But before we discuss this matter, it will be well to state what is known beyond peradventure about the moon.

Its mean distance from the earth is usually called, for the sake of a round number, 240,000 miles, but more accurately stated it is 238,840 miles. This is variable to the extent of more than 31,000 miles, on account of the eccentricity of its...o...b..t, and the eccentricity itself is variable, in consequence of the perturbing attractions of the earth and the sun, so that the distance of the moon from the earth is continually changing. It may be as far away as 253,000 miles and as near as 221,600 miles.

Although the orbit of the moon is generally represented, for convenience, as an ellipse about the earth, it is, in reality, a varying curve, having the sun for its real focus, and always concave toward the latter. This is a fact that can be more readily explained with the aid of a diagram.

[Ill.u.s.tration: THE MOON'S PATH WITH RESPECT TO THE SUN AND THE EARTH.]

In the accompanying cut, when the earth is at _A_ the moon is between it and the sun, in the phase called new moon. At this point the earth's...o...b..t about the sun is more curved than the moon's, and the earth is moving relatively faster than the moon, so that when it arrives at _B_ it is ahead of the moon, and we see the latter to the right of the earth, in the phase called first quarter. The earth being at this time ahead of the moon, the effect of its attraction, combined with that of the sun, tends to hasten the moon onward in its...o...b..t about the sun, and the moon begins to travel more swiftly, until it overtakes the earth at _C_, and appears on the side opposite the sun, in the phase called full moon. At this point the moon's...o...b..t about the sun has a shorter radius of curvature than the earth's. In traveling from _C_ to _D_ the moon still moves more rapidly than the earth, and, having pa.s.sed it, appears at _D_ to the left of the earth, in the phase called third quarter. Now, the earth being behind the moon, the effect of its attraction combined with the sun's tends to r.e.t.a.r.d the moon in its...o...b..t about the sun, with the result that the moon moves again less rapidly than the earth, and the latter overtakes it, so that, upon reaching _E_, the two are once more in the same relative positions that they occupied at _A_, and it is again new moon. Thus it will be seen that, although the real orbit of the moon has the sun for its center of revolution, nevertheless, in consequence of the attraction of the earth, combined in varying directions with that of the sun, the moon, once every month, makes a complete circuit of our globe.

The above explanation should not be taken for a mathematical demonstration of the moon's motion, but simply for a graphical ill.u.s.tration of how the moon appears to revolve about the earth while really obeying the sun's attraction as completely as the earth does.

There is no other planet that has a moon relatively as large as ours.

The moon's diameter is 2,163 miles. Its volume, compared with the earth's, is in the ratio of 1 to 49, and its density is about six tenths of the earth's. This makes its ma.s.s to that of our globe about as 1 to 81. In other words, it would take eighty-one moons to counterbalance the earth. Before speaking of the force of gravity on the moon we will examine the character of the lunar surface.

To the naked eye the moon's face appears variegated with dusky patches, while a few points of superior brilliance shine amid the brighter portions, especially in the southern and eastern quarters, where immense craters like Tycho and Copernicus are visible to a keen eye, gleaming like polished b.u.t.tons. With a telescope, even of moderate power, the surface of the moon presents a scene of astonishing complexity, in which strangeness, beauty, and grandeur are all combined. The half of the moon turned earthward contains an area of 7,300,000 square miles, a little greater than the area of South America and a little less than that of North America. Of these 7,300,000 square miles, about 2,900,000 square miles are occupied by the gray, or dusky, expanses, called in lunar geography, or selenography, _maria_--i.e., "seas." Whatever they may once have been, they are not now seas, but dry plains, bordered in many places by precipitous cliffs and mountains, varied in level by low ridges and regions of depression, intersected occasionally by immense cracks, having the width and depth of our mightiest river canons, and sprinkled with bright points and crater pits. The remaining 4,400,000 square miles are mainly occupied by mountains of the most extraordinary character. Owing partly to roughness of the surface and partly to more brilliant reflective power, the mountainous regions of the moon appear bright in comparison with the dull-colored plains.

Some of the lunar mountains lie in long, ma.s.sive chains, with towering peaks, profound gorges, narrow valleys, vast amphitheaters, and beetling precipices. Looking at them with a powerful telescope, the observer might well fancy himself to be gazing down from an immense height into the heart of the untraveled Himalayas. But these, imposing though they are, do not const.i.tute the most wonderful feature of the mountain scenery of the moon.

Appearing sometimes on the sh.o.r.es of the "seas," sometimes in the midst of broad plains, sometimes along the course of mountain chains, and sometimes in magnificent rows, following for hundreds of miles the meridians of the lunar globe, are tremendous, mountain-walled, circular chasms, called craters. Frequently they have in the middle of their depressed interior floors a peak, or a cl.u.s.ter of peaks. Their inner and outer walls are seamed with ridges, and what look like gigantic streams of frozen lava surround them. The resemblance that they bear to the craters of volcanoes is, at first sight, so striking that probably n.o.body would ever have thought of questioning the truth of the statement that they are such craters but for their incredible magnitude. Many of them exceed fifty miles in diameter, and some of them sink two, three, four, and more miles below the loftiest points upon their walls! There is a chasm, 140 miles long and 70 broad, named Newton, situated about 200 miles from the south pole of the moon, whose floor lies 24,000 feet below the summit of a peak that towers just above it on the east! This abyss is so profound that the shadows of its enclosing precipices never entirely quit it, and the larger part of its bottom is buried in endless night.

One can not but shudder at the thought of standing on the broken walls of Newton, and gazing down into a cavity of such stupendous depth that if Chimborazo were thrown into it, the head of the mighty Andean peak would be thousands of feet beneath the observer.

A different example of the crater mountains of the moon is the celebrated Tycho, situated in lat.i.tude about 43 south, corresponding with the lat.i.tude of southern New Zealand on the earth. Tycho is nearly circular and a little more than 54 miles across. The highest point on its wall is about 17,000 feet above the interior. In the middle of its floor is a mountain 5,000 or 6,000 feet high. Tycho is especially remarkable for the vast system of whitish streaks, or rays, which starting from its outer walls, spread in all directions over the face of the moon, many of them, running, without deviation, hundreds of miles across mountains, craters, and plains. These rays are among the greatest of lunar mysteries, and we shall have more to say of them.

[Ill.u.s.tration: THE LUNAR ALPS, APENNINES, AND CAUCASUS.

Photographed with the Lick Telescope.]

Copernicus, a crater mountain situated about 10 north of the equator, in the eastern hemisphere of the moon, is another wonderful object, 56 miles in diameter, a polygon appearing, when not intently studied, as a circle, 11,000 or 12,000 feet deep, and having a group of relatively low peaks in the center of its floor. Around Copernicus an extensive area of the moon's surface is whitened with something resembling the rays of Tycho, but more irregular in appearance. Copernicus lies within the edge of the great plain named the _Ocea.n.u.s Procellarum_, or "Ocean of Storms," and farther east, in the midst of the "ocean," is a smaller crater mountain, named Kepler, which is also enveloped by a whitish area, covering the lunar surface as if it were the result of extensive outflows of light-colored lava.

In one important particular the crater mountains of the moon differ from terrestrial volcanoes. This difference is clearly described by Nasmyth and Carpenter in their book on The Moon:

"While the terrestrial crater is generally a hollow on a mountain top, with its flat bottom high above the level of the surrounding country, those upon the moon have their lowest points depressed more or less deeply below the general surface of the moon, the external height being frequently only a half or one third of the internal depth."

It has been suggested that these gigantic rings are only "basal wrecks"

of volcanic mountains, whose conical summits have been blown away, leaving vast crateriform hollows where the mighty peaks once stood; but the better opinion seems to be that which a.s.sumes that the rings were formed by volcanic action very much as we now see them. If such a crater as Copernicus or the still larger one named Theophilus, which is situated in the western hemisphere of the moon, on the sh.o.r.e of the "Sea of Nectar," ever had a conical mountain rising from its rim, the height attained by the peak, if the average slope were about 30, would have been truly stupendous--fifteen or eighteen miles!

There is a kind of ring mountains, found in many places on the moon, whose forms and surroundings do not, as the craters heretofore described do, suggest at first sight a volcanic origin. These are rather level plains of an oval or circular outline, enclosed by a wall of mountains.

The finest example is, perhaps, the dark-gray Plato, situated in 50 of north lat.i.tude, near an immense mountain uplift named the Lunar Alps, and on the northern sh.o.r.e of the _Mare Imbrium_, or "Sea of Showers."

Plato appears as an oval plain, very smooth and level, about 60 miles in length, and completely surrounded by mountains, quite precipitous on the inner side, and rising in their highest peaks to an elevation of 6,000 to 7,000 feet. Enclosed plains, bearing more or less resemblance to Plato--sometimes smooth within, and sometimes broken with small peaks and craters or hilly ridges--are to be found scattered over almost all parts of the moon. If our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle. It has been thought that they may once have been filled with water, forming lakes that recall the curious Crater Lake of Oregon.

[Ill.u.s.tration: THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN HEMISPHERES). Photographed with the Lick Telescope.]

It is not my intention to give a complete description of the various lunar features, and I mention but one other--the "clefts" or "rills,"

which are to be seen running across the surface like cracks. One of the most remarkable of these is found in the _Ocea.n.u.s Procellarum_, near the crater-mountain Aristarchus, which is famed for the intense brilliance of its central peak, whose reflective power is so great that it was once supposed to be aflame with volcanic fire. The cleft, or crack, in question is very erratic in its course, and many miles in length, and it terminates in a ringed plain named Herodotus not far east of Aristarchus, breaking through the wall of the plain and entering the interior. Many other similar chasms or canons exist on the moon, some crossing plains, some cleaving mountain walls, and some forming a network of intersecting clefts. Mr. Thomas Gwyn Elger has this to say on the subject of the lunar clefts:

"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 a.s.sertions 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."

Mr. Elger then proceeds to describe his discovery in 1883, in the ring-plain Mersenius, of a cleft never noticed before, and which seems to have been of recent formation.[15]

[Footnote 15: The Moon, a Full Description and Map of its Princ.i.p.al Features, by Thomas Gwyn Elger, 1895.

Those who desire to read detailed descriptions of lunar scenery may consult, in addition to Mr. Elger's book, the following: The Moon, considered as a Planet, a World, and a Satellite, by James Nasmyth and James Carpenter, 1874; The Moon, and the Condition and Configurations of its Surface, by Edmund Neison, 1876. See also Annals of Harvard College Observatory, vol. x.x.xii, part ii, 1900, for observations made by Prof.

William H. Pickering at the Arequipa Observatory.]

We now return to the question of the force of lunar gravity. This we find to be only one sixth as great as gravity on the surface of the earth. It is by far the smallest force of gravity that we have found anywhere except on the asteroids. Employing the same method of comparison that was made in the case of Mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of Adam is on the earth.

Whether this furnishes a sound reason for a.s.suming that the lunar inhabitants, if any exist or have ever existed, should be preposterous giants is questionable; yet such an a.s.sumption receives a certain degree of support from the observed fact that the natural features of the moon are framed on an exaggerated scale as compared with the earth's. We have just observed that the moon is characterized by vast mountain rings, attaining in many cases a diameter exceeding fifty miles. If these are volcanic craters, it is evident, at a glance, that the mightiest volcanoes of the earth fall into insignificance beside them. Now, the slight force of gravity on the moon has been appealed to as a reason why volcanic explosions on the lunar globe should produce incomparably greater effects than upon the earth, where the ejected materials are so much heavier. The same force that would throw a volcanic bomb a mile high on the earth could throw it six miles high on the moon. The giant cannon that we have placed in one of our coast forts, which is said to be able to hurl a projectile to a distance of fifteen miles, could send the same projectile ninety miles on the moon. An athlete who can clear a horizontal bar at a height of six feet on the earth could clear the same bar at a height of thirty-six feet on the moon. In other words, he could jump over a house, unless, indeed, the lunarians really are giants, and live in houses proportioned to their own dimensions and to the size of their mountains. In that case, our athlete would have to content himself with jumping over a lunarian, whose head he could just clear--with the hat off.

These things are not only amusing, but important. There can be no question that the force of gravity on the moon actually is as slight as it has just been described. So, even without calling in imaginary inhabitants to lend it interest, the comparative inability of the moon to arrest bodies in motion becomes a fact of much significance. It has led to the theory that meteorites may have originally been shot out of the moon's great volcanoes, when those volcanoes were active, and may have circulated about the sun until various perturbations have brought them down upon the earth. A body shot radially from the surface of the moon would need to have a velocity of only about a mile and a half in a second in order to escape from the moon's control, and we can believe that a lunar volcano when in action could have imparted such a velocity, all the more readily because with modern gunpowders we have been able to give to projectiles a speed one half as great as that needed for liberation from lunar gravity.

Another consequence of the small gravitative power of the moon bears upon the all-important question of atmosphere. According to the theory of Dr. Johnstone Stoney, heretofore referred to, oxygen, nitrogen, and water vapor would all gradually escape from the moon, if originally placed upon it, because, by the kinetic theory, the maximum velocities of their molecules are greater than a mile and a half per second. The escape would not occur instantly, nor all at once, for it would be only the molecules at the upper surface of the atmosphere which were moving with their greatest velocity, and in a direction radial to the center of the moon, that would get away; but in the course of time this gradual leakage would result in the escape of all of those gases.[16]

[Footnote 16: The discovery of free hydrogen in the earth's atmosphere, by Professor Dewar, 1901, bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. Since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from s.p.a.ce or that it emanates from the earth's crust. In a similar way it may be a.s.sumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.]

After it had been found that, to ordinary tests, the moon offered no evidence of the possession of an atmosphere, and before Dr. Stoney's theory was broached, it was supposed by many that the moon had lost its original supply of air by absorption into its interior. The oxygen was supposed to have entered into combination with the cooling rocks and minerals, thus being withdrawn from the atmosphere, and the nitrogen was imagined to have disappeared also within the lunar crust. For it seems to have always been tacitly a.s.sumed that the phenomenon to be accounted for was not so much the _absence_ of a lunar atmosphere as its _disappearance_. But disappearance, of course, implies previous existence. In like manner it has always been a commonly accepted view that the moon probably once had enough water to form lakes and seas.

These, it has been calculated, could have been absorbed into the lunar globe as it cooled off. But Johnstone Stoney's theory offers another method by which they could have escaped, through evaporation and the gradual flight of the molecules into open s.p.a.ce. Possibly both methods have been in operation, a portion of the const.i.tuents of the former atmosphere and oceans having entered into chemical combinations in the lunar crust, and the remainder having vanished in consequence of the lack of sufficient gravitative force to retain them.

But why, it may be asked, should it be a.s.sumed that the moon ever had things which it does not now possess? Perhaps no entirely satisfactory reply can be made. Some observers have believed that they detected unmistakable indications of alluvial deposits on lunar plains, and of the existence of beaches on the sh.o.r.es of the "seas." Messrs. Loewy and Puiseux, of the Paris Observatory, whose photographs of the moon are perhaps the finest yet made, say on this subject:

"There exists, from the point of view of relief, a general similarity between the 'seas' of the moon and the plateaux which are covered to-day by terrestrial oceans. In these convex surfaces are more frequent than concave basins, thrown back usually toward the verge of the depressed s.p.a.ce. In the same way the 'seas' of the moon present, generally at the edges, rather p.r.o.nounced depressions. In one case, as in the other, we observe normal deformations of a shrinking globe shielded from the erosive action of rain, which tends, on the contrary, in all the abundantly watered parts of the earth to make the concave surfaces predominate. The explanation of this structure, such as is admitted at present by geologists, seems to us equally valid for the moon."[17]

[Footnote 17: Comptes Rendus, June 26, July 3, 1899.]

It might be urged that there is evidence of former volcanic activity on the moon of such a nature that explosions of steam must have played a part in the phenomena, and if there was steam, of course there was water.

But perhaps the most convincing argument tending to show that the moon once had a supply of water, of which some remnant may yet remain below the surface of the lunar globe, is based upon the probable similarity in composition of the earth and the moon. This similarity results almost equally whether we regard the moon as having originated in a ring of matter left off from the contracting ma.s.s that became the earth, or whether we accept the suggestion of Prof. G.H. Darwin, that the moon is the veritable offspring of the earth, brought into being by the a.s.sistance of the tidal influence of the sun. The latter hypothesis is the more picturesque of the two, and, at present, is probably the more generally favored. It depends upon the theory of tidal friction, which was referred to in Chapter III, as offering an explanation of the manner in which the rotation of the planet Mercury has been slowed down until its rotary period coincides with that of its revolution.

The gist of the hypothesis in question is that at a very early period in its history, when the earth was probably yet in a fluid condition, it rotated with extreme rapidity on its axis, and was, at the same time, greatly agitated by the tidal attraction of the sun, and finally huge ma.s.ses were detached from the earth which, ultimately uniting, became the moon.[18]

[Footnote 18: The Tides, by G.H. Darwin, chapter xvi.]

Born in this manner from the very substance of the earth, the moon would necessarily be composed, in the main, of the same elements as the globe on which we dwell, and is it conceivable that it should not have carried with it both air and water, or the gases from which they were to be formed? If the moon ever had enough of these prime requisites to enable it to support forms of life comparable with those of the earth, the disappearance of that life must have been a direct consequence of the gradual vanishing of the lunar air and water. The secular drying up of the oceans and wasting away of the atmosphere on our little neighbor world involved a vast, all-embracing tragedy, some of the earlier scenes of which, if theories be correct, are now reenacted on the half-desiccated planet Mars--a planet, by the way, which in size, ma.s.s, and ability to retain vital gases stands about half-way between the earth and the moon.

One of the most interesting facts about the moon is that its surface affords evidence of a cataclysm which has wiped out many, and perhaps nearly all, of the records of its earlier history, that were once written upon its face. Even on the earth there have been geological catastrophes destroying or burying the acc.u.mulated results of ages of undisturbed progress, but on the moon these effects have been transcendent. The story of the tremendous disaster that overtook the moon is partly written in its giant volcanoes. Although it may be true, as some maintain, that there is yet volcanic action going on upon the lunar surface, it is evident that such action must be insignificant in comparison with that which took place ages ago.

There is a spot in the western hemisphere of the moon, on the border of a placid bay or "sea," that I can never look at without a feeling of awe and almost of shrinking. There, within a s.p.a.ce about 250 miles in length by 100 in width, is an exhibition of the most terrifying effects of volcanic energy that the eye of man can anywhere behold. Three immense craters--Theophilus, 64 miles across and 3-1/2 miles deep; Cyrillus, 60 miles across and 15,000 feet deep; and Catharina, 70 miles across and from 8,000 to 16,000 feet deep--form an interlinked chain of mountain rings, ridges, precipices, chasms, and bottomless pits that take away one's breath.

But when the first impression of astonishment and dismay produced by this overwhelming spectacle has somewhat abated, the thoughtful observer will note that here the moon is telling him a part of her wonderful story, depicted in characters so plain that he needs no instruction in order to decipher their meaning. He will observe that this ruin was not all wrought at once or simultaneously. Theophilus, the crater-mountain at the northwestern end of the chain, whose bottom lies deepest of all, is the youngest of these giants, though the most imposing. For a distance of forty miles the lofty wall of Theophilus has piled itself upon the ruins of the wall of Cyrillus, and the circ.u.mference of the circle of its tremendous crater has been forcibly thrust within the original rim of the more ancient crater, which was thus rudely compelled to make room for its more vigorous rival and successor.

The observer will also notice that Catharina, the huge pit at the southeastern end of the chain, bears evidence of yet greater age. Its original walls, fragments of which still stand in broken grandeur, towering to a height of 16,000 feet, have, throughout the greater part of their circuit, been riddled by the outbreak of smaller craters, and torn asunder and thrown down on all sides.

In the vast enclosure that was originally the floor of the crater-mountain Catharina, several crater rings, only a third, a quarter, or a fifth as great in diameter, have broken forth, and these in turn have been partially destroyed, while in the interior of the oldest of them yet smaller craters, a nest of them, mere Etnas, Cotopaxis, and Kilaueas in magnitude, simple pinheads on the moon, have opened their tiny jaws in weak and ineffective expression of the waning energies of a still later epoch, which followed the truly heroic age of lunar vulcanicity.

This is only one example among hundreds, scattered all over the moon, which show how the surface of our satellite has suffered upheaval after upheaval. It is possible that some of the small craters, not included within the walls of the greater ones, may represent an early stage in the era of volcanic activity that wrecked the moon, but where larger and smaller are grouped together a certain progression can be seen, tending finally to extinction. The internal energies reached a maximum and then fell off in strength until they died out completely.

It can hardly be supposed that the life-bearing phase of lunar history--if there ever was one--could survive the outbreak of the volcanic cataclysm. North America, or Europe, if subjected to such an experience as the continental areas of the moon have pa.s.sed through, would be, in proportion, worse wrecked than the most fearfully battered steel victim of a modern sea fight, and one can readily understand that, in such circ.u.mstances, those now beautiful and populous continents would exhibit, from a distance, scarcely any token of their present topographical features, to say nothing of any relics of their occupation by living creatures.