Part 19

In the majority of cases the meteors appeared white; but many, especially the largest, exhibited a variety of brilliant colors, among which the red, blue, green, yellow and purple were the most common. In general the trails exhibited about the same color as the nucleus, but much fainter, and they were usually pervaded by a greenish tint. In some instances the trails were of quite a different color from the nucleus.

The luminous cloud observed at 5h. 30m. on the morning of November 14th, 1868, after having passed through the series of transformations above described, remained visible for a long while after sunrise, appearing then as a small cirrus cloud, exactly similar in appearance to the hundreds of small cirrus clouds then visible in the sky, which had probably the same meteoric origin. For over three hours after sunrise, these cirrus clouds remained visible in the sky, moving all together with the wind in the high regions of the atmosphere.

Although Plate XII. is intended to represent all the characteristics exhibited by the meteors observed on that night, every form represented having been obtained by direct observation, yet the number is much greater than it was at any single moment during the particular shower of 1868. As regards number, the intention was to give an idea of a great meteoric shower, such as that of 1833, for instance. Although many of the falling stars seem to be close to the Earth's surface, yet this is only an effect of perspective due to their great distance, very few of these meteors ever coming into the lower regions of our atmosphere at all.

The phenomena exhibited during other great meteoric showers have been similar to those presented by the shower just described, the only differences consisting in variations of size and brightness in the meteors, and also in the trails, which sometimes are not so numerous as they were in 1868.

While some shooting-stars move so rapidly that they can hardly be followed in their orbits, others move so slowly that the sight can easily follow them, and even remark the peculiarities of their movements, some remaining visible for half a minute. Some of the falling stars move at the rapid rate of 100 miles a second, but others only 10 miles a second, and even less. In general, they move about half as fast again as the Earth in its orbit. The arcs described by the meteors in the sky are variable. While some extend 80° and even 100°, others are hardly half a degree in length. While some shooting-stars are so faint that they can hardly be seen through the largest telescopes, others are so large and brilliant that they can be seen in the day-time. In general, a shooting-star of average brightness resembles a star of the third or fourth magnitude.

Whatever may be the origin of the shooting-stars, they are, when we see them, not in the celestial spaces, like the planets, the comets, or the stars, but in our atmosphere, through which they travel as long as they remain visible. The height at which they appear and disappear is variable, but in general they are about 80 miles above the surface of our globe when they are first seen, and at about 55 miles when they disappear. In many cases, however, they have been observed at greater elevations, as also at smaller. A meteor simultaneously observed at two different stations first appeared at the height of 285 miles, and was last seen at 192 miles above the Earth's surface; but in rare cases the falling stars have been seen below a layer of clouds completely covering the sky. I myself saw one such shooting-star a few years since. The fact that the meteors are visible at so great elevations, proves that our atmosphere extends much farther than was formerly supposed, although at these great heights it must be extremely rarefied, and very different from what it is in its lower regions.

There is a remarkable difference between the sporadic meteors seen in the sky on every night, and the meteoric showers observed only at comparatively rare intervals. While the first appear from different points in the sky and travel in all directions, being perfectly independent, the meteors of a shower all come from the same point of the heavens, from which they apparently diverge in all directions. This point of divergence of the meteors is called the radiant point of the shower. Although the meteors seem to diverge in all directions from the radiant point, yet they all move in approximately parallel lines, the divergence being an effect of perspective.

Whatever may be the position of the radiant point in the constellations, it remains as fixed in the sky as the stars themselves, and participates with them in the apparent motion which they undergo by the effect of the diurnal motion, and thus rises and sets with the constellation to which it belongs. This fact is sufficient to prove that the orbits of these meteors are independent of the Earth's motion, and that consequently they do not originate in our atmosphere. It has been shown by Encke that the radiant point of the meteoric shower of November 13th is precisely the point towards which our globe moves in space on November 13th; a tangent to the Earth's orbit would pass through this radiant point.

The meteoric showers are particularly remarkable, not merely because of the large number of meteors which are visible and the fact that they all follow a common orbit, but chiefly because they have a periodic return, either after an interval of a year, or after a lapse of several years. At the beginning of the present century only two meteoric showers were known, those of August 10th and of November 13th, and their periodicity had not yet been recognized, although it had begun to be suspected. It was only in 1836 that Quetelet and Olbers ventured to predict the reappearance of the November meteors in the year 1867. Having made further investigations, Prof. Newton, of Yale College, announced their return in the year 1866. In both of these years, as also in 1868, the meteors were very numerous, and were observed in Europe and in America on the night of November 13th. The predictions having thus been fulfilled, the periodicity of the meteors was established. Since then, other periodic showers have been recognized, although they are much less important in regard to number than those of August and November, except that of November 27th, which exhibited so brilliant a display in Europe in 1872. These successive appearances have established the main fact that meteoric showers are more or less visible every year when the Earth occupies certain positions in its orbit.

The meteoric shower of the 10th of August has its radiant point situated in the vicinity of the variable star Algol, in the constellation Perseus, from which its meteors have received the name of Perseids. Although varying in splendor, this meteoric swarm never fails to make its appearance every year. The Perseids move through our atmosphere at the rate of 37 miles per second. The shower usually lasts about six hours.

The meteoric shower of November 13th has its radiant point situated in the vicinity of the star Gamma, in the constellation Leo, from which its meteors have been called Leonids. But while the August meteors recur regularly every year, with slight variations, the shower of November does not occur with the same regularity. During several years it is hardly noticeable, and is even totally absent, while in other years it is very remarkable. Every 33 years an extraordinary meteoric shower occurs on the 13th of November, and the phenomenon is repeated on the two succeeding years at the same date, but with a diminution in its splendor at each successive return. The Leonids move in an opposite direction to that of the Earth, and travel in our atmosphere with an apparent velocity of 45 miles per second, this being about the maximum velocity observed in falling stars. But when the motion of our globe is taken into account, and a deduction is made of the 18 miles which it travels per second, it is found that these meteors move at an actual mean rate of 27 miles a second.

In a meteoric shower the stars do not fall uniformly throughout the night, there being a time when they appear in greater numbers. Usually it is towards morning, between 4 and 6 o'clock, that the maximum occurs. The probable cause of this phenomenon will be explained in its place hereafter.

The orbits of the meteoric showers are not all approximately in the same plane, like those of the planets, but rather resemble those of comets, and have all possible inclinations to the ecliptic. Like the comets, too, the different meteoric showers have either direct or retrograde motion.

The shooting-stars were formerly considered as atmospheric meteors, caused by the combustion of inflammable gases generated at the surface of the Earth, and transported to the high regions of our atmosphere by their low specific gravity. But the considerable height at which they usually appear, the great velocity of their motion, the common orbit followed by the meteors of the same shower, and the periodicity of their recurrence, do not permit us now to entertain these ideas, or to doubt their cosmical origin. But what is their nature?

It is now generally admitted that innumerable minute bodies, moving in various directions around the Sun, are scattered in the interplanetary spaces through which our globe travels. It has been supposed that congregations of such minute bodies form elliptical rings, within which they are all moving in close parallel orbits around the Sun. On the supposition that such rings intersect the orbit of the Earth at the proper places, it was practicable to account for the shooting-stars by the passage through our atmosphere of the numerous minute cosmical bodies composing the rings, and the Leonid and Perseid showers were so explained. But when the elements of the orbits of these two last swarms came to be better known, and were compared with those of other celestial bodies, it was found necessary to alter this theory.

It had for a long while been suspected that some kind of relation existed between the shooting-stars and the comets. This idea, vaguely formulated by Kepler more than two centuries ago, more clearly expressed by Chladni, and still more by Mr. Grey, before the British Association, at Liverpool, in 1855, has recently received a brilliant confirmation by the researches of Professor Schiaparelli, Director of the Observatory of Milan. A thorough investigation of the orbits of the August and November meteors led Schiaparelli to the discovery of a remarkable relation between meteoric and cometary orbits. By comparing the elements of these meteoric orbits with those of comets, he found a very close resemblance between the orbit of the August meteors and that of the comet 1862, III., and again between the orbit of the November meteors and that of Tempel's comet, 1866, I. These resemblances were too striking to be the result of mere chance, and demonstrated the identity of these cometary orbits with those of the Perseid and Leonid showers. In accordance with these new facts, it is now admitted that the meteoric showers result from the passage of our globe through swarms of meteoric particles following the orbits of comets, which intersect the orbit of the Earth.

Professor Schiaparelli has attempted to show how these meteoric swarms were originally scattered along the orbits of comets, by supposing these bodies to originate from nebulous masses, which, in entering the sphere of attraction of the Sun, are gradually scattered along their orbits, and finally form comets followed by long trails of meteoric particles.

It has been shown that in approaching the Sun the comets become considerably elongated, their particles being disseminated over immense distances by the solar repulsion. It seems probable that, owing to its feeble attractive power, the nucleus is incompetent to recall the scattered cometary particles and retain them in its grasp when they are relieved from the solar repulsion, so that they remain free from the nucleus, although they continue to move along its orbit. It is supposable that these cometary particles will scatter more and more in course of time. Forming at first an elongated meteoric cloud, they will finally spread along the whole orbit, and thus form a ring of meteoric particles. Since our globe constantly moves in its orbit and daily occupies a different position, it follows that at any point where such a cometary orbit happens to cross that of the Earth, our globe will necessarily encounter the cometary particles as a shower of meteors. This encounter will take place at a certain time of the year, either yearly, if they form a continuous ring, or after a succession of years, if they simply form an elongated cloud. Such meteoric clouds or rings would not be visible in ordinary circumstances, even through the largest telescopes, except on penetrating the upper regions of our atmosphere, when they would appear as showers of falling stars. It is supposed that in penetrating our atmosphere, even in its most rarefied regions, these meteors are heated by the resistance offered by the air to their motion, first becoming luminous and then being finally vaporized and burnt before they can reach the surface of the Earth.

The orbit of the comet of 1862, III., which so closely corresponds with that of the Perseid meteors, is much more extended than that of Tempel's comet corresponding with that of the Leonids. While the first extends far beyond the orbit of Neptune, the latter only goes a little beyond that of Uranus. The former orbit makes a considerable angle with the plane of the Earth's orbit, but the latter is much nearer to parallelism with it. The period of revolution of the first is 108 years, and that of the last about 33¼ years.

From the fact that the Perseid shower occurs yearly on the 10th of August, when the Earth crosses the orbit of the comet of 1862, III., it is supposed that the cometary particles producing this shower are disseminated along the whole orbit, and form a ring encircling the Sun and Earth. To explain the yearly variations in the number of the shooting-stars observed, these particles are supposed to be unequally distributed over the orbit, being more crowded at one place than they are at another. In order to explain the meteoric shower of Leonids, which appears in all its splendor every 33 years, and then with diminished intensity for two successive years, after which it is without importance, it is supposed that the cometary particles of the comet of 1866, I., have not as yet spread all along the orbit, a sufficient time not having been allowed, but form an elongated meteoric cloud, more dense in its front than in its rear part. From these considerations it has been supposed also that the comet of 1866, I., is of a more recent date than that of 1862, III. While Tempel's comet makes its revolution around the Sun in about 33 years, this meteoric cloud, which has the same period and returns to the same point of its orbit every 33 years, encounters our globe for three successive years. The first year we are passing through its densest parts, and the two following years in less and less crowded parts, from which result the observed phenomena. An idea of the extent of this meteoric cloud may be formed from the fact that, with its cometary velocity of motion, it takes this cloud three years at least to cross the Earth's orbit. From recent researches it would appear that the Leonid cloud is not single, but that at least two others of smaller importance exist, and have periods of 33¼ years.

Biela's comet, which was divided into two parts in 1846, is another of the few comets whose orbit approaches that of the Earth. Possessing this knowledge, and knowing then the close connection existing between meteors and comets, astronomers supposed that there were sufficient reasons to expect a meteoric shower when this comet was passing near the Earth. They consequently expected a meteoric display in 1872, when our globe was to cross its orbit. Their anticipation was plainly fulfilled, and on the night of November 27th, 1872, a splendid meteoric display, having its radiant point in the constellation Andromeda, was observed in Europe, and also in America, but the meteors seen here were not so numerous as in Europe. Other meteoric showers of less importance, such as that of April 20th, for instance, have also been identified with cometary orbits, so that now no doubt seems to remain as to the identity of cometary particles and shooting-stars.

The fact that the maximum number of meteors is always observed in the morning hours, supports the hypothesis of the cosmic origin of the shooting-stars, since the regions of the Earth where it is morning are precisely those fronting the regions towards which our globe is moving in space, and accordingly encounter more directly the meteors moving in their orbit. The greater abundance of falling stars at that time may thus be accounted for.

The number of meteors penetrating our atmosphere must be very great; there is not an hour and probably not a minute during which none fall. From various considerations, some astronomers have estimated at from 65,000,000,000 to 146,000,000,000 the total number of shooting-stars yearly penetrating in our atmosphere. The actual number is undoubtedly great, yet the fact that the meteors are rarely seen through the telescope while employed in observing various celestial objects, does not indicate that they are so numerous as these figures imply. It is only occasionally that one is seen traversing the field of the instrument. Even when the sky is observed with a low power eye-piece for several hours in succession, many nights may pass without disclosing one, although an observer, sweeping the sky more freely with the naked eye, may often perceive four or five during an ordinary night.

About the true nature of these bodies nothing is known with certainty. From spectrum analysis it seems to be established that most of them contain sodium and magnesium, while a few indicate the presence of strontium and iron, and in some rare cases there are traces of coal-gas. Some of the nuclei give a continuous spectrum, and others a spectrum of lines. The trail always gives a spectrum of bright lines which indicates its gaseous state. The traces of coal-gas rarely seen in meteors are, however, of great importance, as it identifies them more closely with the comets, which generally show a similar spectrum. The continuous spectra exhibited by some nuclei would indicate that they are incandescent and either solid or liquid; but it is difficult to conclude from their spectra what is their true nature, since we do not know exactly what part the terrestrial atmosphere may play in producing the results.

The mass of the shooting-stars is not known with certainty, but the fact that during great meteoric showers, none are seen to reach the surface of the Earth, all being consumed in a few seconds, sufficiently indicates that it must be very small. It has been calculated that those equal to Venus in apparent size and brilliancy may weigh several pounds, while the faint ones would weigh only a few grains.

If the shooting-stars have even such a mass as that here attributed to some of them, the extraordinary motions which I have described above seem to be unaccountable. The change of direction of a heavy mass moving swiftly cannot be sudden. The semi-circular, the wavy and the angular orbits observed could not be described, it would seem, by such a mass animated with a great velocity. Although the meteors are said to be ignited by the transformation of part of their progressive motion into molecular motion, yet it is not observed that the velocity of the falling stars diminishes when they are about to disappear. The luminous trails they leave in the atmosphere do not appear to be endowed with any motion, but remain for a time in their original positions. These facts are apparently opposed to the hypothesis that such meteors have any appreciable mass. The extraordinary motions exhibited by some meteors seem to indicate that some unsuspected force resides in these bodies, and causes them to deviate from the laws of ordinary motion.

Although it is very probable that the ordinary shooting-stars have no appreciable mass, yet it is known that very heavy meteoric masses sometimes fall at the surface of the Earth. Such falls are generally preceded by the sudden apparition in the sky of a large, and usually very brilliant fire-ball, which traverses the air at a great speed, sometimes leaving behind it a luminous trail, after which it explodes with a loud sound, and heavy fiery meteoric fragments, diverging in all directions, fall at the surface of the Earth. The name of Aerolites or Meteorolites is given to these ponderous fragments. As these meteors, before they explode and fall to the ground, have many points of resemblance with the shooting-stars, they are generally supposed to be connected with them, and to have a similar cometary origin. The fact that the aerolites differ widely from each other in constitution, and are all composed of substances found on the Earth, associated with other facts given below, would rather seem to indicate a terrestrial than a celestial origin.

If the aerolites belong to the same class of bodies as the falling stars, differing from them only in size and mass, it is difficult to see why so very few should fall upon the Earth during the great meteoric showers, when thousands of shooting-stars traverse our atmosphere. In Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the falls of aerolites and fire-balls which have been observed at the time of the periodic meteoric showers of the 10th of August and the 13th of November, during a period of 221 years for the Perseids, or August showers, and of 318 years for the Leonids, or November showers. During 221 years, 10 falls of aerolites have been witnessed simultaneously with the fall of the Perseids; while during 318 years, only 4 such falls have been recorded as having occurred at the time of the Leonid shower. If there is any close connection between the shooting-stars and the aerolites, we should expect to find a maximum in their fall at the time of the great meteoric displays. So far, no maxima or minima have yet been discovered in the fall of aerolites; they do not seem, like meteoric showers, to be governed by a law of periodicity.