The Story of the Heavens - Sir Robert Stawell Ball (ebook reader for laptop .TXT) 📗

observers. The narrow sharp line leading from the crater to the left is one of those remarkable "clefts" which traverse the moon in so many directions. Another may be seen further to the left. Above Plato are several detached mountains, the loftiest of which is Pico, about 8,000 feet in height. Its long and pointed shadow would at first sight lead one to suppose that it must be very steep; but Schmidt, who specially studied the inclinations of the lunar slopes, is of opinion that it cannot be nearly so steep as many of the Swiss mountains that are frequently ascended. As many as thirty minute craters have been carefully observed on the floor of Plato, and variations have been thought by Mr. W.H. Pickering to be perceptible.

9. _Eratosthenes._--This profound crater, upwards of 37 miles in diameter, lies at the end of the gigantic range of the Apennines. Not improbably, Eratosthenes once formed the volcanic vent for the stupendous forces that elevated the comparatively craterless peaks of these great mountains.

10. _Copernicus._--Of all the lunar craters this is one of the grandest and best known. The region to the west is dotted over with innumerable minute craterlets. It has a central many-peaked mountain about 2,400 feet in height. There is good reason to believe that the terracing shown in its interior is mainly due to the repeated alternate rise, partial congelation, and subsequent retreat of a vast sea of lava. At full moon the crater of Copernicus is seen to be surrounded by radiating streaks.

11. _Kepler._--Although the internal depth of this crater is scarcely less than 10,000 feet, it has but a very low surrounding wall, which is remarkable for being covered with the same glistening substance that also forms a system of bright rays not unlike those surrounding the last object.

12. _Aristarchus_ is the most brilliant of the lunar craters, being specially vivid with a low power in a large telescope. So bright is it, indeed, that it has often been seen on the dark side just after new moon, and has thus given rise to marvellous stories of active lunar volcanoes. To the south-east lies another smaller crater, Herodotus, north of which is a narrow, deep valley, nowhere more than 2-1/2 miles broad, which makes a remarkable zigzag. It is one of the largest of the lunar "clefts."

13. _Grimaldi_ calls for notice as the darkest object of its size in the moon. Under very exceptional circumstances it has been seen with the naked eye, and as its area has been estimated at nearly 14,000 square miles, it gives an idea of how little unaided vision can discern in the moon; it must, however, be added that we always see Grimaldi considerably foreshortened.

14. The great crater _Gassendi_ has been very frequently mapped on account of its elaborate system of "clefts." At its northern end it communicates with a smaller but much deeper crater, that is often filled with black shadow after the whole floor of Gassendi has been illuminated.

15. _Schickard_ is one of the largest walled plains on the moon, about 134 miles in breadth. Within its vast expanse Maedler detected 23 minor craters. With regard to this object Chacornac pointed out that, owing to the curvature of the surface of the moon, a spectator at the centre of the floor "would think himself in a boundless desert," because the surrounding wall, although in one place nearly 10,000 feet high, would lie entirely beneath his horizon.

16. Close to the foregoing is _Wargentin_. There can be little doubt that this is really a huge crater almost filled with congealed lava, as there is scarcely any fall towards the interior.

17. _Clavius._--Near the 60th parallel of lunar south latitude lies this enormous enclosure, the area of which is not less than 16,500 square miles. Both in its interior and on its walls are many peaks and secondary craters. The telescopic view of a sunrise upon the surface of Clavius is truly said by Maedler to be indescribably magnificent. One of the peaks rises to a height of 24,000 feet above the bottom of one of the included craters. Maedler even expressed the opinion that in this wild neighbourhood there are craters so profound that no ray of sunlight ever penetrated their lowest depths, while, as if in compensation, there are peaks whose summits enjoy a mean day almost twice as long as their night.

18. If the full moon be viewed through an opera-glass or any small hand-telescope, one crater is immediately seen to be conspicuous beyond all others, by reason of the brilliant rays or streaks that radiate from it. This is the majestic _Tycho_, 17,000 feet in depth and 50 miles in diameter (Plate X.). A peak 6,000 feet in height rises in the centre of its floor, while a series of terraces diversity its interior slopes; but it is the mysterious bright rays that chiefly surprise us. When the sun rises on Tycho, these streaks are utterly invisible; indeed, the whole object is then so obscure that it requires a practised eye to recognise Tycho amidst its mountainous surroundings. But as soon as the sun has attained a height of about 30 deg. above its horizon, the rays emerge from their obscurity and gradually increase in brightness until the moon becomes full, when they are the most conspicuous objects on her surface. They vary in length, from a few hundred miles to two or, in one instance, nearly three thousand miles. They extend indifferently across vast plains, into the deepest craters, or over the loftiest elevations. We know of nothing on our earth to which they can be compared. As these rays are only seen about the time of full moon, their visibility obviously depends on the light falling more or less closely in the line of sight, quite regardless of the inclination of the surfaces, mountains or valleys, on which they appear. Each small portion of the surface of the streak must therefore be of a form which is symmetrical to the spectator from whatever point it is seen. The sphere alone appears to fulfil this condition, and Professor Copeland therefore suggests that the material constituting the surface of the streak must be made up of a large number of more or less completely spherical globules. The streaks must represent parts of the lunar surface either pitted with minute cavities of spherical figure, or strewn over with minute transparent spheres.[8]

Near the centre of the moon's disc is a fine range of ring plains fully open to our view under all illuminations. Of these, two may be mentioned--_Alphonsus_ (19), the floor of which is strangely characterised by two bright and several dark markings which cannot be explained by irregularities in the surface.--_Ptolemy_ (20). Besides several small enclosed craters, its floor is crossed by numerous low ridges, visible when the sun is rising or setting.

21, 22, 23.--When the moon is five or six days old this beautiful group of three craters will be favourably placed for observation. They are named _Catharina_, _Cyrillus_, and _Theophilus_. Catharina, the most southerly of the group, is more than 16,000 feet deep, and connected with Cyrillus by a wide valley; but between Cyrillus and Theophilus there is no such connection. Indeed, Cyrillus looks as if its huge surrounding ramparts, as high as Mont Blanc, had been completely finished before the volcanic forces commenced the formation of Theophilus, the rampart of which encroaches considerably on its older neighbour. Theophilus stands as a well-defined circular crater about 64 miles in diameter, with an internal depth of 14,000 to 18,000 feet, and a beautiful central group of mountains, one-third of that height, on its floor. Although Theophilus is the deepest crater we can see in the moon, it has suffered little or no deformation from secondary eruptions, while the floor and wall of Catharina show complete sequences of lesser craters of various sizes that have broken in upon and partly destroyed each other. In the spring of the year, when the moon is somewhat before the first quarter, this instructive group of extinct volcanoes can be seen to great advantage at a convenient hour in the evening.

24. _Petavius_ is remarkable not only for its great size, but also for the rare feature of having a double rampart. It is a beautiful object soon after new moon, or just after full moon, but disappears absolutely when the sun is more than 45 deg. above its horizon. The crater floor is remarkably convex, culminating in a central group of hills intersected by a deep cleft.

25. _Hyginus_ is a small crater near the centre of the moon's disc. One of the largest of the lunar chasms passes right through it, making an abrupt turn as it does so.

26. _Triesnecker._--This fine crater has been already described, but is again alluded to in order to draw attention to the elaborate system of chasms so conspicuously shown in Plate VII. That these chasms are depressions is abundantly evident by the shadows inside. Very often their margins are appreciably raised. They seem to be fractures in the moon's surface.

Of the various mountains that are occasionally seen as projections on the actual edge of the moon, those called after Leibnitz (_i_) seem to be the highest. Schmidt found the highest peak to be upwards of 41,900 feet above a neighbouring valley. In comparing these altitudes with those of mountains on our earth, we must for the latter add the depth of the sea to the height of the land. Reckoned in this way, our highest mountains are still higher than any we know of in the moon.

We must now discuss the important question as to the origin of these remarkable features on the surface of the moon. We shall admit at the outset that our evidence on this subject is only indirect. To establish by unimpeachable evidence the volcanic origin of the remarkable lunar craters, it would seem almost necessary that volcanic outbursts should have been witnessed on the moon, and that such outbursts should have been seen to result in the formation of the well-known ring, with or without the mountain rising from the centre. To say that nothing of the kind has ever been witnessed would be rather too emphatic a statement. On certain occasions careful observers have reported the occurrence of minute local changes on the moon. As we have already remarked, a crater named Linne, of dimensions respectable, no doubt, to a lunar inhabitant, but forming a very inconsiderable telescopic object, was thought to have undergone some change. On another occasion a minute crater was thought to have arisen near the well-known object named Hyginus. The mere enumeration of such instances gives real emphasis to the statement that there is at the present time no appreciable source of disturbance of the moon's surface. Even were these trifling cases of suspected change really established--and this is perhaps rather farther than many astronomers would be willing to go--they are still insignificant when compared with the mighty phenomena that gave rise to the host of great craters which cover so large a portion of the moon's surface.

We are led inevitably to the conclusion that our satellite must have once possessed much greater activity than it now displays. We can also give a reasonable, or, at all events, a plausible, explanation of the cessation of that activity in recent times. Let us glance at two other bodies of our system, the earth and the sun, and compare them with the moon. Of the three bodies, the sun is enormously the largest, while the moon is much less than the earth. We have also seen that though the sun must have a very high temperature, there can be no doubt that it is gradually parting with its heat. The surface of the earth,