THE MOON.—Thomas Gwyn Elger
We know, both by tradition and published records, that from the earliest times the faint gray and light spots which diversify the face of our satellite excited the wonder and stimulated the curiosity of mankind, giving rise to superstitions 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.
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 southeastern quadrant, being seen to consist of large gray 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.
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 disk, thus laying the foundation of an accurate system of lunar topography; while the labors of T. Mayer and Schröter in the Eighteenth Century, and of Lohrmann, Mädler, Neison (Nevill), Schmidt, and other observers in the Nineteenth, 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 color and structural character. Somewhat less than half of what we see of it consists of comparatively level dark tracts, some of them 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, etc. (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, has ever been detected on the surface of the moon, the superficial resemblance, in small telescopes, of the large gray 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.
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 southeastern quarter of the Southern Hemisphere—the southwestern 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° in the southeastern 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, or about one-fifth the superficies, covering a large portion of the southeastern 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 Fœcunditatis, in the Western Hemisphere, the greater part of it lying in the southwestern 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.
Among the Maria which exhibit the most remarkable arrangement of ridges is the Mare Humorum, in the southeastern 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 toward 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 neighboring surface.
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.
Walled-plains, approximating more or less to the circular form, though frequently deviating considerably from it, are among the largest inclosures on the moon. They vary from upward of 150 to 160 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 inclosed 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 gray plains of 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.
Ptolemæus, in the third quadrant and not far removed from the centre of the disk, 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 9,000 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-gray interior, Grimaldi, near the eastern limb (perhaps the darkest area on the moon), Schickard, nearly as big on the southeastern limb, and Bailly, larger than either (still further 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 inclosures, 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 infringe on their apparently level floor at an angle of from 1° to 2°, it is seen to be coarse, rough grained, and covered with minute elevations, although an hour or so afterward it appears as smooth as glass.
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 5,000 to 8,000 feet are common. They extend in a southwest direction from Plato to the Caucasus, terminating somewhat abruptly, a little west of the central meridian in about N. lat. 42°. 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 southward, 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 3,000 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 neighboring promontory on the southeast of it is fully 14,000 feet, and another, close by, is still higher above the Mare. Mount Huyghens, again in N. lat. 20°, 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 toward the southwest, and includes some very bright craters and mountain-rings.
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 rills by Gruithuisen, Lohrmann, Mädler, 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. 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.
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 gray perpendicular façade of the Table Mountain, illumined 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 vapor, its effect is alike on both luminaries.” Zöllner’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 neighboring Riccioli. Between these extremes there is every gradation of tone. Proctor, discussing this question on the basis of Zöllner’s experiments respecting the light reflected by various substances, concludes that the dark area just mentioned must be notably darker than the dark gray syenite which figures in his tables, while the floor of Aristarchus is as white as newly fallen snow.