METEORS.—Sir Robert S. Ball
Our present knowledge as to the natural history of the shooting stars has been mainly acquired during the last hundred years. The first important step in the comprehension of these bodies was to recognize that the brilliant flash of light was caused by some object which came from without and plunged into our air. This was known at the end of the Eighteenth Century, largely by the labors of the philosopher Chladni in 1794.
A Portion of the Moon’s Disk
Where Four Mountain Ranges Meet
Could an ordinary shooting star tell us its actual history, the narrative would run somewhat as follows:
“I was a small bit of material, chiefly, if not entirely, composed of substances which are formed from the same chemical elements as those you find on the earth. Not improbably I may have had some iron in my constitution, and also sodium and carbon, to mention only a few of the most familiar elements. I only weighed an ounce or two, perhaps more, perhaps less—but you could probably have held me in your closed hand, or put me into your waistcoat pocket. You would have described me as a sort of small stone, yet I think you would have added that I was very unlike the ordinary stones with which you were familiar. I have led a life of the most extraordinary activity; I have never known what it was to stay still; I have been ever on the move. Through the solitudes of space I have dashed along with a speed which you can hardly conceive. Compare my ordinary motion with your most rapid railway trains; my journey will be done ere the best locomotive ever built could have drawn the train out of the station. Pit me against your rifle bullets, against the shots from your one-hundred-ton guns; before the missile from the mightiest piece of ordnance ever fired shall have gone ten yards I have gone 1,000 yards. I do not assert that my speed has been invariable—sometimes it has been faster, sometimes it has been slower; but I have generally done my million miles a day at the very least. Such has been my career, not for hours or days, but for years and for centuries, probably for untold ages. And the grand catastrophe in which I vanished has been befitting to a life of such transcendent excitement and activity; I have perished instantly, and in a streak of splendor. In the course of my immemorial wanderings I have occasionally passed near some of the great bodies in the heavens; I have also not improbably in former years hurried by that globe on which you live. On those occasions you never saw me, you never could have seen me, not even if you had used the mightiest telescope that has ever been directed to the heavens. But too close an approach to your globe was at last the occasion of my fall. You must remember that you live on the earth buried beneath a great ocean of air. Viewed from outside space, your earth is seen to be a great ball, everywhere swathed with this thick coating of air. Beyond the appreciable limits of the air stretches the open space, and there it is that my prodigious journeys have been performed. Out there we have a freedom to move of which you who live in a dense atmosphere have no conception. Whenever you attempt to produce rapid motion on the earth, the resistance of your air largely detracts from the velocity that would be otherwise attainable. Your quick trains are impeded by air, your artillery ranges are shortened by it. Movements like mine would be impossible in air like yours.
“And this air it is which has ultimately compassed my destruction. So long as I merely passed near your earth, but kept clear of that deadly net which you have spread, in the shape of your atmosphere, to entrap the shooting stars, all went well with me. I felt the ponderous mass of the earth, and I swerved a little in compliance with its attraction; but my supreme velocity preserved me, and I hurried past unscathed. I had many narrow escapes from capture during the lapse of those countless ages in which I have been wandering through space. But at last I approached once too often to the earth. On this fatal occasion my course led me to graze your globe so closely that I could not get by without traversing the higher parts of the atmosphere. Accordingly, a frightful catastrophe immediately occurred. Not to you; it did you no harm; indeed, quite the contrary. My dissolution gave you a pleasing and instructive exhibition. It was then, for the first time, that you were permitted to see me, and you called me a shooting star or a meteor.
“When from the freedom of open space I darted into the atmosphere, I rubbed past every particle of air which I touched in my impetuous flight, and in doing so I experienced the usual consequence of friction—I was warmed by the operation.
“You can readily comprehend the immense quantity of heat that will have been produced ere friction could deprive me of a speed of twenty miles a second. That heat not merely warmed me, but I rapidly became red-hot, white-hot, then I melted, even though composed of materials of a most refractory kind. Still friction had much more to do, and it actually drove me off into vapor, and I vanished. You, standing on your earth many miles below, never saw me—never could have seen me—until this supreme moment, when, glowing with an instantaneous fervor, I for a brief second became visible.
“Nature knows no annihilation, and though I had been driven off into vapor and the trial by fire had scattered and dispersed me, yet in the lofty heights of the atmosphere those vapors cooled and condensed. They did not, they never could again reunite and reproduce my pristine structure. Here and there in wide diffusion I repassed from the vaporous to the solid form, and in this state I wore the appearance of a streak of minute granules distributed all along the highway I had followed. These granules gradually subsided through the air to the earth. On Alpine snows, far removed from the haunts of men and from contamination of chimneys, minute particles have been gathered, many of which have unquestionably been derived from the scattered remains of shooting stars. Into the sea similar particles are forever falling, and they have been subsequently dredged up from profound depths, having subsided through an ocean of water after sinking through an ocean of air.”
Those splendid shooting stars which are often called fire-balls move in every direction. They come from the east, and from the west, from the north, and from the south. There is no hour of the night at which they have not occasionally been seen. Even in daylight it has happened not once or twice, but on several occasions, that a brilliant meteor has forced itself upon our astonished notice. They generally first make their appearance at a height which is between fifty and one hundred miles above the ground. They hurry down their inclined path, but generally become extinguished while still at least twenty miles aloft. In their more ambitious flights meteors have been known to span a kingdom. Nor are even greater strides unrecorded. The length of a continent may be compared with the track of that terrific meteor of 5th September, 1868, which broke into visibility at an appalling height above the Black Sea, and had not expended its stupendous energy until it passed over the smiling vineyards of France.
Great fire-balls are much more numerous than any one would suppose who had not paid attention to the subject. Nor need this be a matter for surprise if it be remembered that when a fire-ball does arrive it is only by a favorable combination of circumstances that any particular individual is privileged to witness the exhibition. As a random example of the yearly crop of fire-balls, I take from the middle of 1877 to the middle of 1878. A list of the fire-balls noticed during this period will be found in that store-house of valuable information, the Reports of the British Association. In the year referred to I see that eighty-six great fire-balls have been recorded. They have appeared in various localities, both in the old hemisphere and in the new. The most arduous observer may think himself fortunate if he has even seen one of them.
As to the brilliant light from some of these great fire-balls, there are numerous statements. We are not infrequently told that even the beams of the full moon are ineffectual in comparison with the blaze of the meteor; and we find a high authority asserting that one of these bodies displayed a flash as “blinding as the sun.” On the 29th July, 1878, a fire-ball was seen which created so splendid an illumination that “the smallest objects were visible at Manchester.”
Fortunate, indeed, would the astronomer have been who, guided by some miraculous prescience, had gone to the ancient city of York on the evening of the 23d of February, 1879, and on the tower of the glorious minster spent the night in observation of the heavens. It would have been his privilege to witness a majestic meteor under circumstances of almost unique magnificence.
It was at seven minutes before three that such few stragglers as the streets of York still contained saw a pear-shaped ball of fire traveling across the sky. It drenched the ancient city with a flood of light. The superb front of the minster never before glowed with a more romantic illumination. The unwonted brilliancy streamed through every aperture in every window in the city; every wakeful eye was instantly on the alert; every light sleeper started up suddenly to know what was the matter. Even those whom the blaze of midnight light had failed to awaken were only permitted to protract their slumbers for another minute and a half—only until an awful crash, like a mighty peal of thunder, burst over the town, shaking the doors, the windows, and even the houses themselves. The whole city was thus alarmed. Every one started at the noise. But that noise was not a clap of thunder. Nor was it produced by an earthquake. It was merely the explosion of the fire-ball which flung itself against the atmosphere after its immeasurable voyage through space.
Perhaps the most remarkable instance of the explosion of a meteor is recorded in the case of the great fire-ball so widely observed in America on the 21st of December, 1876. The movements of this superb object have been carefully studied by Professor H. A. Newton and Professor D. Kirkwood. For the prodigious span of a thousand miles this meteor tore over the American continent with a speed of some ten or fifteen miles a second. It first appeared over Kansas at a height of seventy-five miles. Thence it glided over the Mississippi, over the Missouri; it passed to the south of Lake Michigan; it made a short voyage over Lake Erie, and it can not have been very far from the Falls of Niagara, when by becoming invisible all further traces of its movements were lost. While passing a point midway between Chicago and St. Louis a frightful explosion shivered the meteor into a cluster of brilliant balls of fire, which seemed to chase each other across the sky. This cluster must have been about forty miles long and five miles wide. The detonation by which the explosion was accompanied was a specially notable incident of this meteor. It was not only heard with terrific intensity in the neighborhood, but the volume of sound was borne to great distances.
The glory of a meteor is often so evanescent that we just get a glimpse and it is gone. The sky resumes its ordinary aspect; the familiar stars are there, and even the very situation of the brilliant streak has become unrecognizable. But this is not always so; it sometimes happens that the brief career of the meteor leaves a notable trace behind it, so that for seconds and for minutes the sky is diversified by an unwonted spectacle. The path of the meteor leaves a stain of pearly light on the sky to mark the highway pursued by our celestial visitor.
In its fearful career the meteor is often rent to fragments, reduced to dust, dissolved into vapor. The glowing atoms of the wreck lie strewn along the path, just as the ghastly remnants of Napoleon’s mighty army limned out the awful retreat from Moscow.
A pencil-shaped cloud of meteoric débris, perhaps eighty or a hundred miles in length, and four or five miles in diameter, thus hangs poised in air. It is at night. The sun has sunk so far below the horizon that there is no trace of the feeblest twilight glow. An ordinary cloud would, of course, be invisible except as concealing the stars; no beams of light fall upon it; there is nothing to render it luminous. So, too, the meteoric streak will often pass instantly into invisibility, but, as I have said, this is not always the case. There is a well-authenticated instance in which the trail of a superb meteor remained visible for nearly an hour. I have endeavored up to the present to explain the various phenomena presented to us in the fall of a meteor, but here, for the first time, we have to note a circumstance for which it is not easy to account. We can explain why it is that the long meteoric cloud should be there, but we can not so easily explain why we should be able to see it. Whence comes this beautiful pearly luminosity? It seems that the meteoric dust must glow with some intrinsic luminosity.
We have spoken of dazzling fire-balls which generate for a brief moment a light which eye-witnesses, with possibly a pardonable exaggeration, have ventured to compare with the beams of the sun himself. Other meteors are described as being as bright as the full moon. Descending still lower in the scale of splendor, we read of fire-balls as bright as Venus or Jupiter, as bright as Sirius, or as a star of the first magnitude. With each step downward in brilliancy we find the meteors to increase in numerical abundance. Shooting stars as bright as the stars of the second or third magnitude are comparatively frequent; they are still more numerous of the fourth and fifth magnitudes. Every night brings its tale of shooting stars whose brightness is just sufficient to impress the unaided eye. Nor do the shooting stars which even the most attentive eye can detect represent a fraction of their entire number. As there are telescopic stars which the unaided eye can not see, so it might fairly be conjectured that, as we can trace meteors of successive stages of brightness down to the limit of unaided eye visibility, so there may be meteors still and still smaller which would be detected could we only direct a telescope toward them.
If we reflect that for every one that is seen there must be thousands which dart in unseen, we obtain an imposing idea of the myriads of shooting stars that daily rain in upon our globe.
The world is thus pelted on all sides day and night, year after year, century after century, by troops and battalions of shooting stars of every size, from objects not much larger than grains of sand up to mighty masses which can only be expressed in tons. In the lapse of ages our globe must thus be gradually growing by the everlasting deposit of meteoric débris. Looking back through the vistas of time past, it becomes impossible to estimate how much of the solid earth may not owe its origin to this celestial source.
The first and most important truth with regard to the recurrence of the meteors is their occasional appearance in what are known as “meteoric showers.” During such displays it sometimes happens that shooting stars in shoals break forth simultaneously, so as to produce a spectacle which we now regard as of the utmost beauty and interest, but which in earlier times has often been the source of the direst terror and dismay.
Let me, for the sake of illustration, give some account of one of these great showers of shooting stars.
In the year 1866 I occupied the position of astronomer to the late Earl of Rosse. The memorable night between November 13th and 14th, 1866, was a very fine one; the moon was absent—a very important consideration in regard to the effectiveness of the display. The stars shone out clearly, and I was diligently examining some faint nebulæ in the eye-piece of the great telescope, when a sudden exclamation from the attendant caused me to look up from the eye-piece just in time to catch a glimpse of a fine shooting star, which, like a great sky-rocket, but without its accompanying noise, shot across the sky over our heads. The great shooting star which had already appeared was merely the herald announcing the advent of a mighty host. At first the meteors came singly, and then, as the hours wore on, they arrived in twos and in threes, in dozens, in scores, in hundreds. Our work at the telescope was forsaken; we went to the top of the castellated walls of the great telescope and abandoned ourselves to the enjoyment of the gorgeous spectacle.
To number the meteors baffled all our arithmetic; while we strove to count on the one side many of them hurried by on the other. The vivid brilliance of the meteors was sharply contrasted with the silence of their flight. We heard on that marvelous night no sounds save those with which we were familiar. The flights of the celestial rockets were attended with no noises that we could hear. The meteors were no doubt somewhat various as to size, but the characteristic feature of this shower, as contrasted with another great shower I have also seen, was the remarkable brilliance of the shooting stars. It was their exceptional splendor even more than their innumerable profusion that gave to the shower its peculiarity. As to the actual brilliancy of the meteors, I am enabled to give the accurate estimate made by Mr. Baxendell at Manchester, where the shower was well seen. Out of every hundred of these meteors ten were brighter than a first magnitude star, and two or three of them were brighter than Sirius. Fifteen out of each hundred were between the first and second magnitudes, and twenty-five were between the second and third magnitudes, while the remainder were smaller.
Some important facts with regard to ancient shooting-star showers have survived the thousand and one casualties to which historical records are exposed. A careful discussion of those which are sufficiently accurate to be intelligible discloses to us the startling fact that in general every thirty-three years a grand shooting-star shower has rained down on our earth. It sometimes happens that two consecutive years are rendered memorable by great showers. At present the day of the year on which this particular shower is wont to appear is about the 13th November; but in earlier ages we find the date to shift slowly toward the commencement of the year. Thus the display which took place in A. D. 1698 was on the 9th of November; while, looking back still further to one of the very earliest records, viz., that of the year 934, we find the date has receded to October 14th. This change of the day on which the shower occurs is of profound theoretical importance in connection with the discovery of the orbit which these meteors pursue. The advance of the date is, however, so slow that for the past few generations, as well as for the next, we may sufficiently define this particular shower by the meteors which enliven the skies between the 12th and the 14th of November. In fact, the poetaster has parodied the well-known lines for the days of the month by a similar effort, which will serve to remind us also of another periodic shower of shooting stars which occurs in August. He writes:
“If you November’s stars would see,
From twelfth to fourteenth watching be.
In August too stars shine from heaven,
On nights between nine and eleven.”
These lines are intended to imply that the days named will usually bring, in every November, a few meteors at all events belonging to the grand shower. These are stragglers, as it were, from the mighty host which visits us three times in the century.
Astronomers have a special name for this group of November meteors. They are called the “Leonids.” To explain why this name has been given, and why it is appropriate, we must dwell on an important part of the phenomena of the shower.
Fig. 23.—Position of Leo, Source of the Leonids
Among the constellations there is a fine sickle-shaped group, forming a part of Leo, one of the signs of the Zodiac. That part of the sky defined by Leo is curiously related to the meteors of the 12th to the 14th of November. Every shooting star truly belonging to that great shower pursued a track across the heavens, the direction of which, if carried back far enough, was always found to pierce through the sickle of Leo. Indeed, the paths of all the meteors formed a set of rays spreading away from that one point in the constellation. An invariable characteristic of this particular shower is its connection with the constellation of Leo, hence the appropriateness of the name of Leonids.
It must be borne in mind that we can never see the meteors until the fatal moment when they dive into our atmosphere. We could, indeed, at any time point our telescope to the spot in the heavens where we know the great shoal must certainly be located. But the mightiest telescope in the world does not disclose the shoal to us. In fact, we would never have seen these Leonids at all, we would never have become conscious that such a shoal of meteors existed, had it not been for a certain circumstance, which, for want of a better expression, I must speak of as accidental.
Our globe pursues a certain definite track around the sun. Year after year, with undeviating regularity, the earth performs the stages of its journey. If it reaches certain points on the 1st of January and the 12th of October in one year, then it reaches the same points on the 1st of January and the 12th of October respectively on next year, or any other year.
The Leonids and the earth have each a certain track. It might of course have happened that one of these tracks lay quite outside or quite inside the other. In the case of the Leonids, it has chanced that their orbit does intersect the orbit of the earth, and to this circumstance we are indebted for the glorious displays every thirty-three years.
There are many other periodic showers of shooting stars besides those notable Leonids. None of the other showers, however, possess the same importance as the Leonids, nor do they ever manifest celestial splendor comparable with that of those of the 13th of November. The Perseids, for example, which appear from the 9th to the 11th of August, are tolerably constant in their appearance, but have little spectacular interest. There is also another shower called the Andromedes, which occurs on the 27th of November. It has produced certain displays, one of the most remarkable of which took place in 1872. The meteors were excessively numerous on that occasion, but they were so short in their paths, and so insignificant as to brilliance, that the spectacle, though of great scientific interest, could not be compared as to splendor with that of the Leonids in 1866.
There are also several other showers which appear with greater or less regularity. Each of these possesses two distinct characteristics by which its meteors can be identified. One of these characters is the date on which the shower appears. The other is the constellation or point on the heavens from which all the meteors appear to radiate. Thus when we speak of the Andromedes on the 27th of November, we express that the shower on the 27th November comes from the part of the heavens marked by the constellation of Andromeda.
A striking discovery has been made which points to a curious connection between comets and shooting stars. It has been found that the track followed by a great shower of meteors is often identical with the track pursued by a comet. It is wholly beyond the province of mere chance that an orbit such as that of the Leonids should, both as to its size and its position in space, be likewise that of a comet, unless the comet and the meteor swarm were objects related to each other.
The great sun guides our world through its long annual journey. The mighty mass of the earth yields compliance to the potent sway of the ruler of our system. But the sun does not merely exercise a control over the vast planets which circulate around him. The supreme law of gravitation constrains the veriest mote that ever floated in a sunbeam, with the same unremitting care that it does the mightiest of planets. Thus it is that each little meteor is guided in its journeys for untold ages. Each of these little objects hurries along, deflected at every moment, to follow its beautifully curved path by the incessant attraction of the sun. At last, however, the fatal plunge is taken. The long wanderings of the meteor have come to an end and it vanishes in a streak of splendor.