall, to from 10 deg. to 2 deg. at the bottom, where it meets the floor; the former extends for a great distance at a very flat gradient before it sinks to the general level of the surrounding country. It differs likewise from the inner descent, in the fact that, though often traversed by valleys, intersected by deep gullies and irregular depressions, and covered with humpy excrescences and craters, it is only rarely that any features comparable to the terraces, usually present on the inner escarpment, can be traced upon it.
Elongated depressions of irregular outline, and very variable in size and depth, are frequently found on the outer slopes of the border. Some of them consist of great elliptical or sub-circular cavities, displaying many expansions and contractions, called "pockets," and suggesting the idea that they were originally distinct cup-shaped hollows, which from some cause or other have coalesced like rows of inosculating craters. While many of these features are so deep that they remain visible for a considerable time under a low sun, others, though perhaps of greater extent, vanish in an hour or so.
As in the case of the walled-plains, the ramparts of the ring-plains exhibit gaps and are broken by craters and depressions, but to a much less extent. Often the lofty crest, surmounted by aiguilles or by blunter peaks, towering in some cases to nearly double its altitude above the interior, is perfectly continuous (like Copernicus), or only interrupted by narrow passes. It is a suggestive circumstance that gaps, other than valleys, are almost invariably found either in the north or south walls, or in both, and seldom in other positions. The buttress, or long-extending spur, is a feature frequently associated with the ring- plain rampart, as are also numbers of what, for the lack of a better name, must be termed little hillocks, which generally radiate in long rows from the outer foot of the slope. The spurs usually abut on the wall, and, either spreading out like the sticks of a fan or running roughly parallel to each other, extend for long distances, gradually diminishing in height and width till they die out on the surrounding surface. They have been compared to lava streams, which those round Aristillus, Aristoteles, and on the flank of Clavius a, certainly somewhat resemble, though, in the two former instances, they are rather comparable to immense ridges. In addition to the above, the spurs radiating from the south-eastern rampart of Condamine and the long undulating ridges and rows of hillocks running from Cyrillus over the eastern glacis of Theophilus, may be named as very interesting examples.
Neison and some other selenographers place in a distinct class certain of the smaller ring-plains which usually have a steeper outer slope, and are supposed to present clearer indications of a volcanic origin than the ring-plains, terming them "Crater-plains."
CRATERS.—Under this generic name is placed a vast number of formations exhibiting a great difference in size and outward characteristics, though generally (under moderate magnification) of a circular or sub-circular shape. Their diameter varies from 15 miles or more to 3, and even less, and their flanks rise much more steeply to the summit, which is seldom very lofty, than those of the ring-plains, and fall more gradually to the floor. There is no portion of the moon in which they do not abound, whether it be on the ramparts, floors, and outer slopes of walled and ring plains, the summits and escarpments of mountain ranges, amid the intricacies of the highlands, or on the grey surface of the Maria. In many instances they have a brighter and newer aspect than the larger formations, often being the most brilliant points on their walls, when they are found in this position. Very frequently too they are not only very bright themselves, but stand on bright areas, whose borders are generally concentric with them, which shine with a glistening lustre, and form a kind of halo of light around them. Euclides and Bessarion A, and the craters east of Landsberg, are especially interesting examples. It seems not improbable that these areas may represent deposits formed by some kind of matter ejected from the craters, but whether of ancient or modern date, it is, of course, impossible to determine. Future observers will perhaps be in a better position to decide the question without cavil, if such eruptions should again take place. Like the larger enclosures, these smaller objects frequently encroach upon each other— crater-ring overlapping crater-ring, as in the case of Thebit, where a large crater, which has interfered with the continuity of the east wall, has, in its turn, been disturbed by a smaller crater on its own east wall. The craters in many cases, possibly in the majority if we could detect them, have central mountains, some of them being excellent tests for telescopic definition—as, for example, the central peaks of Hortensius, Bessarion, and that of the small crater just mentioned on the east wall of Thebit A. A tendency to a linear arrangement is often displayed, especially among the smaller class, as is also their occurrence in pairs.
CRATER-CONES.—These objects, plentifully distributed on the lunar surface, are especially interesting from their outward resemblance to the parasitic cones found on the flanks of terrestrial volcanoes (Etna, for instance). In the larger examples it is occasionally possible to see that the interiors are either inverted cones without a floor, or cup-shaped depressions on the summit of the object. Frequently, however, they are so small that the orifice can only be detected under oblique illumination. Under a high sun they generally appear as white spots, more or less ill- defined, as on the floors of Archimedes, Fracastorius, Plato, and many other formations, which include a great number, all of which are probably crater cones, although only a few have been seen as such. It is a significant fact that in these situations they are always found to be closely associated with the light streaks which traverse the interior of the formations, standing either on their surface or close to their edges. The instrumental and meteorological requirements necessary for a successful scrutiny of the smallest type of these features, are beyond the reach of the ordinary observer in this country, as they demand direct observation in large telescopes under the best atmospheric conditions.
Some years ago Dr. Klein of Cologne called attention to some very interesting types of crater-cones, which may be found on certain dark or smoky-grey areas on the moon. These, he considers, may probably represent active volcanic vents, and urges that they should be diligently examined and watched by observers who possess telescopes adequate to the task. The most noteworthy examples of these objects are in the following positions:—(1) West of a prominent ridge running from Beaumont to the west side of Theophilus, and about midway between these formations; (2) in the Mare Vaporum, south of Hyginus; (3) on the floor of Werner, near the foot of the north wall; (4) under the east wall of Alphonsus, on the dusky patch in the interior; (5) on the south side of the floor of Atlas. I have frequently described elsewhere with considerable detail the telescopic appearance of these features under various phases, and have pointed out that though large apertures and high powers are needed to see these cones to advantage, the dusky areas, easily traced on photograms, might be usefully studied by observers with smaller instruments, as if they represent the ejecta from the crater-cones which stand upon them, changes in their form and extent could very possibly be detected. In addition to those already referred to, a number of mysterious dark spots were discovered by Schmidt in the dusky region about midway between Copernicus and Gambart, which Klein describes as perforated like a sieve with minute craters. A short distance south-west of Copernicus stands a bright crater-cone surrounded by a grey nimbus, which may be classed with these objects. It is well seen under a high light, as indeed is the case with most of these features.
CRATERLETS, CRATER-PITS.—To a great extent the former term is needless and misleading, as the so-called craters merge by imperceptible gradations into very minute objects, as small as half a mile in diameter, and most probably, if we could more accurately estimate their size, still less. The crater-pit, however, has well-marked peculiarities which distinguish it from all other types, such as the absence of a distinguishable rim and extreme shallowness. They appear to be most numerous on the high-level plains and plateaus in the south-western quadrant, and may be counted by hundreds under good atmospheric conditions on the outer slopes of Walter, Clavius, and other large enclosures. In these positions they are often so closely aggregated that, as Nasmyth remarks, they remind one of an accumulation of froth. Even in an 8 1/2 inch reflector I have frequently seen the outer slope of the large ring-plain on the north-western side of Vendelinus, so perforated with these objects that it resembled pumice or vesicular lava, many of the little holes being evidently not circular, but square shaped and very irregular. The interior of Stadius and the region outside abounds in these minute features, but the well-known crater-row between this formation and Copernicus seems rather to consist of a number of inosculating crater-cones, as they stand very evidently on a raised bank of some altitude.
MOUNTAIN RANGES, ISOLATED MOUNTAINS, &c.—The more massive and extended mountain ranges of the moon are found in the northern hemisphere, and (what is significant) in that portion of it which exhibits few indications of other superficial disturbances. The most prominently developed systems, the Alps, the Caucasus, and the Apennines, forming a mighty western rampart to the Mare Imbrium and giving it all the appearance of a vast walled plain, present few points of resemblance to any terrestrial chain. The former include many hundred peaks, among which, Mont Blanc rises to a height of 12,000 feet, and a second, some distance west of Plato, to nearly as great an altitude; while others, ranging from 5000 to 8000 feet, are common. They extend in a south-west direction from Plato to the Caucasus, terminating somewhat abruptly, a little west of the central meridian, in about N. lat. 42 deg. One of the most interesting features associated with this range is the so-called great Alpine valley, which cuts through it west of Plato. The Caucasus consist of a massive wedge-shaped mountain land, projecting southwards, and partially dividing the Mare Imbrium from the Mare Serenitatis, both of which they flank. Though without peaks so lofty as those pertaining to the Alps, there is one, immediately east of the ring-plain Calippus, which, towering to 19,000 feet, surpasses any of which the latter system can boast. The Apennines, however, are by far the most magnificent range on the visible surface, including as they do some 3000 peaks, and extending in an almost continuous curve of more than 400 miles in length from Mount Hadley, on the north, to the fine ring-plain Eratosthenes, which forms a fitting termination, on the south. The great headland Mount Hadley rises more than 15,000 feet, while a neighbouring promontory on the south-east of it is fully 14,000 feet, and another, close by, is still higher above the Mare. Mount Huygens, again, in N. lat. 20 deg., and the square-shaped mass Mount Wolf, near the southern end of the chain, include peaks standing 18,000 and 12,000 feet respectively above the plain, to which their flanks descend with a steep declivity. The counterscarp of the Apennines, in places 160 miles in width from east to west, runs down to the Mare Vaporum with a comparatively gentle inclination. It is everywhere traversed by winding valleys of a very intricate type, all trending towards the south-west, and includes some bright craters and mountain-rings. The Carpathians, forming in part the southern border of the Mare Imbrium, extend for a length of more than 180 miles eastward of E., long. 16 deg., and, embracing the ring-plain Gay- Lussac, terminate west of Mayer. They present a less definite front to the Mare than the Apennines, and are broken up and divided by irregular valleys and gaps; their loftiest peak, situated on a very projecting promontory north-west of Mayer, rising to a height of 7000 feet. Notwithstanding their comparatively low altitude, the region they occupy forms a fine telescopic picture at lunar sunrise. The Sinus Iridum highlands, bordering the beautiful bay on the north-east side of the Mare Imbrium, rank among the loftiest and most intricate systems on the moon, and, like the Apennines, present a steep face to the grey plain from which they rise, though differing from them in other respects. They include many high peaks, the loftiest, in the neighbourhood of the ring- plain Sharp, rising 15,000 feet. There are probably some still higher mountains in the vicinity, but the difficulties attending their measurement render it impossible to determine their altitude with any approach to accuracy.
The Taurus Mountains extend from the west side of the Mare Serenitatis, near Le Monnier and Littrow, in a north-westerly direction towards Geminus and Berselius, bordering the west side of the Lacus Somniorum. They are a far less remarkable system than any of the preceding, and consist rather of a wild irregular mountain region than a range. In the neighbourhood of Berselius are some peaks which, according to Neison, cannot be less than 10,000 feet in height.
On the north side of the Mare Imbrium, east of Plato, there is a beautiful narrow range of bright outlying heights, called the Teneriffe Mountains, which include many isolated objects of considerable altitude, one of the loftiest rising about 8000 feet. Farther towards the east lies another group of a very similar character, called the Straight Range, from its linear regularity. It extends from west to east for a distance of about 60 miles, being a few miles shorter than the last, and includes a peak of 6000 feet.
The Harbinger Mountains.—A remarkable group, north-west of Aristarchus, including some peaks as high as 7000 feet, and other details noticed in the catalogue.
The above comprise all the mountain ranges in the northern hemisphere of any prominence, or which have received distinctive names, except the Hercynian Mountains, on the north-east limb, east of the walled plain Otto Struve. These are too near the edge to be well observed, but, from what can be seen of them, they appear to abound in lofty peaks, and to bear more resemblance to a terrestrial chain than any which have yet been referred to.
The mountain systems of the southern hemisphere, except the ranges visible on the limb, are far less imposing and remarkable than those just described. The Pyrenees, on the western side of the Mare Nectaris, extend in a meridional direction for nearly 190 miles, and include a peak east of Guttemberg of nearly 12,000 feet, and are traversed in many places by fine valleys.
The Altai Mountains form a magnificent chain, 275 miles in length, commencing on the outer eastern slope of Piccolomini, and following a tolerably direct north-east course, with a few minor bendings, to the west side of Fermat, where they turn more towards the north, ultimately terminating about midway between Tacitus and Catherina. The region situated on the south-east is a great table-land, without any prominent features, rising gently towards the mountains, which shelve steeply down to an equally barren expanse on the north-west, to which they present a lofty face, having an average altitude of about 6000 feet. The loftiest peak, over 13,000 feet, rises west of Fermat.
The Riphaean Mountains, a remarkably bright group, occupying an isolated position in the Mare Procellarum south of Landsberg, and extending for more than 100 miles in a meridional direction. They are most closely aggregated at a point nearly due west of Euclides, from which they throw off long-branching arms to the north and south, those on the north bifurcating and gradually sinking to the level of the plain. The loftiest peaks are near the extremity of this section, one of them rising to 3000 feet. Two bright craters are associated with these mountains, one nearly central, and the other south of it.
The Percy Mountains.—This name is given to the bright highlands extending east of Gassendi towards Mersenius, forming the north-eastern border of the Mare Humorum. They abound in minute detail—bright little mountains and ridges—and include some clefts pertaining to the Mersenius rill-system; but their most noteworthy feature is the long bright mountain-arm, branching out from the eastern wall of Gassendi, and extending for more than 50 miles towards the south-east.
The principal ranges on the limb are the Leibnitz Mountains, extending from S. lat. 70 deg. on the west to S. lat. 80 deg. on the east limb. They include some giant peaks and plateaus, noteworthy objects in profile, some of which, according to Schroter and Madler, rise to 26,000 feet. The Dorfel Mountains, between S. lat. 80 deg. and 57 deg. on the eastern limb, include, if Schroter's estimate is correct, three peaks which exceed 26,000 feet. On the eastern limb, between S. lat. 35 deg. and 18 deg., extend the Rook Mountains, which have peaks, according to Schroter, as high as 25,000 feet. Next in order come the Cordilleras, which extend to S. lat. 8 deg., and the D'Alembert Mountains, lying east of Rocca and Grimaldi, closely associated with them, and probably part of the same system. Some of the peaks approach 20,000 feet. In addition to these mountain ranges there are others less prominent on the limb in the northern hemisphere, which have not been named.
ISOLATED MOUNTAINS are very numerous in different parts of the moon, the most remarkable are referred to in the appendix. Many remain unnamed.
CLEFTS OR RILLS.—Though Fontenelle, in his Entretiens sur la Pluralite des Mondes, informs his pupil, the Marchioness, that "M. Cassini discovered in the moon something which separates, then reunites, and finally loses itself in a cavity, which from its appearance seemed to be a river," it can hardly be supposed that what the French astronomer saw, or fancied he saw, with the imperfect telescopes of that day, was one of the remarkable and enigmatical furrows termed clefts or rills, first detected by the Hanoverian selenographer Schroter; who, on October 7, 1787, discovered the very curious serpentine cleft near Herodotus, having a few nights before noted for the first time the great Alpine valley west of Plato, once classed with the clefts, though it is an object of a very different kind. Between 1787 and 1797 Schroter found ten rills; but twenty years elapsed before an addition was made to this number by the discoveries of Gruithuisen, and, a short time after, by those of Lohrmann, who in twelve months (1823-24) detected seventy. Kinau, Madler, and finally Schmidt, followed, till, in 1866, when the latter published his work, Ueber Rillen auf dem Monde, the list was thus summarised:—
In the 1st or N.W. quadrant 127 rills In the 2nd or N.E. quadrant 75 rills In the 3rd or S.E. quadrant 141 rills In the 4th or S.W. quadrant 82 rills
or 425 in all. Since the date of this book the number of known rills has been more than doubled; in fact, scarcely a lunation passes without new discoveries being made.
The significance of the word rille in German, a groove or furrow, describes with considerable accuracy the usual appearance of the objects to which it is applied, consisting as they do of long narrow channels, with sides more or less steep, and sometimes vertical. They often extend for hundreds of miles in approximately straight lines over portions of the moon's surface, frequently traversing in their course ridges, craters, and even more formidable obstacles, without any apparent check or interruption, though their ends are sometimes marked by a mound or crater. Their length ranges from ten or twelve to three hundred miles or more (as in the great Sirsalis rill), their breadth, which is very variable within certain limits, from less than half a mile to more than two, and their depth (which must necessarily remain to a great extent problematical) from 100 to 400 yards. They exhibit in the telescope a gradation from somewhat coarse grooves, easily visible at suitable times in very moderately sized instruments, to striae so delicate as to require the largest and most perfect optical means and the best atmospheric conditions to be glimpsed at all. Viewed under moderate amplification, the majority of rills resemble deep canal-like channels with roughly parallel sides, displaying occasionally local irregularities, and fining off to invisibility at one or both ends. But, if critically scrutinised in the best observing weather with high powers, the apparent evenness of their edges entirely disappears, and we find that the latter exhibit indentations, projections, and little flexures, like the banks of an ordinary stream or rivulet, or, to use a very homely simile, the serrated edges and little jagged irregularities of a biscuit broken across. In some cases we remark crateriform hollows or sudden expansions in their course, and deep sinuous ravines, which render them still more unsymmetrical and variable in breadth. With regard to their distribution on the lunar surface; they are found in almost every region, but perhaps not so frequently on the surface of the Maria as elsewhere, though, as in the case of the Triesnecker and other systems, they often abound in the neighbourhood of disturbed regions in these plains, and in many cases along their margins, as, for example, the Gassendi-Mersenius and the Sabine-Ritter groups. The interior of walled plains are frequently intersected by them, as in Gassendi, where nearly forty, more or less delicate examples, have been seen; in Hevel, where there is a very interesting system of crossed clefts, and within Posidonius. If we study any good modern lunar map, it is evident how constantly they appear near the borders of mountain ranges, walled-plains, and ring-plains; as, for instance, at the foot of the Apennines; near Archimedes, Aristarchus, Ramsden, and in many other similar positions. Rugged highlands also are often traversed by them, as in the case of those lying west of Le Monnier and Chacornac, and in the region west of the Mare Humorum. It may be here remarked, however, as a notable fact, that the neighbourhood of the grandest ring-mountain on the moon, Copernicus, is, strange to say, devoid of any features which can be classed as true clefts, though it abounds in crater-rows. The intricate network of rills on the west of Triesnecker, when observed with a low power, reminds one of the wrinkles on the rind of an orange or on the skin of a withered apple. Gruithuisen, describing the rill-traversed region between Agrippa and Hyginus, says that "it has quite the look of a Dutch canal map." In the subjoined catalogue many detailed examples will be given relating to the course of these mysterious furrows; how they occasionally traverse mountain arms, cut through, either completely or partially (as in Ramsden), the borders of ring-plains and other enclosures, while not unfrequently a small mound or similar feature appears to have caused them to swerve suddenly from their path, as in the case of the Ariadaeus cleft, and in that of one member of the Mercator-Campanus system.
Of the actual nature of the lunar rills we are, it must be confessed, supremely ignorant. With some of the early observers it was a very favourite notion that they are artificial works, constructed presumably by Kepler's sub-volvani, or by other intelligences. There is perhaps some excuse to be made for the freaks of an exuberant fancy in regard to objects which, if we ignore for a moment their enormous dimensions, judged by a terrestrial standard, certainly have, in their apparent absence of any physical relation to neighbouring objects, all the appearance of being works of art rather than of nature. The keen-sighted and very imaginative Gruithuisen believed that in some instances they represent roads cut through interminable forests, and in others the dried-up beds of once mighty rivers. His description of the Triesnecker rill-system reads like a page from a geographical primer. A portion of it is compared to the river Po, and he traces its course mile by mile up to the "delta" at its place of disemboguement into the Mare Vaporum. From the position of some rills with respect to the contour of the surrounding country, it is evident that if water were now present on the moon, they, being situated at the lowest level, would form natural channels for its reception; but the exceptions to this arrangement are so numerous and obvious, that the idea may be at once dismissed that there is any analogy between them and our rivers. The eminent selenographer, the late W.K. Birt, compared many of them to "inverted river-beds" from the fact that, as often as not, they become broader and deeper as they attain a higher level. The branches resemble rivers more frequently than the main channels; for they generally commence as very fine grooves, and, becoming broader and broader, join them at an acute angle. An attempt again has been made to compare the lunar clefts with those vast gorges, the marvellous results of aqueous action, called canyons, which attain their greatest dimensions in North America; such as the Great Canyon of the Colorado, which is at least 300 miles in length, and in places 2000 yards in depth, with perpendicular or even overhanging sides; but the analogy, at first sight specious, utterly breaks down under closer examination. Some selenographers consider them to consist of long-extending rows of confluent craters, too minute to be separately distinguished, and to be thus due to some kind of volcanic action. This is undoubtedly true in many instances, for almost every lunar region affords examples of crater- rows merging by almost imperceptible gradations into cleft-like features, and crater-rows of considerable size resemble clefts under low powers. Still it seems probable that the greater number of these features are immense furrows or cracks in the surface and nothing more; for the higher the magnifying power employed in their examination, the less reason there is to object to this description. Dr. Klein of Cologne believes that rills of this class are due to the shrinkage of parts of the moon's crust, and that they are not as a rule the result of volcanic causes, though he admits that there may be some which have a seismic origin. No good reason has as yet been given for the fact that they so frequently cross small craters and other objects in their course, though it has been suggested that the route followed by a rill from crater to crater in these instances may be a line of least surface resistance, an explanation far from being satisfactory.
Whether variations in the visibility of lunar details, when observed under apparently similar conditions, actually occur from time to time from some unknown cause, is one of those vexed questions which will only be determined when the moon is systematically studied by experienced observers using the finest instruments at exceptionally good stations; but no one who examines existing records of observations of rills by Gruithuisen, Lohrmann, Madler, Schmidt, and other observers, can well avoid the conclusion that the anomalies brought to light therein point strongly to the probability of the existence of some agency which occasionally modifies their appearance or entirely conceals them from view.
The following is one illustration out of many which might be quoted. At a point in its course, nearly due north of the ring-plain Agrippa, the great Ariadaeus cleft sends out a branch which runs into the well-known Hyginus cleft, reminding one, as Dr. Klein remarks, of two rivers connected in the shortest way by a canal. This uniting furrow was detected by Gruithuisen, who observed it several times. On some occasions it appeared perfectly straight, at others very irregular; but, what is very remarkable, although two such accurate observers as Lohrmann and Madler frequently scrutinised the region, neither of them saw a trace of this object; and but for its rediscovery by Schmidt in 1862, its existence would certainly have been ignored by selenographers as a mere figment of Gruithuisen's too lively imagination. Dr. Klein has frequently seen this rill with great distinctness, and at other times sought for it in vain; though on each occasion the conditions of illumination, libration, and definition were practically similar. I have sometimes found this cleft an easy object with a 4 inch achromatic. Again, many rills described by Madler as very delicate and difficult to trace, may now be easily followed in "common telescopes." In short, the more direct telescopic observations accumulate, and the more the study of minute detail is extended, the stronger becomes the conviction, that in spite of the absence of an appreciable atmosphere, there may be something resembling low-lying exhalations from some parts of the surface which from time to time are sufficiently dense to obscure, or even obliterate, the region beneath them.
If, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now "a changeless world," that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. Should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for. Many instances might be adduced tending to confirm this supposition, to one of which, as coming under my notice, I will briefly refer. On the evening of November 11, 1883, when examining the interior of the great ring-plain Mersenius with a power of 350 on an 8 1/2 inch reflector; in addition to the two closely parallel clefts discovered by Schmidt, running from the inner foot of the north-eastern rampart towards the centre, I remarked another distinct cleft crossing the northern part of the floor from side to side. Shortly afterwards, M. Gaudibert, one of our most experienced selenographers, who has discovered many hitherto unrecorded clefts, having seen my drawing, searched for this object, and, though the night was far from favourable, had distinct though brief glimpses of it with the moderate magnifying power of 100. Mersenius is a formation about 40 miles in diameter, with a prominently convex interior, containing much detail which has received more than ordinary attention from observers. It has, moreover, been specially mapped by Schmidt and others, yet no trace of this rill was noted, though objects much more minute and difficult have not been overlooked. Does not an instance of this kind raise a well-grounded suspicion of recent change which it is difficult to explain away?
To see the lunar clefts to the best advantage, they must be looked for when not very far removed from the terminator, as when so situated the black shadow of one side, contrasted with the usually brightly- illuminated opposite flank, renders them more conspicuous than when they are viewed under a higher sun. Though, as a rule, invisible at full moon, some of the coarser clefts—as, for example, a portion of the Hyginus furrow, and that north of Birt—may be traced as delicate white lines under a nearly vertical light.
For properly observing these objects, a power of not less than 300 on telescopes of large aperture is needed; and in studying their minute and delicate details, we are perhaps more dependent on atmospheric conditions than in following up any other branch of observational astronomy. Few indeed are the nights, in our climate at any rate, when the rough, irregular character of the steep interior of even the coarser examples of these immense chasms can be steadily seen. We can only hope to obtain a more perfect insight into their actual structural peculiarities when they are scrutinised under more perfect climatic circumstances than they have been hitherto. When observing the Hyginus cleft, Dr. Klein noticed that at one place the declivities of the interior displayed decided differences of tint. At many points the reflected sunlight was of a distinctly yellow hue, while in other places it was white, as if the cliffs were covered with snow. He compares this portion of the rill to the Rhine valley between Bingen and Coblentz, but adds that the latter, if viewed from the moon, would probably not present so fresh an appearance, and would, of course, be frequently obscured by clouds.
Since the erection of the great Lick telescope on Mount Hamilton, our knowledge of the details of some of the lunar clefts has been greatly increased, as in the case of the Ariadaeus cleft, and many others. Professor W.H. Pickering, also, at Arequipa, has made at that ideal astronomical site many observations which, when published, will throw more light upon their peculiar characteristics.
A few years ago M.E.L. Trouvelot of Meudon drew attention to a curious appearance which he noted in connection with certain rills when near the terminator, viz., extremely attenuated threads of light on their sites and their apparent prolongations. He observed it in the ring-plain Eudoxus, crossing the southern side of the floor from wall to wall; and also in connection with the prominent cleft running from the north side of Burg to the west of Alexander, and in some other situations. He terms these phenomena Murs enigmatiques. Apparent prolongations of clefts in the form of rows of hillocks or small mounds are very common.
FAULTS.—These sudden drops in the surface, representing local dislocations, are far from unusual: the best examples being the straight wall, or "railroad," west of Birt; that which strikes obliquely across Plato; another which traverses Phocylides; and a fourth that has manifestly modified the mountain arm north of Cichus. They differ from the terrestrial phenomena so designated in the fact that the surface indications of these are destroyed by denudation or masked by deposits of subsequent date. In many cases on the moon, though its course cannot be traced in its entirety by its shadow, yet the existence of a fault may be inferred by the displacement and fracture of neighbouring objects.
VALLEYS.—Features thus designated, differing greatly both in size and character, are met with in almost every part of the surface, except on the grey plains. While the smallest examples, from their delicacy, tenuity, and superficial resemblance to rills, are termed rill-valleys, the larger and more conspicuous assume the appearance of coarse chasms, gorges, or trough-like depressions. Between these two extremes, are many objects of moderate dimensions—winding or straight ravines and defiles bounded by steep mountains, and shallow dales flanked by low rounded heights. The rill valleys are very numerous, only differing in many instances from the true rills in size, and are probably due to the same cause. Among the most noteworthy valleys of the largest class must, of course, be placed the great valley of the Alps, one of the most striking objects in the northern hemisphere, which also includes the great valley south-east of Ukert. The Rheita valley, the very similar chasm west of Reichenbach, and the gorge west of Herschel, are also notable examples in the southern hemisphere. The borders of some of the Maria (especially that of the Mare Crisium) and of many of the depressed rimless formations, furnish instances of winding valleys intersecting their borders: the hilly regions likewise often abound in long branching defiles.
BRIGHT RAY-SYSTEMS.—Reference has already been made to the faint light streaks and markings often found on the floors of the ring-mountains and in other situations, and to the brilliant nimbi surrounding some of the smaller craters; but, in addition to these, many objects on the moon's visible surface are associated with a much more remarkable and conspicuous phenomenon—the bright rays which, under a high sun, are seen either to radiate from them as apparent centres to great distances, or, in the form of irregular light areas, to environ them, and to throw out wide-spreading lucid beams, extending occasionally many hundreds of miles from their origin. The more striking of these systems were recognised and drawn at a very early stage of telescopic observation, as may be seen if we consult the quaint old charts of Hevel, Riccioli, Fontana, and other observers of the seventeenth century, where they are always prominently, though very inaccurately, portrayed. The principal ray-systems are those of Tycho, Copernicus, Kepler, Anaxagoras, Aristarchus, Olbers, Byrgius A, and Zuchius; while Autolycus, Aristillus, Proclus, Timocharis, Furnerius A, and Menelaus are grouped as constituting minor systems. Many additional centres exist, a list of which will be found in the appendix.
The rays emanating from Tycho surpass in extent and interest any of the others. Hundreds of distinct light streaks originate round the grey margin of this magnificent object, some of them extending over a greater part of the moon's visible superficies, and "radiating," in the words of Professor Phillips, "like false meridians, or like meridians true to an earlier pole of rotation." No systematic attempt has yet been made to map them accurately as a whole on a large scale, for their extreme intricacy and delicacy would render the task a very difficult one, and, moreover, would demand a long course of observation with a powerful telescope in an ideal situation; but Professor W.H. Pickering, observing under these conditions at Arequipa, has recently devoted considerable attention both to the Tycho and other rays, with especially suggestive and important results, which may be briefly summarised as follows:—
(1.) That the Tycho streaks do not radiate from the apparent centre of this formation, but point towards a multitude of minute craterlets on its south-eastern or northern rims. Similar craterlets occur on the rims of other great craters, forming ray-centres. (2.) Speaking generally, a very minute and brilliant crater is located at the end of the streak nearest the radiant point, the streak spreading out and becoming fainter towards the other end. The majority of the streaks appear to issue from one or more of these minute craters, which rarely exceed a mile in diameter. (3.) The streaks which do not issue from minute craters, usually lie upon or across ridges, or in other similar exposed situations, sometimes apparently coming through notches in the mountain walls. (4.) Many of the Copernicus streaks start from craterlets within the rim, flow up the inside and down the outside of the walls. Kepler includes two such craterlets, but here the flow seems to have been more uniform over the edges of the whole crater, and is not distinctly divided up into separate streams. (5.) Though there are similar craters within Tycho, the streaks from them do not extend far beyond the walls. All the conspicuous Tycho streaks originate outside the rim. (6.) The streaks of Copernicus, Kepler, and Aristarchus are greyish in colour, and much less white than those associated with Tycho: some white lines extending south-east from Aristarchus do not apparently belong to the system. In the case of craterlets lying between Aristarchus and Copernicus the streaks point away from the latter. (7.) There are no very long streaks; they vary from ten to fifty miles in length, and are rarely more than a quarter of a mile broad at the crater. From this point they gradually widen out and become fainter. Their width, however, at the end farthest from the crater, seldom exceeds five miles.
These statements, especially those relating to the length of the streaks, are utterly opposed to prevailing notions, but Professor Pickering specifies the case of the two familiar parallel rays extending from the north-east of Tycho to the region east of Bullialdus. His observations show that these streaks, originating at a number of little craters situated from thirty to sixty miles beyond the confines of Tycho, "enter a couple of broad slightly depressed valleys. In these valleys are found numerous minute craters of the kind above described, with intensely brilliant interiors. When the streaks issuing from those craters near Tycho are nearly exhausted, they are reinforced by streaks from other craters which they encounter upon the way, the streaks becoming more pronounced at these points. These streaks are again reinforced farther out. These parallel rays must therefore not be considered as two streaks, but as two series of streaks, the components of which are placed end to end."
Thus, according to Professor Pickering, we must no longer regard the rays emanating from the Tycho region and other centres as continuous, but as consisting of a succession of short lengths, diminishing in brilliancy but increasing in width, till they reach the next crater lying in their direction, when they are reinforced; and the same process of gradual diminution in brightness and reinforcement goes on from one end to the other.
The following explanation is suggested to account for the origin of the rays:—"The earth and her satellite may differ not so much as regards volcanic action as in the densities of their atmospheres. Thus if the craterlets on the rim of Tycho were constantly giving out large quantities of gas or steam, which in other regions was being constantly absorbed or condensed, we should have a wind uniformly blowing away from that summit in all directions. Should other summits in its vicinity occasionally give out gases, mixed with any fine white powder, such as pumice, this powder would be carried away from Tycho, forming streaks."
The difficulty surrounding this very ingenious hypothesis is, that though, assuming the existence of pumice-emitting craters and regions of condensation, there might be a more or less lineal and streaky deposition of this white material over large areas of the moon, why should this deposit be so definitely arranged, and why should these active little craters happen to lie on these particular lines?
The confused network of streaks round Copernicus seem to respond more happily to the requirements of Professor Pickering's hypothesis, for here there is an absence of that definiteness of direction so manifestly displayed in the case of the Tycho rays, and we can well imagine that with an area of condensation surrounding this magnificent object beyond the limits of the streaks, and a number of active little craters on and about its rim, the white material ejected might be drawn outwards in every direction by wind currents, which possibly once existed, and, settling down, assume forms such as we see.
Nasmyth's well-known hypothesis attributes the radiating streaks to cracks in the lunar globe caused by the action of an upheaving force, and accounts for their whiteness by the outwelling of lava from them which has spread to a greater or less distance on either side. If the moon has been fractured in this way, we can easily suppose that the craters formed on these fissures, being in communication with the interior, might eject some pulverulent white matter long after the rest of the surface with its other types of craters had attained a quiescent stage.
The Tycho rays, when viewed under ordinary conditions, appear to extend in unbroken bands to immense distances. One of the most remarkable, strikes along the eastern side of Fracastorius, across the Mare Nectaris to Guttemberg, while another, more central, extends, with local variations in brightness, through Menelaus, over the Mare Serenitatis nearly to the north-west limb. This is the ray that figures so prominently in rude woodcuts of the moon, in which the Mare Serenitatis traversed by it is made to resemble the Greek letter PHI. The Kepler, Aristarchus, and Copernicus systems, though of much smaller extent, are very noteworthy from the crossing and apparent interference of the rays; while those near Byrgius, round Aristarchus, and the rays from Proclus, are equally remarkable.
[Nichol found that the rays from Kepler cut through rays from Copernicus and Aristarchus, while rays from the latter cut through rays from the former. He therefore inferred that their relative ages stand in the order,—Copernicus, Aristarchus, Kepler.]
As no branch of selenography has been more neglected than the observation of these interesting but enigmatical features, one may hope that, in spite of the exacting conditions as to situation and instrumental requirements necessary for their successful scrutiny, the fairly equipped amateur in this less favoured country will not be deterred from attempting to clear up some of the doubts and difficulties which at present exist as to their actual nature.
THE MOON'S ALBEDO, SURFACE BRIGHTNESS, &c.—Sir John Herschel maintained that "the actual illumination of the lunar surface is not much superior to that of weathered sandstone rock in full sunshine." "I have," he says, "frequently compared the moon setting behind the grey perpendicular facade of the Table Mountain, illuminated by the sun just risen in the opposite quarter of the horizon, when it has been scarcely distinguishable in brightness from the rock in contact with it. The sun and moon being at nearly equal altitudes, and the atmosphere perfectly free from cloud or vapour, its effect is alike on both luminaries." Zollner's elaborate researches on this question are closely in accord with the above observational result. Though he considers that the brightest parts of the surface are as white as the whitest objects with which we are acquainted, yet, taking the reflected light as a whole, he finds that the moon is more nearly black than white. The most brilliant object on the surface is the central peak of the ring-plain Aristarchus, the darkest the floor of Grimaldi, or perhaps a portion of that of the neighbouring Riccioli. Between these extremes, there is every gradation of tone. Proctor, discussing this question on the basis of Zollner's experiments respecting the light reflected by various substances, concludes that the dark area just mentioned must be notably darker than the dark grey syenite which figures in his tables, while the floor of Aristarchus is as white as newly fallen snow.
The estimation of lunar tints in the usual way, by eye observations at the telescope, involving as it does physiological errors which cannot be eliminated, is a method far too crude and ambiguous to form the basis of a scientific scale or for the detection of slight variations. An instrument on the principle of Dawes' solar eyepiece has been suggested; this, if used with an invariable and absolute scale of tints, would remove many difficulties attending these investigations. The scale which was adopted by Schroter, and which has been used by selenographers up to the present time, is as follows:—
0 deg. = Black. 1 deg. = Greyish black. 2 deg. = Dark grey. 3 deg. = Medium grey. 4 deg. = Yellowish grey. 5 deg. = Pure light grey. 6 deg. = Light whitish grey. 7 deg. = Greyish white. 8 deg. = Pure white. 9 deg. = Glittering white. 10 deg. = Dazzling white.
The following is a list of lunar objects published in the Selenographical Journal, classed in accordance with this scale:—
0 deg. Black shadows. 1 deg. Darkest portions of the floors of Grimaldi and Riccioli. 1 1/2 deg. Interiors of Boscovich, Billy, and Zupus. 2 deg. Floors of Endymion, Le Monnier, Julius Caesar, Cruger, and Fourier a. 2 1/2 deg. Interiors of Azout, Vitruvius, Pitatus, Hippalus, and Marius. 3 deg. Interiors of Taruntius, Plinius, Theophilus, Parrot, Flamsteed, and Mercator. 3 1/2 deg. Interiors of Hansen, Archimedes, and Mersenius. 4 deg. Interiors of Manilius, Ptolemaeus, and Guerike. 4 1/2 deg. Surface round Aristillus, Sinus Medii. 5 deg. Walls of Arago, Landsberg, and Bullialdus. Surface round Kepler and Archimedes. 5 1/2 deg. Walls of Picard and Timocharis. Rays from Copernicus. 6 deg. Walls of Macrobius, Kant, Bessel, Mosting, and Flamsteed. 6 1/2 deg. Walls of Langrenus, Theaetetus, and Lahire. 7 deg. Theon, Ariadaeus, Bode B, Wichmann, and Kepler. 7 1/2 deg. Ukert, Hortensius, Euclides. 8 deg. Walls of Godin, Bode, and Copernicus. 8 1/2 deg. Walls of Proclus, Bode A, and Hipparchus c. 9 deg. Censorinus, Dionysius, Mosting A, and Mersenius B and c. 9 1/2 deg. Interior of Aristarchus, La Peyrouse DELTA. 10 deg. Central peak of Aristarchus.
TEMPERATURE OF THE MOON'S SURFACE.—Till the subject was undertaken some years ago by Lord Rosse, no approach was made to a satisfactory determination of the surface temperature of the moon. From his experiments he inferred that the maximum temperature attained, at or near the equator, about three days after full moon, does not exceed 200 deg. C., while the minimum is not much under zero C. Subsequent experiments, however, both by himself and Professor Langley, render these results more than doubtful, without it is admitted that the moon has an atmospheric covering. Langley's results make it probable that the temperature never rises above the freezing-point of water, and that at the end of the prolonged lunar night of fourteen days it must sink to at least 200 deg. below zero. Mr. F.W. Verey of the Alleghany Observatory has recently conducted, by means of the bolometer, similar researches as to the distribution of the moon's heat and its variation with the phase, by which he has deduced the varying radiation from the surface in different localities of the moon under various solar altitudes.
LUNAR OBSERVATION.—In observing the moon, we enjoy an advantage of which we cannot boast when most other planetary bodies are scrutinised; for we see the actual surface of another world undimmed by palpable clouds or exhalations, except such as exist in the air above us; and can gaze on the marvellous variety of objects it presents much as we contemplate a relief map of our own globe. But inasmuch as the manifold details of the relief map require to be placed in a certain light to be seen to the best advantage, so the ring-mountains, rugged highlands, and wide-extending plains of our satellite, as they pass in review under the sun, must be observed when suitable conditions of illumination prevail, if we wish to appreciate their true character and significance.
As a general rule, lunar objects are best seen when they are at no great distance from "the terminator," or the line dividing the illumined from the unillumined portion of the spherical surface. This line is constantly changing its position with the sun, advancing slowly onwards towards the east at a rate which, roughly speaking, amounts to about 30.5 min. in an hour, or passing over 10 deg. of lunar longitude in about 19 hrs. 40 mins. When an object is situated on this line, the sun is either rising or setting on the neighbouring region, and every inequality of the surface is rendered prominent by its shadow; so that trifling variations in level and minor asperities assume for the time being an importance to which they have no claim. If we are observing an object at lunar sunrise, a very short time, often only a few minutes, elapses before the confusion caused by the presence of the shadows of these generally unimportant features ceases to interfere with the observation, and we can distinguish between those details which are really noteworthy and others which are trivial and evanescent. Every formation we are studying should be observed, and drawn if possible, under many different conditions of illumination. It ought, in fact, to be examined from the time when its loftiest heights are first illumined by the rising sun till they disappear at sunset. This is, of course, practically impossible in the course of one lunation, but by utilising available opportunities, a number of observations may be obtained under various phases which will be more or less exhaustive. It cannot be said that much is known about any object until an attempt has been made to carry out this plan. Features which assume a certain appearance at one phase frequently turn out to be altogether different when viewed under another; important details obscured by shadows, craters masked by those of neighbouring objects, or by the shadows of their own rims, are often only revealed when the sun has attained an altitude of ten degrees or more. In short, there is scarcely a formation on the moon which does not exemplify the necessity of noting its aspect from sunrise to sunset. Regard must also be had to libration, which affects to a greater or less degree every object; carrying out of the range of observation regions near the limb at one time, and at another bringing into view others beyond the limits of the maps, which represent the moon in the mean state of libration. The area, in fact, thus brought into view, or taken out of it, is between 1/12th and 1/13th of the entire area of the moon, or about the 1/6th part of the hemisphere turned away from the earth. It is convenient to bear in mind that we see an object under nearly the same conditions every 59 d. 1 h. 28 m., or still more accurately, after the lapse of fifteen lunations, or 442 d. 23 h. Many observers avoid the observation of objects under a high light. This, however, should never be neglected when practicable, though in some cases it is not easy to carry out, owing to the difficulty in tracing details under these circumstances.