EVERYTHING around us on this Earth we see is subject to one inevitable cycle of birth, growth, decay. Nothing that begins but comes at last to end. Not less is this true of the Earth as a whole and of each of its sister planets. Though our own lives are too brief even to mark the slow nearing to that eventual goal, the past history of the Earth written in its rocks and the present aspects of the several planets that circle similarly round the Sun alike assure us of the course of aging as certainly as if time, with all it brings about, passed in one long procession before our very eyes.

Death is a distressing thing to contemplate under any circumstances, and not less so to a philosopher when that of a whole world is concerned. To think that this fair globe with all it has brought forth must lapse in time to nothingness; that the generations of men shall cease to be, their very records obliterated, is something to strike a chill into the heart of the most callous and numb endeavor at its core. That Æons must roll away before that final day is to the mind of the far-seeing no consolation for the end. Not only that we shall pass, but that everything to show we ever were shall perish too, seems an extinction too overpowering for words.

But vain regret avails not to change the universe’s course. What is concerns us and what will be too. From facing it we cannot turn away. We may alleviate its poignancy by the thought that our interest is after all remote, affecting chiefly descendants we shall never know, and commend to ourselves the altruistic example so nobly set us by doctors of medicine who, on the demise of others at which—and possibly to which—they have themselves assisted, show a fortitude not easily surpassed, a fortitude extending even to their bills. If they can act thus unshaken at sight of their contemporaries, we should not fall behind them in heroism toward posterity.

Having in our last chapter run the gantlet of the geologists, we are in some sort fortified to face death—in a world—in this. The more so that we have some millenniums of respite before the execution of the decree. By the death of a planet we may designate that stage when all change on its surface, save disintegration, ceases. For then all we know as life in its manifold manifestations is at an end. To this it may come by many paths. For a planet, like a man, is exposed to death from a variety of untoward events.

Of these the one least likely to occur is death by accident. This, celestially speaking, is anything which may happen to the solar system from without, and is of the nature of an unforeseen catastrophe. Our Sun might, as we remarked, be run into. For so far as we know at present the stars are moving among themselves without any too careful regard for one another. The swarm may be circling a central Sun as AndrÉ states, but the individual stars behave more like the random particles of a gas with licensed freedom to collide; whereas we may liken the members of the solar system to molecules in the solid state held to a centre from which they can never greatly depart. Their motions thus afford a sense of security lacking in the universe at large.

Such an accident, a collision actual or virtual with another sun, would probably occur with some dark star; of which we sketched the ultimate results in our first chapter. The immediate ones would be of a most disastrous kind. For prefatory to the new birth would be the dissolution to make such resurrection possible. Destruction might come direct, or indirectly through the Sun. For though the Sun would be the tramp’s objective point, we might inadvertently find ourselves in the way. The choice would be purely academic; between being powdered, or deorbited and burnt up.

So remote is this contingency that it need cause us no immediate alarm, as I carefully pointed out. But so strong is the instinct of self-preservation and so pleasurable the sensation of spreading appalling news, that the press of America, and incidentally Europe, took fire, with the result, so I have been written, that by the time the pictured catastrophe reached the Pacific “it had assumed the dimensions of a first magnitude fact.”

This is the first way in which our world may come by its death. It is possible, but unlikely. For our Earth, long before that, is morally certain to perish otherwise.

The second mode is one, incident to the very constitution of our solar system. It follows as a direct outcome of that system’s mechanical evolution, and may be properly designated, therefore, as due to natural causes. It might be diagnosed as death by paralysis. For such it resembles in human beings, palsy of individual movement afflicting a planet instead of a man.

Tidal friction is the slow undermining cause; a force which is constantly at work in the action of every body in the universe upon every other. As we previously explained, the pull of one mass upon another is inevitably differential. Not only is the second drawn in its entirety toward the first, falling literally as it circles round, but the nearer parts are drawn more than the centre and the centre more than those farthest away. We may liken the result to a stretched rotating rubber ball, with, however, one important difference,—that each layer is more or less free to shear over the others. The bulge, solicited by the rotation to keep up, by the disturber to lag behind, is torn two ways, and the friction acts as a break upon the body’s rotation, tending first to turn it over if it be rotating backward and then to slow it down till the body presents the same face in perpetuity to its primary. The tides are the bulge, not simply those superficial ones which we observe in our oceans, and know to be so strong, but substantial ones of the whole body which we must conceive thus as egg-shaped through the action that goes on—the long diameter of the egg pointing somewhat ahead of the line joining its centre to the distorting mass. All the bodies in the solar system are thus really egg-shaped, though the deformation is so slight as to escape detection observationally. The knowledge is an instance of how much more perceptive the brain is than the eye. For we are certain of the fact, and yet to see it with our present means is impossible, and may long remain so.

Two concomitant symptoms follow the friction of the tidal ansÆ: a shift of the plane in which the rotation takes place, and a loss of speed in the spin itself. The first tends to bring the plane of rotation down to the orbital plane, with rotation and revolution in the same sense. This effect takes place quicker than the other, and in consequence different stages may be noted in the creeping paralysis by which the body is finally overcome. Loss of seasons characterizes the first. For the coincidence of the two planes means invariability in the Sun’s declination throughout the year for a given latitude. This reduces all its days to one dead level in which summer and winter, spring and autumn, are always and everywhere the same. There is thus a return at the end of the planet’s career to an uneventful condition reminiscent of its start; a senility in planets comparable to second childhood in man.

In large planets this outgrowing of seasons occurs before they have any, while the planet is yet cloud-wrapped. Such planets know nothing of some attributes of youth, like those unfortunate men who never were boys; just as reversely the meteorites are boys that never grew up. For if the planet be large, the action of the tidal forces is proportionately more powerful; while on the other hand the self-aging of the planet is greatly prolonged, and thus it may come about that the former process outstrips the latter to the missing of seasons entirely. This is sure to be the case with Jupiter, as the equator has already got down to within 3° of the orbit, and threatens to be the case with Saturn. These bodies, then, when they shall have put off their swaddling clothes of cloud, will wake to climates without seasons; globes where conditions are always the same on the same belts of latitude, and on which these alter progressively from equator to pole. Variety other than diurnal is thus excluded from their surfaces and from their skies. For the Sun and stars will rise always the same, in punctual obedience only to the slowly shifting year.

The next stage of deprivation is the parting with the day. Although the day disappears, the result is too much day or too little, depending on where you choose to consider yourself upon the afflicted orb. For tidal friction proceeds to lengthen the twenty-four or other hours first to weeks, then months, then years, and at last to infinity; thus bringing the sun to a stock-still on the meridian, to flood one side of the world with perpetual day and plunge the other in eternal night.

Which of these two hemispheres would be the worse abode, is matter of personal predilection; dust or glacier, deserts both. Everlasting unshielded noon would cause a wind circulation from all points of the enlightened periphery to the centre, whence a funnel-shaped current would rise to overflow back into the antipodes, thence to return by the horizon again. As the night side would be several hundred degrees at least colder than the noon one, all the moisture would be evaporated on the sunlit hemisphere, to be carried round and deposited as ice on the other, there to stay. Life would be either toasted or frappÉ. A Sahara backed by polar regions would be the obverse and the reverse of the shield.

The reader may deem the picture a fancy sketch which possibly may not appeal to him. Nevertheless, it not only is possible, but one which has overtaken our nearest of neighbors. To this pass the Mater Amorum, Venus herself, has already been brought. She betrays it by the wrinkles which modern observation has revealed upon her face. Innocent critics, with a gallantry one would hardly have credited them,—which shows how one may wrong even the humblest of creatures,—have denied the existence of these marks of age, on the chivalrous a priori assumption that it could not possibly be true because never seen before. Their negation, in naÏve ignorance of the facts, partakes the logic of the gallant captain, who, when asked by a lady to guess her age, replied: “’Pon my word, I haven’t the slightest idea,” hastily adding, “But you don’t look it!” Less commendable than this conventional nescience, but unfortunately more to the point, is the evidence of prying scientific curiosity. Shrewdly divined as much as detected by Schiaparelli, made more certain by the crow’s-feet disclosed at Flagstaff, and corroborated by the testimony of the spectroscope there, her isochronism of rotation and revolution lies beyond a doubt. Attraction to her lord has conquered at last her who was the cynosure of all. Venus, in her old age, stares forever at the Sun, and we all know how ill an aging beauty can support a garish light.

Mercury has been brought to a like pass. This was evident even before the facts came out about Venus, for Venus, true to her instincts, shields herself with a veil of air which largely baffles man’s too curious gaze. Mercury, on the other hand, offers no objection to observation. When looked for at the proper time, his markings are quite distinct, dark, broken lines suggesting cracks. Schiaparelli, again, was the first to perceive the true state of the case, and his observations were independently confirmed and extended at Flagstaff in 1896. In so doing the latter disclosed a very interesting fact. It was evident that the markings held in general a definite fixed position upon the illuminated part of the disk, showing that the planet kept the same face always to the Sun. But systematic observation, continued day after day for weeks, disclosed a curious shift, which, though slight, was unmistakable. Upon thought the cause suggested itself, and on being subjected to calculation proved equal to such accounting. In this singular systematic sway stood revealed the libration in longitude caused by the eccentricity of the planet’s orbit.

Mercury revolves about the Sun in an ellipse more eccentric than that of any other principal planet. At times he is half as far off again from him as he is at others. When near, he travels faster than when far. For both reasons, nearness and speed, his angular revolution about the Sun varies greatly from point to point according to where he finds himself in his orbit. His rotation, however, is necessarily uniform. For even the Sun has no power at once to change the enormous moment of momentum of his axial spin. In consequence, at times his angular velocity of revolution gains on his rotation, at other times loses, both coming out together at the end of a complete Mercurial year. The result is a superb rhythmic oscillation, a true mercurial pendulum compensated by celestial laws to perfect isochronism of swing.

The outward sign of this shows in the movement of the markings. To observers in space like ourselves, the planet seems to sway his head as he travels along his orbit. For weeks he turns his face, as shown by the markings on it, more and more over to the left; then turns it back again as far over to the right. It is as if he were looking furtively around as he hastens over his planetary path.

Venus, of course, is equally subject to this law of distraction, but owing to the almost perfect circularity of her orbit she is less visibly affected. In fact, it is not possible to detect her lapse from a fixed regard to the Sun. At most it is no more than a glance out of the corner of her eyes—her slight deviation from perfect rectitude of demeanor. Knowledge of the laws governing such action alone permits us to recognize its occurrence.

Mercury and Venus are the only planets as yet that turn a constant face to their overruling lord. The reason for this appears when one goes into the matter analytically. The tidal force is not the direct pull of the Sun on a particle of the body, but the difference in the pulls upon a particle at the centre and one at the circumference. Being differential, it depends directly upon the radius of the distorted body and inversely upon the third power of its distance away. As the space through which the force acts is proportional to the force itself, the effect is as the squares of the quantities mentioned, or, inversely, as the sixth power of the distance and as the square of the body’s radius. The result thus proves greatest on the planets nearest to the Sun, and diminishes rapidly as we pass outward from him. If, then, the solar force had had time enough to produce its effects, it would be first in Mercury and then in Venus that it should be seen. And this is precisely where we observe it.

The Moon presents us a well-known case of such filial regard, resulting in permanent incompetency of action on its own account. It turns always the same face to us, following us about with the mute attention of a dog to its master. Here again the libration may be detected, for no dog but makes excursions on the road. This case differs from those of Mercury and Venus in that the body to which the regard is paid is not also the dispenser of light and warmth. In consequence, though the side of the Moon with which we are presented remains always the same, we do not always see it; the light creeping over it with the progress of the lunation, from new to full. On this account the worst that happens to our Moon in its old age is that its day becomes its month.

Our Moon is not peculiar in having its day and its month the same. On the contrary, it is now the rule with satellites thus to protract their days. So far as we can observe, all the large satellites of Jupiter turn the same face to him; those of Saturn pay him a like regard; while about those of Uranus and Neptune we are too far off to tell. Their direct respect for their primary, with only secondary recognition of the Sun, keeps them from the full consequences of their fatal yielding to attraction. It is bad enough to have the day half a month long, but worse to have one that never ends, or, still worse, perpetual night.

In our diagnosis of the cause of death in planets, we now pass from paralysis to heart failure. For so we may speak of the next affection which ends in their taking off, since it is due to want of circulation and lack of breath. It comes of a planet’s losing first its oceans and then its air.

To understand how this distressing condition comes about, we must consider one of the interesting scientific legacies of the nineteenth century to the twentieth: the kinetic theory of gases.

The kinetic theory of gases supposes them to be made up of minute particles all alike, which are perfectly elastic and are travelling hither and thither at great speeds in practically straight lines. In consequence, these are forever colliding among themselves, giving and taking velocities with bewildering rapidity, resulting in a state of confusion calculated to drive a computer mad. Somebody has likened a quiet bit of air to a boiler full of furious bees madly bent on getting out. The simile flatters the bees. To follow the vicissitudes of any one molecule in this hurly-burly would be out of the question; still more, it would seem, that of all of them at once. Yet no less Herculean a task confronts us. To find out about their motions, we are therefore driven to what is called the statistical method of inquiry,—which is simply a branch of the doctrine of probabilities. It is the method by which we learn how many people are going to catch cold in Boston next week when we know nothing about the people, or about colds, or about catching them. At first sight it might seem as if we could never discover anything in this hopelessly ignorant way, and as if we had almost better call in a doctor. But in the multitude of colds—not of counsellors—lies wisdom. So in other things not hygienic. As you cannot possibly divine, for instance, what each boy in town is going to do during the year, nor what is his make of mind, how can you say whether he will accidentally discharge a firearm and shoot his playmate or not! And yet if you take all the boys of Boston, you can predict to a nicety how many will thus let off a gun and “not know that it was loaded.”

In this only genuine method of prophecy, complete ignorance of all the actual facts, we are able without knowing anything whatever about each of the molecules to predicate a good deal about them all. To begin with, the pressure a gas exerts upon the sides of a vessel containing it must be the bombardment the sides receive from the little molecules; and the heating due this rain of blows, or the temperature to which the vessel is raised, must measure their energy of translation. On this supposition it is found that the laws of Avogadro and of Boyle are perfectly accounted for, besides many more properties of gases which the theory explains, and as nothing yet has been encountered seriously contradicting it, we may consider it as almost as surely correct as the theory of gravitation. To three great geniuses of the last century we owe this remarkable discovery—Clausius, Clerk Maxwell, and Boltzmann.

By determining the density of a gas at a given temperature and under a given pressure, we can find by the statistical method the average speed of its molecules. It depends on the most probable distribution of their energy. For hydrogen at the temperature of melting ice, and under atmospheric pressure, this speed proves to be a little over a mile a second—a speed, curiously enough, which is to that of light almost exactly as centimetres to miles. But some of the molecules are going at speeds much above the mean; fewer and fewer as the speed gets higher. Just how many there are for any assigned speed, we can calculate by the same ingenious application of unknown quantities.

These speeds have been found for a temperature of freezing, and as the speed varies as the square root of the absolute temperature, we might suppose that when an adventurous or lucky molecule arrived at practically the limit of the atmosphere, where the cold is intense, it would become numbly sluggish. But let us consider this. When we enclose a gas in a cooler vessel, the molecules bombard the sides more than they are bombarded back. In consequence, they lose energy; as we say, are cooled. But in free air if a molecule be fortunate enough to elude its neighbors, there is nothing to take away its motion but the ether through radiation, and this is a very slow process. Thus the escaping fugitive must arrive at the confines of the air with the speed it had at its last encounter. We reach, then, this result: In space there is no such thing as temperature; temperature being simply the aggregate effect of molecular temperament. The reason we should consider it uncommonly cold up there is that fewer molecules would strike us. Quantity, therefore, in our estimation replaces quality,—a possible substitution which also accounts for some reputations, literary or otherwise. The only forces which could affect this lonely molecule would be the heating by the Sun, the repellent force of light, and gravity.

Now the speed which gravity on the Earth can control is 6.9 miles a second. It can impart this to a body falling freely to it from infinite space, and can therefore annul it on the way up, and no more. If, then, any of the molecules reach the outer boundary of the air going at more than this speed, they will pass beyond the Earth’s power to restrain. They will become little rovers in space on their own account, and dart off on interstellar travels of their own. This extension of the kinetic theory and of the consequent voyages of the molecules is due to Dr. Johnstone Stoney, who has since, humorously enough, tried to stop the very balls he set rolling. First thoughts are usually the best, after all.

As among the molecules some are already travelling at speeds in excess of this critical velocity, molecules must constantly be attaining to this emancipation, and thus be leaving the Earth for good. In consequence there is a steady drain upon its gaseous covering. Furthermore, as we know from comets’ tails, the repellent power of the light-waves, what we may call the levity of light, much exceeds upon such volatile vagrants the heat excitement or even the gravity of the Sun, so that we arrive at this interesting conclusion—their escape is best effected under cover of the night.

Again, the heavier the gas, the less its molecular speed at a given temperature, because its kinetic energy which measures that temperature is one-half the molecule’s mass into the square of its speed. Thus their ponderosity prevents as many of them from following their more agile cousins of a different constitution. So that the lighter gases are sooner gone. Water-vapor leaves before oxygen. Nor is there any escape from this escape of the gases. It may take excessively long, but go they must until a solitary individual who happens to have had the wrong end of the last collision is alone left hopelessly behind.

Another factor also is concerned. The smaller the planet, the lower the utmost velocity it can control, and the quicker, therefore, it must lose its atmosphere. For a greater number of molecules must at every instant reach the releasing speed. Thus those bodies that are little shall, perforce, have less to cover themselves withal.

Now this inevitable depletion of their atmospheric envelopes, the aspects of the various planets strikingly attest. They do so in most exemplary fashion, according to law. The larger, the major planets, as we have already remarked, have a perfect plethora of atmosphere, more than we at least know what to do with in the way of cataloguing yet. The medium-sized, like our own Earth, have a very comfortable amount; Mars, an uncomfortable one, as we consider, and the smallest none at all. All the smaller bodies of our system are thus painfully deprived so far as we can discover. We are certain of it in the case of our Moon and Mercury, the only ones we can see well enough to be sure. In further evidence it has been shown at the Yerkes and at Flagstaff that no perceptible effect of air betrays itself in the spectroscopic imprint of the rings of Saturn, those tiny satellites of his, and very recently a spectrogram of Ganymede, Jupiter’s third moon, made at Flagstaff for the purpose by Mr. E. C. Slipher has proved equally void of atmospheric hint.

With the loss of water and of air, all possibility of development departs. Not only must every organism die, but even the inorganic can no longer change its state. In the extinction thus not only of inhabitants but of the habitat that made them possible, occurs a curious inversion of the order we are familiar with in the life history of organisms. In planets it is the grandchildren that die first, then the children, and lastly their surviving parent. And this is not accidental, but inevitably consequent upon their respective origins. For the offspring, as we may spell it with a hyphen, of any cosmic mass is of necessity smaller than that from which it issued. Being smaller, it must age quicker. In the natural order of events, then, its end must be reached first.

Such has been the course taken, or still taking, by the bodies of our solar family. The latest generation has already succumbed to this ebbing of vitality with time. Every one of the satellites of the planets—those of Neptune, Uranus, Saturn, Jupiter, and our own Moon—is practically dead; born so the smaller which never were alive. Our own Moon carries its decrepitude on its face. To all intents and purposes its life is past; and that it had at one time a very fiery existence, the great lunar craters amply testify. It is now, for all its flooding with radiance our winter nights, the lifeless statue of its former self.

The same inevitable end, in default of others, is now overtaking the planetary group. Its approach is stamped on the face of Mars. There we see a world dying of exhaustion. The signs of it are legible in the markings we descry. How long before its work is done, we ignore. But that it is a matter of time only, our study of the laws of the inexorable lead us to conclude. Mars has been spared the fate of Mercury and Venus to perish by this other form of planetary death.

Last in our enumeration of the causes by which the end of a world may be brought about, because the last to occur in order of time, is the extinction of the Sun itself. Certain to come and conclude the solar system’s history as the abode of life, if all the others should by any chance fail to precede it, it fittingly forms the climax, grand in its very quietude, of all that went before.

By the same physical laws that caused our Earth once to be hot, the Sun shines to-day. Only its greater size has given it a life and a brilliancy denied to smaller orbs. The falling together of the scattered particles of which it is composed, caused, and still is causing, the dazzling splendor it emits. And so long as it remains gaseous, its temperature must increase, in spite of its lavish expenditure of heat, as Homer Lane discovered forty years ago.

But the Sun’s store of heat, immense as it is to-day, and continued as it is bound to be for untold Æons by means of contraction of its globe upon itself, and possibly by other causes, must some day give out. From its present gaseous condition it must gradually but eventually contract to a solid one, and this in turn radiate all its heat into space. Slowly its lustre must dim as it becomes incapable of replenishing its supply of motive power by further shrinkage in size. Fitfully, probably, like Mira Ceti to-day, it will show temporary bursts of splendor as if striving to regain the brightness it had lost, only to sink after each effort into more and more impotent senility. At last some day must come, if we may talk of days at all when the great event occurs when all days shall be blotted out, that the last flicker shall grow extinct in the orb that for so long has made the hearth of the whole system. For, presciently enough, the Latin word focus means hearth, and the body which includes within it the focus about which all the planets revolve also constitutes the hearth from which they all are lighted and warmed.

When this ultimate moment arrives and the last spark of solar energy goes out, the Sun will have reverted once more to what it was when the cataclysm of the foretime stranger awoke it into activity. It will again be the dark body it was when our peering into the past first descries it down the far vista of unrecorded time. Ghostlike it will travel through space, unknown, unheralded, till another collision shall cause it to take a place again among the bright company of heaven. Thus, in our account of the career of a solar system, we began by seeing with the mind’s eye a dark body travelling incognito in space, and a dark body we find ourselves again contemplating at the end.

In this kaleidoscopic biograph of the solar system’s life, each picture dissolves into its successor by the falling together of its parts to fresh adjustments of stability, as in that instrument of pleasure which so witched our childish wonder in early youth. Just as when a combination had proved so pretty, once gone, to our sorrow no turning of the handle could ever bring it back, so in the march of worlds no retrace is possible of steps that once are past. Inexorable permutations lead from one state to the next, till the last of all be reached.

Yet, unlike our childhood’s toy, reasoning can conjure up beside the present picture far vistas of what preceded it and of what is yet to come. Hidden from thought only by the distraction of the day, as the universe to sight lies hid by the day’s overpowering glare, both come out on its withdrawal till we wonder we never gazed before. Our own surroundings shut out the glories that lie beyond. Our veil of atmosphere cloaks them from our view. But wait, as an astronomer, till the Sun sinks behind the hills and his gorgeous gold of parting fades to amber amid the tender tapestry of trees. The very air takes on a meaning which the flood of day had swamped. Seen itself, no longer imperfectly seen through, it wakes to semi-sentient existence, a spirit come to life aloft to shield us from the too immediate vacancy of space. The perfumes of the soil, the trees, the flowers, steal out to it, as the twilight glow itself exhales to heaven. In the hushed quiet of the gloaming Earth holds her breath, prescient of a revelation to come.

Then as the half-light deepens, the universe appears. One by one the company of heaven stand forth to human sight. Venus first in all her glory brightens amid the dying splendor of the west, growing in lustre as her setting fades. From mid-heaven the Moon lets fall a sheen of silvery light, the ghostly mantle of her ghostlike self, over the silent Earth. Eastward Jupiter, like some great lantern of the system’s central sweep, swings upward from the twilight bow to take possession of the night. Beyond lies Saturn, or Uranus perchance dim with distance, measuring still greater span. All in order in their several place the noble cortÈge of the Sun is exposed to view, seen now by the courtesy of his withdrawal, backgrounded against the immensity of space. Great worlds, these separate attendants, and yet as nothings in the void where stare the silent stars, huge suns themselves with retinues unseen, so vast the distances ’twixt us and them.

No less a revelation awaits the opening of the shutters of the mind. If night discloses glimpses of the great beyond, knowledge invests it with a meaning unfolding and extending as acquaintance grows. Sight is human; insight seems divine. To know those points of light for other worlds themselves, worlds the telescope approaches as the years advance, while study reconstructs their past and visions forth their future, is to be made free of the heritage of heaven. Time opens to us as space expands. We stand upon the Earth, but in the sky, a vital portion not only of our globe, but of all of which it, too, forms part. To feel it is to enter upon another life; and if to realization of its beauty, its grandeur, and its sublimity of thought these chapters of its history have proved in any wise the portal, they have not been penned in vain.