The chief point to determine is the height of the meteor above the earth. For this we must have two observations at least, made in places as far asunder as possible. Suppose an observer at London and an observer at York were both witnesses of a splendid meteor; if they find, on subsequent comparison, that their observations were made at the same moment, there is no reasonable doubt that it was the same object they both saw. The observer at York describes the meteor as lying to the south, halfway down from the point directly over his head towards the horizon. The London observer speaks of the meteor as being to the north; and to him also it appeared that the object was halfway down towards his horizon from the point directly over his head. If you know a little Euclid, you can easily show from these facts that the height of the meteor must have been half the distance between London and York, that is, 85 miles (Fig. 78).
I do not mean to say that the mode of discovering the meteor’s height will be always quite such a simple process as it has been in the case of the London and York observations. The principle is, however, the same—that whenever from two sufficiently distant positions the direction of the meteor has been observed, its path is known—just as on p. 21 we showed how the height of the suspended ball was obtained from observations at each end of the table. Generally speaking, bright meteors begin at an elevation of between fifty and one hundred miles, and they become extinguished before they are within twenty miles of the ground.
Sometimes a tremendous explosion will take place during the passage of a meteor through the air. There was a celebrated instance in America on the 21st of December, 1876, which will give an idea of one of these objects possessing exceptional magnificence. It began in Kansas about seventy-five miles high, and thence it flew for a thousand miles at a speed of ten or fifteen miles a second, until it disappeared somewhere near Lake Ontario. Over a certain region between Chicago and St. Louis, the great ball of fire burst into a number of pieces, and formed a cluster of glowing stars that seemed to chase each other over the sky. This cluster must have been about forty miles long and five miles wide, and when the explosion occurred a most terrific noise was produced, so loud that many thought it was an earthquake. A remarkable circumstance illustrates the tremendous height at which this explosion occurred. The meteor had burst into pieces, the display was all over, and was beginning to be forgotten, and yet nothing had been heard. It was not until a quarter of an hour after the explosion had been seen that a fearful crash was heard at Bloomington. The explosion actually occurred 180 miles from the spot, and as sound takes five seconds to travel a mile, you can easily calculate that the noise required a quarter of an hour for its journey. What a tremendous noise it must have been!
Shooting stars are of every grade of brightness. Beginning with the more gorgeous objects which have been compared with the moon or even with the sun himself, we descend to others as bright as Venus or as Jupiter; others are as bright as stars of various degrees of brilliancy. Fainter shooting stars are much more numerous than the conspicuous ones; in fact, there are multitudes of these objects so extremely feeble that the unaided eye would not show them. They only become perceptible in a telescope. It is not uncommon while watching the heavens at night to notice a faint streak of light dashing across the field of the instrument. This is a shooting star which is invisible except through the telescope.
THE GREAT NOVEMBER SHOWERS.
Occasionally we have the superb spectacle of a shower of shooting stars. None of you, my young friends, can as yet have had the good fortune to witness one of the specially grand displays, but you may live in hope; there are still showers to come. Astronomers have ventured on the prophecy that in or about the year 1899 you will have the opportunity of seeing a magnificent exhibition of this kind. There is only one ground for anxiety, and that is as to whether the clouds will keep out of the way for the occasion. I think I cannot explain my subject better than by taking you into my confidence and showing you the reasons on which we base this prediction. The last great shooting-star shower took place in the year 1866, or, perhaps, I should rather say that this was the last display from the same shooting-star system as that about which we are now going to speak. On the night of the 13th of November, 1866, astronomers were everywhere delighted by a superb spectacle. Enjoyment of the wondrous sight was not only for astronomers. Every one who loves to see the great sights of nature will have good reason for remembering that night. I certainly shall never forget it. It was about ten o’clock when a brilliant meteor or two first flashed across the sky, then presently they came in twos and threes, and later on in dozens, in scores, in hundreds. These meteors were brilliant objects, any one of which would have extorted admiration on an ordinary night. What, then, was the splendor of the display when they came on in multitudes? For two or three hours the great shower lasted, and then gradually subsided.
We were not taken unawares on this occasion, for the shower was expected, and had been, in fact, awaited with eager anticipation. It should first be noticed that each year some shooting stars may always be looked for on or about the 13th of November. Every thirty-three years, or thereabouts, the ordinary spectacle breaks out into a magnificent display. It has also been found that for nearly 1000 years there have been occasional grand showers of meteors at the time of year mentioned, and all these incidents agree with the supposition that they are merely repetitions of the regular thirty-three-year shower. The first was in the year A.D. 902, which an old chronicle speaks of as the “year of the stars,” from the extraordinary display which then took place. I do not think the good people 1000 years ago fully appreciated the astronomical interest of such spectacles; in fact, they were often frightened out of their wits, and thought the end of the world had come. Doubtless many ancient showers have taken place of which we have no record whatever. In more modern days we have had somewhat fuller information; for example, on the night between the 12th and 13th of November, 1833, a shower was magnificently seen in America. Mr. Kirkwood tells us that a gentleman of South Carolina described the effect on the negroes of his plantation as follows: “I was suddenly awakened by the most distressing cries that ever fell on my ears. Shrieks of horror and cries for mercy I could hear from most of the negroes of the three plantations, amounting in all to about 600 or 800. While earnestly listening for the cause, I heard a faint voice near the door calling my name. I arose, and taking my sword, stood at the door. At this moment I heard the same voice still beseeching me to arise, and crying out that the world was on fire. I then opened the door, and it is difficult to say which excited me the most—the awfulness of the scene or the distressed cries of the negroes. Upwards of a hundred lay prostrate on the ground, some speechless, and some with the bitterest cries, but with their hands raised praying for mercy. The scene was truly awful, for never did rain fall much thicker than the meteors fell towards the earth.”
By the study of many records of great showers it was learned that the interval at which these grand displays succeeded one another was about thirty-three years; and when it was remembered that the last great shower was in 1833 it was confidently expected that another similar display would take place in 1866. This was fully confirmed. Yet another thirty-three years brings us to 1899, when we have good reason for looking forward to a grand shower of these bodies. It may be expected to occur on November 14 or November 15. It may, however, possibly be that a shower will occur on the same days of the succeeding year.
We know a good deal now as regards the movements of these little objects. I want you to think of a vast swarm, something like a flock of birds, which I dare say you have often seen flying high in the air; the difference, however, is that the flock of meteors is enormously greater than any flock of birds ever was; and the meteors, too, are scattered so widely apart, that each one may be miles away from its next neighbors. Usually the meteoric shoal is many millions of miles long, and perhaps a hundred thousand miles in width. The great flock of meteors travels through space in a certain definite track. We have learned how the sun guides a planet, and forces the planet to move around him in an ellipse. But our sun will also condescend to guide an object no bigger than a shooting star. A bullet, a pea, or even a grain of sand will be held to an elliptic course around the sun as carefully as the great Jupiter himself. The entire shoal of meteors may therefore pursue their common journey around the sun as if inspired by a common purpose, each individual member of the host being, however, guided by the sun, and performing its path in real independence of its neighbors. The orbit followed by this shoal of meteors is enormously large and wide. Here is a sketch of the path (Fig. 80), and I have laid down the position of the orbit of the earth, but not on the same scale. The ellipse is elongated, so that while the shoal approaches comparatively close to the sun at one end of its journey, at the other end it goes out to an enormous distance, far beyond the orbit of the earth—beyond, indeed, the orbit of Jupiter or Saturn; in fact, it reaches to the path of Uranus. To accomplish so vast a journey as this thirty-three years and a quarter are required, and now you will be easily able to see why we get periodical visits from the shoal.
It is, however, a mere piece of good fortune that we ever encounter the November meteors. Probably there are numerous other shoals of meteors quite as important which we never see, just in the same way as there are many shoals of fish in the sea that never come into our net. The earth moves round the sun in a path which is very nearly a circle, and the shoal moves round in this long oval. We cannot easily represent the true state of things by mere diagrams which show all these objects on the same plane. This does not give an accurate representation of the orbits. I think you will better understand what I mean by means of some wire rings. Make a round one to represent the path of the earth, and a long oval one to represent the path of the meteors. There is to be a small opening in the circular ring so that we can slip one of the orbits inside the other. If we are to see the meteors, it is of course necessary that they should strike the earth’s atmosphere, for they are not visible to us when they lie at a distance like the moon or like the planets. It is necessary that there be a collision between the earth and the shoal of meteors. But there never could be a collision between two trains unless the lines on which these trains run meet each other; therefore, it is necessary that this long ellipse shall actually cross the earth’s track; it will not do to have it pass inside like the two links of a chain; our earth would then miss the meteors altogether, and we should never see them. There are very likely many of such shoals of meteors revolving in this way, and thus escaping our notice entirely.
You will also understand why there is no use in looking for these showers except on a particular day of November. On that day, and on that day alone, the earth appears at that particular point of its route where the latter crosses the track of the shoal. On the 1st of November, for instance, the earth has not yet reached the point where it could meet with these bodies. By the end of November it has passed too far. But even supposing that the earth is crossing the track of the meteors on the 13th of November, it is still possible that only a few, or none at all, shall be seen. The shoal may not happen to be at that spot at the right time. For a display of meteors to occur, it is therefore necessary that the shoal shall happen to be passing this particular stage of the journey on the 13th of November. In 1866 the earth dipped through the shoal and caught a great many of these meteors in its net. For a few hours the earth was engaged in the capture, until it emerged on the other side of the shoal, and the display was at an end.
Sometimes it happens that in two years following each other, grand showers of meteors are seen. The reason of this is that the shoal is very long and thin, and consequently if the earth passes through the beginning of the shoal one year, it may have returned to the same point next year before the whole length of the shoal has completely passed. In this case we shall have two great showers in consecutive years. Thus a very fine display was seen in America on the proper day in 1867, while many stragglers were also observed during the three subsequent recurrences of the same date.
Whenever the 13th of November comes round we generally meet with at least five shooting stars belonging to this same system, and we must explain how this occurs. Suppose there is a small racecourse so that the competitors will have to run a great many times round before the race is over. Let there be a very large number of entries, and let the majority of the athletes be fairly good runners, while a few are exceptionally good with varying degrees of excellence, and a few are very bad, some being worse than others. The whole group starts together in a cluster at the signal, and perhaps for the first round or two they may keep tolerably well together. It will be noticed the cluster begins to elongate as one circuit after another is made; the better runners draw out to the front, and the slower runners lag further and further behind; at last it may happen that those at the head will have gained a whole round on those at the tail, while the other runners of varying degrees of speed will be scattered all round the course. The majority of the runners, if of nearly equal speed, may continue in a pretty dense group.
Precisely similar has been the great celestial race which these meteors are running. They started on their grand career centuries ago, and ever since then they have been flying round and round their mighty course. The greater proportion of the meteors still stay close together, and their pace is nearly uniform. The exceptionally smart ones have shot ahead, the exceptionally slow ones have lagged behind, and thus it happens that, after fifty or more revolutions have been completed, the shape of the original swarm has become considerably modified. Its length has been drawn out, while the stragglers and the fastest runners have been scattered all around the path. Across this course our earth carries us every November; there we usually encounter some of the members of this swarm which have strayed from the great host; they flash into the air, and thus it is that some of these bodies are generally seen every November.
During a shooting-star shower it is interesting to notice that all the meteors seem to diverge from a single point. In the adjoining figure (Fig. 81), which shows the directions of a number of meteors’ tracks, you will notice that every one seems to radiate from a certain point of the sky. In the case of the shower of the 13th–15th of November this point lies in the constellation Leo. I must refer you to the Appendix for a description of the way to find Leo or the Lion. The radiant point, as we term it, of this system of meteors is there situated. It is true that the meteors themselves do not generally seem to come all the way from this place. It is the direction of their luminous trails produced backward that carries the eye to the radiant (Fig. 81). If a meteor were actually seen there, it would be certainly coming straight towards us; it would not then appear as a streak of light at all; it would merely seem like a star which suddenly blazed into splendor and then again sank down into invisibility. Every meteor which appeared near this point would be directed very nearly at the observer, and its path would therefore seem very much foreshortened. I can illustrate this with a long straight rod. If I point it directly at you, you can only see the end. If I point it nearly at you, it will seem very much shortened. During the great shower in 1866 many of the meteors could be observed so close to the radiant in Leo that they seemed merely like very short marks in the sky; some of them, indeed, seemed to be merely starlike points swelling up into brilliance and then going out. Hence it is that we call this system of shooting stars the “Leonids.” They bear this name because their radiant lies in the constellation Leo, and unless the direction of a shooting star emanates from this point it does not belong to the Leonids. Even if it did so, the meteor would not be a Leonid unless the date was right, namely, on the 13th of November, or within a day thereof. We thus have two characteristics which belong to a system of shooting stars; there is the date on which they occur and the point from which they radiate.
OTHER GREAT SHOWERS.
To illustrate what I have said, we will speak about another system of shooting stars; they are due every August, from the 9th to the 11th, and their directions diverge from a point in the constellation of Perseus. I may remind you of the dates of the recurrence of this shower as well as of the November meteors of which we have just spoken, by quoting the following production:—
About the fourteenth watching be.
In August, too, stars shine through heaven,
On nights between nine and eleven.
It may be worth your while to remember these lines, and always to keep a look-out on the days named. The August meteors, the Perseids we often call them, do not make gorgeous displays, in particular years, with the regularity of the Leonids. There have been, no doubt, some exceptionally grand showers between the 9th and the 11th of August, but we cannot predict when the next splendid one is due. There are vast numbers of stragglers all round the track of the Perseids. In fact, it would seem as if the great race had gone on for such a long period that the cluster had to a great extent broken up, and that a large proportion of the meteors were now scattered the whole way around the course with tolerable uniformity. This being so, it follows that every time we cross the track we are nearly certain to fall in with a few of the stragglers, though we may never enjoy the tremendous spectacle of a plunge through a dense host of meteoroids.
There are many other showers besides the two I have mentioned. Some shooting stars are to be seen almost every fine night, and those astronomers who pay particular attention to this subject are able to make out scores of small showers which might not interest you. Each of these is fully defined by the night of the year on which it occurs and the position of the point in the heavens from which the meteors radiate. Of these I must mention one. It is not usually very attractive, but it has a particular interest, as I shall now explain.
On the 27th of November, 1872, a beautiful meteoric shower took place. You will notice that though the month is the same, the day is entirely different from that on which the Leonids appear. This shower of the 27th is called the Andromedes, because the lines of direction of the shooting stars of which it is composed seem to diverge from a point in the constellation Andromeda. Ordinarily speaking, there is no special display of meteors connected with the annual return of this day; but in 1872 astronomers were astonished by an exhibition of shooting stars belonging to this system. They were not at all bright when compared with the Leonid meteors. They were, however, sufficiently numerous to arrest the attention of very many, even among those who do not usually pay much attention to the heavens.
The chief interest of the shower of Andromedes centers in a remarkable discovery connecting meteors and comets. There is a comet which was discovered by the astronomer Biela. It is a small object, requiring a telescope to show it. This comet completes each revolution in a period of about seven years; or rather, I should say, that was the time which the comet used to spend on its journey, for a life of trouble and disaster seems of late to have nearly extinguished the unfortunate object. In 1872 the comet was due in our neighborhood, and on the night of the 27th of November, in the same year, the earth crossed the track, and, in doing so, the shower of shooting stars was seen. This was a remarkable coincidence. We crossed the path of the comet at the time when we knew the comet ought to be there; and though we did not then see the comet, we saw a shower of shooting stars, and a wonderful shower too. A circumstance so peculiar suggested at once that the comet and the shooting stars must in some way or other be connected together. This is a suggestion we can test in another manner. We know the history of the comet, and we are aware that at the very time of the shower, the comet was approaching from the direction of the constellation of Andromeda. It was coming, in fact, from the very quarter whence the shooting stars have themselves travelled. Taking all these things together, it seems impossible to doubt that the shoal of shooting stars was, if not actually the comet itself, something closely connected with that famous body.
METEORITES.
Some years ago, a farmer living near Rowton, in Shropshire, noticed on a path in a field a hole which had been suddenly made by some mysterious and unknown agent. The laborers who were near told him they had just heard a remarkable noise; and when the farmer put his hand down into the hole, he felt something hot at the bottom of it. He took a spade and dug up the strange body, and found it to be a piece of iron, weighing about seven pounds. He was naturally amazed at such an occurrence, and brought the body home with him.
Where did that piece of iron come from? It is plain that it could not have been always in the ground. The noise and the recently made hole showed that was not the case, and that is confirmed by the fact that the iron was hot. A piece of iron within a few feet of the earth’s surface cannot have remained warm for any length of time. It is therefore clear that the iron must have tumbled from the sky. This is a marvellous notion; in fact, it seems so incredible that at first people refused to believe that such things as stones or solid lumps of iron could have fallen from the heavens to the earth. But they had to believe it; the evidence was too conclusive. Fortunately, however, the occurrence is a comparatively rare one; indeed, our life on this globe would have an intolerable anxiety added to it if showers of iron hailstones like that at Rowton were at all of frequent occurrence. We should want umbrellas of a more substantial description than those which suffice for the rains we actually experience. There are, indeed, instances on record of persons having been killed by the fearful blows given by these bodies in falling.
The Rowton siderite is a comparatively small one; pieces weighing hundredweights, and even tons, have been collected together in our museums. I would recommend you to pay a visit to that interesting room in our great British Museum in which these meteorites are exhibited. There we see actual specimens of celestial bodies which we can feel or weigh, and which our chemists can analyze. It may be noticed that they only contain substances that we already know on this earth. This celestial iron has often been made use of in primitive times before man understood how to smelt iron from its ore and how to transform it from cast iron to wrought iron. Nature seems to have taken heed of their wants, and occasionally to have thrown down a lump or two for the benefit of those who were so fortunate as to secure them.
That these stones or irons drop from the sky is absolutely certain, but when we try to find out their earlier history we become involved in not a few difficulties. Nobody really knows the true history of these objects, but the view of their origin which seems to me to possess fewer difficulties than any other view is that which we may call the Columbiad Theory. I use this expression because every boy or girl listening to me ought to have read Jules Verne’s wonderful book, “From the Earth to the Moon,” and if any of you have not read it, the sooner you do so the better. It is there narrated how the gun club of Baltimore designed a magnificent cannon which was sunk deep into the ground, and then received a terrific charge of guncotton, on which a great hollow projectile was carefully lowered, containing inside the three adventurous explorers who desired to visit the moon. Calculations were produced with a view of showing that by firing on a particular day the explosion would drive the projectile up to the moon. There was, however, the necessary condition that the speed of projection should be great enough. The gun club were accurate in saying that if the cannon were able to discharge the projectile with a speed twenty or thirty times as great as that which had ever been obtained with any other cannon, then the missile would ascend up and up forever if no further influence were exerted on it. No doubt we have to overlook the resistance on the air and a few other little difficulties, but to this extent, at all events, the gun club were right: that a velocity of about six or seven miles a second would suffice to carry a body away from the gravitation of the earth.
No one supposes that there were ever Columbiad cannons on our globe by which projectiles were shot up into space; but it seems possible that there may have been in very ancient days volcanoes on the earth with a shooting power as great as that which President Barbicane designed for the big cannon.
Even now we have some active volcanoes of great energy on our earth, and we know that in former days the volcanoes must have been still more powerful; that, in fact, the Vesuvius of the present must be merely a popgun in comparison with volcanoes which have shaken the earth in those primitive days when it had just cooled down from its original fiery condition. Some of these early volcanoes, in the throes of their mighty eruptions, appear to have shot forth pieces of iron and volcanic substances with a violence great enough to carry them off into space.
Suppose that a missile were projected upwards, it would ascend higher and higher, and gravity would, of course, tend to drag it back again down to earth. It can be shown that with an initial speed of six or seven miles a second the missiles would never return to the earth if only influenced by its attraction. The subsequent history of such a projectile would be guided by the laws according to which a planet moves. The body is understood to escape the destination which was aimed at by the Columbiad. I mean, of course, that it is not supposed to hit the moon. Of course, this might conceivably happen; but most of the projectiles would go quite wide of the mark, and would travel off into space.
Though the earth would be unable to recall the projectile, the attraction of the sun would still guide it, whether it was as big as a paving-stone or ever so much larger or smaller. The body would be constrained to follow a path like a little planet around the sun. This track it would steadily pursue for ages. The wanderer would, however, cross the earth’s track once during each of its revolutions at the point from which it was projected. Of course, it will generally happen that the earth will not be there at the time the meteorite is crossing, and the meteorite will not be there at the time the earth is crossing. Nothing will therefore happen, and the object goes again on its long rounds. But sometimes it must occur that a meteor does not get past the junction without coming so close to the earth that it plunges into the air, often producing a noise and generating a streak of light like a shooting star. Then it tumbles down, and is restored to that earth whence it originally came.
If this be the true view—and I think there are less weighty objections to it than to any other I know of—then the history of the piece of iron that was found in Shropshire would be somewhat as follows. Many millions of years ago, when the fires of our earth were much more vigorous than they are in these dull times, a terrific volcanic outbreak took place, and vast quantities of material were shot into space, of which this is one of the fragments. During all the ages that have since elapsed this piece of iron has followed its lonely track. In a thousandth part of the time rust and decay would have destroyed it had it lain on the earth, but in the solitudes of space there was found no air or damp to produce corrosion. At last, after the completion of its long travels, it again crashed down on the earth.
We have now briefly surveyed the extent of the solar system. We began with the sun, which presides over all, and then we discussed the various planets with their satellites, next we considered the eccentric comets, and finally the minute bodies which, as shooting stars or meteorites, must be regarded as forming part of the Sun’s system. In our closing lecture we shall have to deal with objects of a far more magnificent character.