Saturn seen with a Five-Inch Telescope.
Saturn ranks next to Jupiter in attractiveness for the observer with a telescope. The rings are almost as mystifyingto-day as they were in the time of Herschel. There is probably no single telescopic view that can compare in the power to excite wonder with that of Saturn when the ring system is not so widely opened but that both poles of the planet project beyond it. One returns to it again and again with unflagging interest, and the beauty of the spectacle quite matches its singularity. When Saturn is in view the owner of a telescope may become a recruiting officer for astronomy by simply inviting his friends to gaze at the wonderful planet. The silvery color of the ball, delicately chased with half-visible shadings, merging one into another from the bright equatorial band to the bluish polar caps; the grand arch of the rings, sweeping across the planet with a perceptible edging of shadow; their sudden disappearance close to the margin of the ball, where they go behind it and fall straightway into night; the manifest contrast of brightness, if not of color, between the two principal rings; the fine curve of the black line marking the 1,600-mile gap between their edges—these are some of the elements of a picture that can never fade from the memory of any one who has once beheld it in its full glory.
Saturn's moons are by no means so interesting to watch as are those of Jupiter. Even the effect of their surprising number (raised to nine by Professor Pickering's discovery in 1899 of a new one which is almost at the limit of visibility, and was found only with the aid of photography) is lost, because most of them are too faint to be seen with ordinary telescopes, or, if seen, to make any notable impression upon the eye. The two largest—Titanand Japetus—are easily found, and Titan is conspicuous, but they give none of that sense of companionship and obedience to a central authority which strikes even the careless observer of Jupiter's system. This is owing partly to their more deliberate movements and partly to the inclination of the plane of their orbits, which seldom lies edgewise toward the earth.
Polar View of Saturn's System.The orbits of the five nearest satellites are shown. The dotted line outside the rings shows Roche's limit.
But the charm of the peerless rings is abiding, and the interest of the spectator is heightened by recalling whatscience has recently established as to their composition. It is marvelous to think, while looking upon their broad, level surfaces—as smooth, apparently, as polished steel, though thirty thousand miles across—that they are in reality vast circling currents of meteoritic particles or dust, through which run immense waves, condensation and rarefaction succeeding one another as in the undulations of sound. Yet, with all their inferential tumult, they may actually be as soundless as the depths of interstellar space, for Struve has shown that those spectacular rings possess no appreciable mass, and, viewed from Saturn itself, their (to us) gorgeous seeming bow may appear only as a wreath of shimmering vapor spanning the sky and paled by the rivalry of the brighter stars.
In view of the theory of tidal action disrupting a satellite within a critical distance from the center of its primary, the thoughtful observer of Saturn will find himself wondering what may have been the origin of the rings. The critical distance referred to, and which is known as Roche's limit, lies, according to the most trustworthy estimates, just outside the outermost edge of the rings. It follows that if the matter composing the rings were collected into a single body that body would inevitably be torn to pieces and scattered into rings; and so, too, if instead of one there were several or many bodies of considerable size occupying the place of the rings, all of these bodies would be disrupted and scattered. If one of the present moons of Saturn—for instance, Mimas, the innermost hitherto discovered—should wander within the magic circle of Roche's limit it would suffer a similar fate, and its particles would be disseminated among the rings. One can hardly help wondering whether the rings have originated from the demolition of satellites—Saturn devouring his children, as the ancient myths represent, andencircling himself, amid the fury of destruction, with the dust of his disintegrated victims. At any rate, the amateur student of Saturn will find in the revelations of his telescope the inspirations of poetry as well as those of science, and the bent of his mind will determine which he shall follow.
Professor Pickering's discovery of a ninth satellite of Saturn, situated at the great distance of nearly eight million miles from the planet, serves to call attention to the vastness of the "sphere of activity" over which the ringed planet reigns. Surprising as the distance of the new satellite appears when compared with that of our moon, it is yet far from the limit where Saturn's control ceases and that of the sun becomes predominant. That limit, according to Prof. Asaph Hall's calculation, is nearly 30,000,000 miles from Saturn's center, while if our moon were removed to a distance a little exceeding 500,000 miles the earth would be in danger of losing its satellite through the elopement of Artemis with Apollo.
Although, as already remarked, the satellites of Saturn are not especially interesting to the amateur telescopist, yet it may be well to mention that, in addition to Titan and Japetus, the satellite named Rhea, the fifth in order of distance from the planet, is not a difficult object for a three-or four-inch telescope, and two others considerably fainter than Rhea—Dione (the fourth) and Tethys (the third)—may be seen in favorable circumstances. The others—Mimas (the first), Enceladus (the second), and Hyperion (the seventh)—are beyond the reach of all but large telescopes. The ninth satellite, which has received the name of Phœbe, is much fainter than any of the others, its stellar magnitude being reckoned by its discoverer at about 15.5.
Mars, the best advertised of all the planets, is nearlythe least satisfactory to look at except during a favorable opposition, like those of 1877 and 1892, when its comparative nearness to the earth renders some of its characteristic features visible in a small telescope. The next favorable opposition will occur in 1907.
Mars seen with a Five-Inch Telescope.
When well seen with an ordinary telescope, say a four-or five-inch glass, Mars shows three peculiarities that may be called fairly conspicuous—viz., its white polar cap, its general reddish, or orange-yellow, hue, and its dark markings, one of the clearest of which is the so-called Syrtis Major, or, as it was once named on account of its shape, "Hourglass Sea." Other dark expanses in the southern hemisphere are not difficult to be seen, although their outlines are more or less misty and indistinct. The gradual diminution of the polar cap, which certainly behaves in this respect as a mass of snow and ice would do, is a most interesting spectacle. As summer advances in the southern hemisphere of Mars, the white circular patch surrounding the pole becomes smaller, night after night, until it sometimes disappears entirely even from the ken of the largest telescopes. At the same time the dark expanses become more distinct, as if the melting of the polar snows had supplied them with a greater depth of water, or the advance of the season had darkened them with a heavier growth of vegetation.
The phenomena mentioned above are about all that asmall telescope will reveal. Occasionally a dark streak, which large instruments show is connected with the mysterious system of "canals," can be detected, but the "canals" themselves are far beyond the reach of any telescope except a few of the giants handled by experienced observers. The conviction which seems to have forced its way into the minds even of some conservative astronomers, that on Mars the conditions, to use the expression of Professor Young, "are more nearly earthlike than on any other of the heavenly bodies which we can see with our present telescopes," is sufficient to make the planet a center of undying interest notwithstanding the difficulties with which the amateur is confronted in his endeavors to see the details of its markings.
The Illumination of Venus's Atmosphere at the beginning of her Transit across the Sun.
In Venus "the fatal gift of beauty" may be said, as far as our observations are concerned, to be matched by the equally fatal gift of brilliance. Whether it be due to atmospheric reflection alone or to the prevalence of clouds, Venus is so bright that considerable doubt exists as to the actual visibility of any permanent markings on her surface. The detailed representations of the disk of Venus by Mr. Percival Lowell, showing in some respects a resemblance to the stripings of Mars, can not yet be accepted as decisive. More experienced astronomers than Mr. Lowell have been unable to see atall things which he draws with a fearless and unhesitating pencil. That there are some shadowy features of the planet's surface to be seen in favourable circumstances is probable, but the time for drawing a "map of Venus" has not yet come.
The previous work of Schiaparelli lends a certain degree of probability to Mr. Lowell's observations on the rotation of Venus. This rotation, according to the original announcement of Schiaparelli, is probably performed in the same period as the revolution around the sun. In other words, Venus, if Schiaparelli and Lowell are right, always presents the same side to the sun, possessing, in consequence, a day hemisphere and a night hemisphere which never interchange places. This condition is so antagonistic to all our ideas of what constitutes habitability for a planet that one hesitates to accept it as proved, and almost hopes that it may turn out to have no real existence. Venus, as the twin of the earth in size, is a planet which the imagination, warmed by its sunny aspect, would fain people with intelligent beings a little fairer than ourselves; but how can such ideas be reconciled with the picture of a world one half of which is subjected to the merciless rays of a never-setting sun, while the other half is buried in the fearful gloom and icy chill of unending night?
Any amateur observer who wishes to test his eyesight and his telescope in the search of shades or markings on the disk of Venus by the aid of which the question of its rotation may finally be settled should do his work while the sun is still above the horizon. Schiaparelli adopted that plan years ago, and others have followed him with advantage. The diffused light of day serves to take off the glare which is so serious an obstacle to the successful observation of Venus when seen against a dark sky.Knowing the location of Venus in the sky, which can be ascertained from the Ephemeris, the observer can find it by day. If his telescope is not permanently mounted and provided with "circles" this may not prove an easy thing to do, yet a little perseverance and ingenuity will effect it. One way is to find, with a star chart, some star whose declination is the same, or very nearly the same, as that of Venus, and which crosses the meridian say twelve hours ahead of her. Then set the telescope upon that star, when it is on the meridian at night, and leave it there, and the next day, twelve hours after the star crossed the meridian, look into your telescope and you will see Venus, or, if not, a slight motion of the tube will bring her into view.
For many amateurs the phases of Venus will alone supply sufficient interest for telescopic observation. The changes in her form, from that of a round full moon when she is near superior conjunction to the gibbous, and finally the half-moon phase as she approaches her eastern elongation, followed by the gradually narrowing and lengthening crescent, until she is a mere silver sickle between the sun and the earth, form a succession of delightful pictures.
Not very much can be said for Mercury as a telescopic object. The little planet presents phases like those of Venus, and, according to Schiaparelli and Lowell, it resembles Venus in its rotation, keeping always the same side to the sun. In fact, Schiaparelli's discovery of this peculiarity in the case of Mercury preceded the similar discovery in the case of Venus. There are markings on Mercury which have reminded some astronomers of the moon, and there are reasons for thinking that the planet can not be a suitable abode for living beings, at least for beings resembling the inhabitants of the earth.
Uranus and Neptune are too far away to present any attraction for amateur observers.
"... the Moon, whose orbThe Tuscan artist views through optic glassAt evening from the top of Fesolé,Or in Valdarno, to descry new lands,Rivers or mountains in her spotty globe."—Paradise Lost.
"... the Moon, whose orbThe Tuscan artist views through optic glassAt evening from the top of Fesolé,Or in Valdarno, to descry new lands,Rivers or mountains in her spotty globe."—Paradise Lost.
The moon is probably the most interesting of all telescopic objects. This arises from its comparative nearness to the earth. A telescope magnifying 1,000 diameters brings the moon within an apparent distance of less than 240 miles. If telescopes are ever made with a magnifying power of 10,000 diameters, then, provided that atmospheric difficulties can be overcome, we shall see the moon as if it were only about twenty miles off, and a sensitive astronomer might be imagined to feel a little hesitation about gazing so closely at the moon—as if he were peering into a neighbor world's window.
But a great telescope and a high magnifying power are not required to interest the amateur astronomer in the study of the moon. Our three-inch telescope is amply sufficient to furnish us with entertainment for many an evening while the moon is running through its phases, and we shall find delight in frequently changing the magnifying power as we watch the lunar landscapes, because every change will present them in a different aspect.
It should be remembered that a telescope, unless a terrestrial eyepiece or prism is employed, reverses such anobject as the moon top for bottom. Accordingly, if the moon is on or near the meridian when the observations are made, we shall see the north polar region at the bottom and the south polar region at the top. In other words, the face of the moon as presented in the telescope will be upside down, north and south interchanging places as compared with their positions in a geographical map. But east and west remain unaltered in position, as compared with such a map—i. e., the eastern hemisphere of the moon is seen on the right and the western hemisphere on the left. It is the moon's western edge that catches the first sunlight when "new moon" begins, and, as the phase increases, passing into "first quarter" and from that to "full moon," the illumination sweeps across the disk from west to east.
Lunar Chart No. 1, Northwest Quarter.
The narrow sickle of the new moon, hanging above the sunset, is a charming telescopic sight. Use a low power, and observe the contrast between the bright, smooth round of the sunward edge, which has almost the polish of a golden rim, and the irregular and delicately shaded inner curve, where the adjacent mountains and plains picturesquely reflect or subdue the sunshine. While the crescent grows broader new objects are continually coming into view as the sun rises upon them, until at length one of the most conspicuous and remarkable of the lunar "seas," theMare Crisium, or Sea of Crises, lies fully displayed amid its encircling peaks, precipices, and craters. TheMare Crisiumis all in the sunlight between the third and fourth day after "new moon." It is about 350 by 280 miles in extent, and if ever filled with water must have been a very deep sea, since its arid bed lies at a great but not precisely ascertained depth below the general level of the moon. There are a few small craters on the floor of theMare Crisium, the largest bearing the name of Picard,and its borders are rugged with mountains. On the southwestern side is a lofty promontory, 11,000 feet in height, called Cape Agarum. At the middle of the eastern side a kind of bay opens deep in the mountains, whose range here becomes very narrow. Southeast of this bay lies a conspicuous bright point, the crater mountain Proclus, on which the sun has fully risen in the fourth day of the moon, and which reflects the light with extraordinary liveliness. Adjoining Proclus on the east and south is acurious, lozenge-shaped flat, broken with short, low ridges, and possessing a most peculiar light-brown tint, easily distinguished from the general color tone of the lunar landscapes. It would be interesting to know what was passing in the mind of the old astronomer who named this singular regionPalus Somnii. It is not the only spot on the moon which has been called a "marsh," and to which an unexplained connection with dreams has been ascribed.
Nearly on the same meridian with Proclus, at a distance of about a hundred miles northward, lies a fine example of a ring mountain, rather more than forty miles in diameter, and with peak-tipped walls which in some places are 13,000 feet in height, as measured from the floor within. This is Macrobius. There is an inconspicuous central mountain in the ring.
North of theMare Crisium, and northwest of Macrobius, we find a much larger mountain ring, oblong in shape and nearly eighty miles in its greatest diameter. It is named Cleomenes. The highest point on its wall is about 10,000 feet above the interior. Near the northeast corner of the wall yawns a huge and very deep crater, Tralles, while at the northern end is another oblong crater mountain called Burckhardt.
From Cleomenes northward to the pole, or to the northern extremity of the crescent, if our observations are made during new moon, the ground appears broken with an immense number of ridges, craters, and mountain rings, among which we may telescopically wander at will. One of the more remarkable of these objects, which may be identified with the aid ofLunar Chart No. 1, is the vast ringed plain near the edge of the disk, named Gauss. It is more than a hundred and ten miles in diameter. Owing to its situation, so far down the side of the lunar globe, it is foreshortened into a long ellipse, although in reality itis nearly a circle. A chain of mountains runs north and south across the interior plain. Geminus, Berzelius, and Messala are other rings well worth looking at. The remarkable pair called Atlas and Hercules demand more than passing attention. The former is fifty-five and the latter forty-six miles in diameter. Each sinks 11,000 feet below the summit of the loftiest peak on its encircling wall. Both are full of interesting detail sufficient to occupy the careful observer for many nights. The broad ring bearing the name of Endymion is nearly eighty miles in diameter, and has one peak 15,000 feet high. The interior plain is flat and dark. Beyond Endymion on the edge of the disk is part of a gloomy plain called theMare Humboltianum.
After glancing at the crater-shaped mountains on the western and southern border of theMare Crisium, Alhazen, Hansen, Condorcet, Firmicus, etc., we pass southward into the area covered inLunar Chart No. 2. The long dark plain south of theMare Crisiumis theMare Fecunditatis, though why it should have been supposed to be particularly fecund, or fertile, is by no means clear. On the western border of this plain, about three hundred miles from the southern end of theMare Crisium, is the mountain ring, or circumvallation, called Langrenus, about ninety miles across and in places 10,000 feet high. There is a fine central mountain with a number of peaks. Nearly a hundred miles farther south, on the same meridian, lies an equally extensive mountain ring named Vendelinus. The broken and complicated appearance of its northern walls will command the observer's attention. Another similar step southward, and still on the same meridian brings us to a yet finer mountain ring, slightly larger than the others, and still more complicated in its walls, peaks, and terraces, and in its surroundings of craters, gorges, and broken ridges. This is Petavius. West of Petavius, on the veryedge of the disk, is a wonderful formation, a walled plain named Humboldt, which is looked down upon at one point near its eastern edge by a peak 16,000 feet in height. About a hundred and forty miles south of Petavius is the fourth great mountain ring lying on the same meridian. Its name is Furnerius. Look particularly at the brilliantly shining crater on the northeast slope of the outer wall of Furnerius.
Lunar Chart No. 2, Southwest Quarter.
Suppose that our observations are now interrupted, to be resumed when the moon, about "seven days old," is in its first quarter. If we had time, it would be a most interesting thing to watch the advance of the lunar sunrise every night, for new beauties are displayed almost from hour to hour; but, for the purposes of our description it is necessary to curtail the observations. At first quarter one half of the lunar hemisphere which faces the earth is illuminated by the sun, and the line of sunrise runs across some of the most wonderful regions of the moon.
We begin, referring once more toLunar Chart No. 1, in the neighborhood of the north pole of the moon. Here the line along which day and night meet is twisted and broken, owing to the roughness of the lunar surface. About fifteen degrees southwest of the pole lies a remarkable square-cornered, mountain-bordered plain, about forty miles in length, called Barrow. Very close to the pole is a ring mountain, about twenty-five miles in diameter, whose two loftiest peaks, 8,000 to 9,000 feet high, according to Neison, must, from their situation, enjoy perpetual day.
The long, narrow, dark plain, whose nearest edge is about thirty degrees south of the pole, is theMare Frigoris, bordered on both sides by uplands and mountains. At its southern edge we find the magnificent Aristoteles, a mountain ring, sixty miles across, whose immense wall is composed of terraces and ridges running up to lofty peaks, which rise nearly 11,000 feet above the floor of the valley. About a hundred miles south of Aristoteles is Eudoxus, another fine mountain ring, forty miles in diameter, and quite as deep as its northern neighbor. These two make a most striking spectacle.
We are now in the neighborhood of the greatest mountainchains on the moon, the lunar Alps lying to the east and the lunar Caucasus to the south of Aristoteles and Eudoxus, while still farther south, separated from the Caucasus by a strait not more than a hundred miles broad, begins the mighty range of the lunar Apennines. We first turn the telescope on the Alps. As the line of sunrise runs directly across their highest peaks, the effect is startling. The greatest elevations are about 12,000 feet. The observer's eye is instantly caught by a great valley, running like a furrow through the center of the mountain mass, and about eighty or ninety miles in length. The sealike expanse south and southeast of the Alps is theMare Imbrium, and it is along the coast of this so-called sea that the Alps attain their greatest height. The valley, or gorge, above mentioned, appears to cut through the loftiest mountains and to reach the "coast," although it is so narrowed and broken among the greater peaks that its southern portion is almost lost before it actually reaches theMare Imbrium. Opening wider again as it enters theMare, it forms a deep bay among precipitous mountains.
The Caucasus Mountains are not so lofty nor so precipitous as the Alps, and consequently have less attraction for the observer. They border the dark, oval plain of theMare Serenitatison its northeastern side. The great bay running out from theMaretoward the northwest, between the Caucasus and the huge mountain ring of Posidonius, bears the fanciful name ofLacus Somniorum. In the old days when the moon was supposed to be inhabited, those terrestrial godfathers, led by the astronomer Riccioli, who were busy bestowing names upon the "seas" and mountains of our patient satellite, may have pleased their imagination by picturing this arm of the "Serene Sea" as a peculiarly romantic sheet of water, amid whose magicalinfluences the lunar gentlefolk, drifting softly in their silver galleons and barges, and enjoying the splendors of "full earth" poured upon their delightful little world, were accustomed to fall into charming reveries, as even we hard-headed sons of Adam occasionally do when the waters under the keel are calm and smooth and the balmy air of a moonlit night invokes the twin spirits of poetry and music.
Posidonius, the dominating feature of the shore line here, is an extraordinary example of the many formations on the moon which are so different from everything on the earth that astronomers do not find it easy to bestow upon them names that truly describe them. It may be called a ring mountain or a ringed plain, for it is both. Its diameter exceeds sixty miles, and the interior plain lies about 2,000 feet below the outer surface of the lunar ground. The mountain wall surrounding the ring is by no means remarkable for elevation, its greatest height not exceeding 6,000 feet, but, owing to the broad sweep of the curved walls, the brightness of the plain they inclose, and the picturesque irregularity of the silhouette of shadow thrown upon the valley floor by the peaks encircling it, the effect produced upon the observer is very striking and attractive.
Having finished with Posidonius and glanced across the broken region of the Taurus Mountains toward the west, we turn next to consider theMare Serenitatis. This broad gray plain, which, with a slight magnifying power, certainly looks enough like a sea to justify the first telescopists in thinking that it might contain water, is about 430 by 425 miles in extent, its area being 125,000 square miles. Running directly through its middle, nearly in a north and south line, is a light streak, which even a good opera glass shows. This streak is the largest and mostwonderful of the many similar rays which extend on all sides from the great crater, or ring, of Tycho in the southern hemisphere. The ray in question is more than 2,000 miles long, and, like its shorter congeners, it turns aside for nothing; neither "sea," nor peak, nor mountain range, nor crater ring, nor gorge, nor cañon, is able to divert it from its course. It ascends all heights and drops into all depths with perfect indifference, but its continuity is not broken. When the sun does not illuminate it at a proper angle, however, the mysterious ray vanishes. Is it a metallic vein, or is it volcanic lava or ash? Was the globe of the moon once split open along this line?
TheMare Serenitatisis encircled by mountain ranges to a greater extent than any of the other lunar "seas." On its eastern side the Caucasus and the Apennines shut it in, except for a strait a hundred miles broad, by means of which it is connected with theMare Imbrium. On the south the range of the Hæmus Mountains borders it, on the north and northwest the Caucasus and the Taurus Mountains confine it, while on the west, where again it connects itself by a narrow strait with another "sea," theMare Tranquilitatis, it encounters the massive uplift of Mount Argæus. Not far from the eastern strait is found the remarkable little crater named Linné, not conspicuous on the gray floor of theMare, yet easily enough found, and very interesting because a considerable change of form seems to have come over this crater some time near the middle of the nineteenth century. In referring to it as a crater it must not be forgotten that it does not form an opening in the top of a mountain. In fact, the so-called craters on the moon, generally speaking, are simply cavities in the lunar surface, whose bottoms lie deep below the general level, instead of being elevated on the summit of mountains, and inclosed in a conical peak.In regard to the alleged change in Linné, it has been suggested, not that a volcanic eruption brought it about, but that a downfall of steep walls, or of an unsupported rocky floor, was the cause. The possibility of such an occurrence, it must be admitted, adds to the interest of the observer who regularly studies the moon with a telescope.
Just on the southern border of theMare, the beautiful ring Menelaus lies in the center of the chain of the Hæmus Mountains. The ring is about twenty miles across, and its central peak is composed of some highly reflecting material, so that it shines very bright. The streak or ray from Tycho which crosses theMare Serenitatispasses through the walls of Menelaus, and perhaps the central peak is composed of the same substance that forms the ray. Something more than a hundred miles east-southeast from Menelaus, in the midst of the darkMare Vaporum, is another brilliant ring mountain which catches the eye, Manilius. It exceeds Menelaus in brightness as well as in size, its diameter being about twenty-five miles. There is something singular underlying the dark lunar surface here, for not only is Manilius extraordinarily brilliant in contrast with the surrounding plain, but out of that plain, about forty miles toward the east, projects a small mountain which is also remarkable for its reflecting properties, as if the gray ground were underlain by a stratum of some material that flashes back the sunlight wherever it is exposed. The crater mountain, Sulpicius Gallus, on the border of theMare, north of Manilius and east of Menelaus, is another example of the strange shining quality of certain formations on the moon.
Follow next the Hæmus range westward until the attention falls upon the great ring mountain Plinius, more than thirty miles across, and bearing an unusual resemblance to a fortification. Mr. T. G. Elger, the celebratedEnglish selenographer, says of Plinius that, at sunrise, "it reminds one of a great fortress or redoubt erected to command the passage between theMare Tranquilitatisand theMare Serenitatis." But, of course, the resemblance is purely fanciful. Men, even though they dwelt in the moon, would not build a rampart 6,000 feet high!
Mount Argæus, at the southwest corner of theMare Serenitatis, is a very wonderful object when the sun has just risen upon it. This occurs five days after the new moon.
Returning to the eastern extremity of theMare, we glance, in passing, at the precipitous Mount Hadley, which rises more than 15,000 feet above the level of theMareand forms the northern point of the Apennine range. Passing into the region of theMare Imbrium, whose western end is divided into thePalus Putredinison the south and thePalus Nebularumon the north, we notice three conspicuous ring mountains, Cassini near the Alps, and Aristillus and Autolycus, a beautiful pair, nearly opposite the strait connecting the twoMaria. Cassini is thirty-six miles in diameter, Aristillus thirty-four, and Autolycus twenty-three. The first named is shallow, only 4,000 feet in depth from the highest point of its wall, while Aristillus carries some peaks on its girdle 11,000 feet high. Autolycus, like Cassini, is of no very great depth.
Westward from the middle of an imaginary line joining Aristillus and Cassini is the much smaller crater Theætetus. Outside the walls of this are a number of craterlets, and a French astronomer, Charbonneaux, of the Meudon Observatory, reported in December, 1900, that he had repeatedly observed white clouds appearing and disappearing over one of these small craters.
South of theMare Vaporumare found some of the most notable of those strange lunar features that are called"clefts" or "rills." Two crater mountains, in particular, are connected with them, Ariadæus at the eastern edge of theMare Tranquilitatisand Hyginus on the southern border of theMare Vaporum. These clefts appear to be broad and deep chasms, like the cañons cut by terrestrial rivers, but it can not be believed that the lunar cañons are the work of rivers. They are rather cracks in the lunar crust, although their bottoms are frequently visible. The principal cleft from Ariadæus runs eastward and passes between two neighboring craters, the southern of which is named Silberschlag, and is noteworthy for its brightness. The Hyginus cleft is broader and runs directly through the crater ring of that name.
The observer will find much to interest him in the great, irregular, and much-broken mountain ring called Julius Cæsar, as well as in the ring mountains, Godin, Agrippa, and Triesnecker. The last named, besides presenting magnificent shadows when the sunlight falls aslant upon it, is the center of a complicated system of rills, some of which can be traced with our five-inch glass.
We next take upLunar Chart No. 2, and pay a telescopic visit to the southwestern quarter of the lunar world. TheMare Tranquilitatismerges through straits into two southern extensions, theMare Fecunditatisand theMare Nectaris. The great ring mountains or ringed plains, Langrenus, Vendelinus, Petavius, and Furnerius, all lying significantly along the same lunar meridian, have already been noticed. Their linear arrangement and isolated position recall the row of huge volcanic peaks that runs parallel with the shore of the Pacific Ocean in Oregon and Washington—Mount Jefferson, Mount Hood, Mount St. Helen's, Mount Tacoma—but these terrestrial volcanoes, except in elevation, are mere pins' heads in the comparison.
In the eastern part of theMare Fecunditatislies a pair of relatively small craters named Messier, which possess particular interest because it has been suspected, though not proved, that a change of form has occurred in one or other of the pair. Mädler, in the first half of the nineteenth century, represented the two craters as exactly alike in all respects. In 1855 Webb discovered that they are not alike in shape, and that the easternmost one is the larger, and every observer easily sees that Webb's description is correct. Messier is also remarkable for the light streak, often said to resemble a comet's tail, which extends from the larger crater eastward to the shore of theMare Fecunditatis.
Goclenius and Guttemberg, on the highland between theMare Fecunditatisand theMare Nectaris, are intersected and surrounded by clefts, besides being remarkable for their broken and irregular though lofty walls. Guttemberg is forty-five miles and Goclenius twenty-eight miles in diameter. The short mountain range just east of Guttemberg, and bordering a part of theMare Nectarison the west, is called the Pyrenees.
TheMare Nectaris, though offering in its appearance no explanation of its toothsome name—perhaps it was regarded as the drinking cup of the Olympian gods—is one of the most singular of the dark lunar plains in its outlines. At the south it ends in a vast semicircular bay, sixty miles across, which is evidently a half-submerged mountain ring. But submerged by what? Not water, but perhaps a sea of lava which has now solidified and forms the floor of theMare Nectaris. The name of this singular formation is Fracastorius. Elger has an interesting remark about it.
"On the higher portion of the interior, near the center," he says, "is a curious object consisting apparently offour light spots, arranged in a square, with a craterlet in the middle, all of which undergo notable changes of aspect under different phases."
Other writers also call attention to the fine markings, minute craterlets, and apparently changeable spots on the floor of Fracastorius.
We go now to the eastern side of theMare Nectaris, where we find one of the most stupendous formations in the lunar world, the great mountain ring of Theophilus, noticeably regular in outline and perfect in the completeness of its lofty wall. The circular interior, which contains in the center a group of mountains, one of whose peaks is 6,000 feet high, sinks 10,000 feet below the general level of the moon outside the wall! One of the peaks on the western edge towers more than 18,000 feet above the floor within, while several other peaks attain elevations of 15,000 to 16,000 feet. The diameter of the immense ring, from crest to crest of the wall, is sixty-four miles. Theophilus is especially wonderful on the fifth and sixth days of the moon, when the sun climbs its shining pinnacles and slowly discloses the tremendous chasm that lies within its circles of terrible precipices.
On the southeast Theophilus is connected by extensions of its walls with a shattered ring of vast extent called Cyrillus; and south from Cyrillus, and connected with the same system of broken walls, lies the still larger ring named Catharina, whose half-ruined walls and numerous crater pits present a fascinating spectacle as the shadows retreat before the sunrise advancing across them. These three—Theophilus, Cyrillus, and Catharina—constitute a scene of surpassing magnificence, a glimpse of wonders in another world sufficient to satisfy the most riotous imagination.
South of theMare Nectaristhe huge ring mountain ofPiccolomini attracts attention, its massive walls surrounding a floor nearly sixty miles across, and rising in some places to an altitude of nearly 15,000 feet. It should be understood that wherever the height of the mountain wall of such a ring is mentioned, the reference level is that of the interior plain or floor. The elevation, reckoned from the outer side, is always very much less.
The entire region south and east of Theophilus and its great neighbors is marvelously rough and broken. Approaching the center of the moon, we find a system of ringed plains even greater in area than any of those we have yet seen. Hipparchus is nearly a hundred miles long from north to south, and nearly ninety miles broad from east to west. But its walls have been destroyed to such an extent that, after all, it yields in grandeur to a formation like Theophilus.
Albategnius is sixty-five miles across, with peaks from 10,000 to 15,000 feet in height. Sacrobosco is a confused mass of broken and distorted walls. Aliacensis is remarkable for having a peak on the eastern side of its wall which is more than 16,000 feet high. Werner, forty-five miles in diameter, is interesting because under its northeastern wall Mädler, some seventy years ago, saw a light spot of astonishing brightness, unmatched in that respect by anything on the moon except the peak of Aristarchus, which we shall see later. This spot seems afterward to have lost brilliance, and the startling suggestion has been made that its original brightness might have been due to its then recent deposit from a little crater that lies in the midst of it. Walter is of gigantic dimensions, about one hundred miles in diameter. Unlike the majority of the ringed plains, it departs widely from a circle. Stöfler is yet larger than Walter; but most interesting of all thesegigantic formations is Maurolycus, whose diameter exceeds one hundred and fifty miles, and which has walls 13,000 or 14,000 feet high. Yet, astonishing though it may seem, this vast and complicated mass of mountain walls, craters, and peaks, is virtually unseen at full moon, owing to the perpendicularity of the sunlight, which prevents the casting of shadows.
We shall next suppose that another period of about seven days has elapsed, the moon in the meantime reaching its full phase. We refer for guidance toLunar Chart No. 3. The peculiarity of the northeastern quadrant which immediately strikes the eye is the prevalence of the broad plains calledMaria, or "seas." The northern and central parts are occupied by theMare Imbrium, the "Sea of Showers" or of "Rains," with its dark bay theSinus Æstuum, while the eastern half is covered by the vastOceanus Procellarum, the "Ocean of Storms" or of "Tempests."
Toward the north a conspicuous oval, remarkably dark in hue, immediately attracts our attention. It is the celebrated ringed plain of Plato, about sixty miles in diameter and surrounded by a saw-edged rampart, some of whose pinnacles are 6,000 or 7,000 feet high. Plato is a favorite subject for study by selenographers because of the changes of color which its broad, flat floor undergoes as the sun rises upon it, and also because of the existence of enigmatical spots and streaks whose visibility changes. South of Plato, in theMare Imbrium, rises a precipitous, isolated peak called Pico, 8,000 feet in height. Its resemblance in situation to the conical mountain Pico in the Azores strikes the observer.
Eastward of Plato a line of highlands, separating theMare Imbriumfrom theMare Frigoris,carries the eye to the beautiful semicircularSinus Iridum, or "Bay of Rainbows."The northwestern extremity of this remarkable bay is guarded by a steep and lofty promontory called Cape Laplace, while the southeastern extremity also has its towering guardian, Cape Heraclides. The latter is interesting for showing, between nine and ten days after full moon, a singularly perfect profile of a woman's face looking out across theMare Imbrium. The winding lines, like submerged ridges, delicately marking the floor of theSinus Iridumand that of theMarebeyond, are beautifultelescopic objects. The "bay" is about one hundred and thirty-five miles long by eighty-four broad.
Lunar Chart No. 3, Northeast Quarter.
TheMare Imbrium, covering 340,000 square miles, is sparingly dotted over with craters. All of the more conspicuous of them are indicated in thechart. The smaller ones, like Caroline Herschel, Helicon, Leverrier, Délisle, etc., vary from eight to twelve miles in diameter. Lambert is seventeen miles in diameter, and Euler nineteen, while Timocharis is twenty-three miles broad and 7,000 feet deep below its walls, which rise only 3,000 feet above the surface of theMare.
Toward the eastern border of the sea, south of the Harbinger Mountains, we find a most remarkable object, the mountain ring, or crater plain, called Aristarchus. This ring is not quite thirty miles in diameter, but there is nothing on the moon that can compare with it in dazzling brilliance. The central peak, 1,200 or 1,300 feet high, gleams like a mountain of crusted snow, or as if it were composed of a mass of fresh-broken white metal, or of compacted crystals. Part of the inner slope of the east wall is equally brilliant. In fact, so much light is poured out of the circumvallation that the eye is partially blinded, and unable distinctly to see the details of the interior. No satisfactory explanation of the extraordinary reflecting power of Aristarchus has ever been offered. Its neighbor toward the east, Herodotus, is somewhat smaller and not remarkably bright, but it derives great interest from the fact that out of a breach in its northern wall issues a vast cleft, or chasm, which winds away for nearly a hundred miles across the floor of theMare, making an abrupt turn when it reaches the foot of the Harbinger Mountains.
The comparatively small crater, Lichtenberg, near the northeastern limb of the moon, is interesting because Mädlerused to see in its neighborhood a pale-red tint which has not been noticed since his day.
Returning to the western side of the quadrant represented inLunar Chart No. 3, we see the broad and beautifully regular ringed plain of Archimedes, fifty miles in diameter and 4,000 feet deep.
A number of clefts extend between the mountainous neighborhood of Archimedes and the feet of the gigantic Apennine Mountains on the southwest. The little double crater, Beer, between Archimedes and Timocharis, is very bright.
The Apennines extend about four hundred and eighty miles in a northwesterly and southeasterly direction. One of their peaks near the southern end of the range, Mount Huygens, is at least 18,000 feet high, and the black silhouettes of their sharp-pointed shadows thrown upon the smooth floor of theMare Imbriumabout the time of first quarter present a spectacle as beautiful as it is unique. The Apennines end at the southeast in the ring mountain, Eratosthenes, thirty-eight miles across and very deep, one of its encircling chain of peaks rising 16,000 feet above the floor, and about half that height above the level of theMare Imbrium. The shadows cast by Eratosthenes at sunrise are magnificent.
And now we come to one of the supreme spectacles of the moon, the immense ring or crater mountain Copernicus. This is generally regarded as the grandest object that the telescope reveals on the earth's satellite. It is about fifty-six miles across, and its interior falls to a depth of 8,000 feet below theMare Imbrium. Its broad wall, composed of circle within circle of ridges, terraces, and precipices, rises on the east about 12,000 feet above the floor. On the inner side the slopes are very steep, cliff falling below cliff, until the bottom of the fearful abyss isattained. To descend those precipices and reach the depressed floor of Copernicus would be a memorable feat for a mountaineer. In the center of the floor rises a complicated mountain mass about 2,400 feet high. All around Copernicus the surface of the moon is dotted with countless little crater pits, and splashed with whitish streaks. Northward lie the Carpathian Mountains, terminating on the east in Tobias Mayer, a ring mountain more than twenty miles across. The mountain ring Kepler, which is also the center of a great system of whitish streaks and splashes, is twenty-two miles in diameter, and notably brilliant.
Finally, we turn to the southeastern quadrant of the moon, represented inLunar Chart No. 4. The broad, dark expanse extending from the north is theMare Nubiumon the west and theOceanus Procellarumon the east. Toward the southeast appears the notably dark, rounded area of theMare Humoruminclosed by highlands and rings. We begin with the range of vast inclosures running southward near the central meridian, and starting with Ptolemæus, a walled plain one hundred and fifteen miles in its greatest diameter and covering an area considerably exceeding that of the State of Massachusetts. Its neighbor toward the south, Alphonsus, is eighty-three miles across. Next comes Arzachel, more than sixty-five miles in diameter. Thebit, more than thirty miles across, is very deep. East of Thebit lies the celebrated "lunar railroad," a straight, isolated wall about five hundred feet high and sixty-five miles long, dividing at its southern end into a number of curious branches, forming the buttresses of a low mountain. Purbach is sixty miles broad, and south of that comes a wonderful region where the ring mountains Hell, Ball, Lexell, and others, more or less connected with walls, inclose an area even larger thanPtolemæus, but which, not being so distinctly bordered as some of the other inclosed plains, bears no distinctive name.