Part 14

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.

A very remarkable peculiarity of the aerolites is that they seem to have a tendency to fall in certain regions. Such are the southern part of France, the north of Italy, Hindostan, the central states of North America, and Mexico and Brazil. There is a curious contrast existing between the quick cometary motion of the aerolites before their explosion, and the comparatively slow motion of their fragments as they reach the Earth; motion which seems to be no greater than that corresponding to their natural fall impeded by the resistance of the air. In general, their penetration into the soil upon which they fall does not at all correspond to the great velocity with which they move in the atmosphere. The fragmentary structures of the aerolites, their identity of substance with that of our globe, their great resemblance to the volcanic minerals of the Earth, and the fractures and faults which some of them exhibit, do not correspond at all with the idea that they are cometary particles fallen on the Earth. As far as their structure and appearance is concerned, they seem rather to be a volcanic product of the interior of the Earth than parts of disintegrated comets. It must be admitted that their identity with the shooting-stars is far from established, and that they are still involved in mystery.

The so-called meteoric dust gathered at sea and on high mountains may have various origins, and may be partly furnished by volcanic dust carried to great distances in the atmosphere.

Since millions of shooting-stars penetrate our atmosphere every year and remain in it, becoming definitively a part of the Earth, it follows that, no matter how small may be the quantity of matter of which they are composed, they must gradually increase the volume and mass of our globe, although the increase may be exceedingly slow. Supposing every one of the shooting-stars penetrating our atmosphere to contain one cubic millimeter of matter, it has been calculated that it would take nearly 35,000 years to make a deposit one centimeter in thickness all over the surface of our globe. Insignificant as this may appear, it is probable that the quantity of matter of meteoric origin which is added to our globe is much less than has just been supposed.

THE MILKY-WAY OR GALAXY

PLATE XIII

During clear nights, when the Moon is below the horizon, the starry vault is greatly adorned by an immense belt of soft white light, spanning the heavens from one point of the horizon to the opposite point, and girdling the celestial sphere in its delicate folds. Every one is familiar with this remarkable celestial object, called the _Milky-way_ or _Galaxy_.

Seen with the naked eye, the Galaxy appears as an irregular, narrow, nebulous belt, apparently composed of cloud-like luminous masses of different forms and sizes, separated by comparatively dark intervals. These cloud-like masses vary much in luminous intensity, and while some among them are very bright and conspicuous, others are so faint that they are hard to recognize. In general, the brightest parts of the Milky-way are situated along the middle of its belt, while its borders, which are usually very faint, gradually vanish in the sky. Some parts of the Galaxy, however, show very little of the cloudy structure so characteristic of other parts, being almost uniform throughout, except towards the borders, which are always fainter. These parts showing greater uniformity are also the faintest.

Such is the general appearance of the Milky-way on ordinary nights, but on rare occasions, when the atmosphere is particularly pure, it presents one of the grandest sights that can be imagined. At such favorable moments I have seen the Galaxy gleaming with light, and appearing as if composed of star-dust or of precious stones. The strange belt then appeared all mottled over and fleecy, its large cloud-like masses being subdivided into numerous small, irregular cloudlets of great brilliancy, which appeared projected upon a soft luminous background.

The width of the Galaxy is far from being uniform; while in some places it is only 4° or 5°, in others it is 15° and even more. In some places it appears wavy in outline, at others quite straight; then it contracts, to expand a few degrees distant; while at other places it sends off branches and loops, varying in form, size and direction, some of which are quite prominent, while others are very faint.

Although very irregular in form, the general appearance of the galactic belt is that of a regular curve occupying one of the great circles of the celestial sphere. The Milky-way completely encircles the heavens, but, of course, only one-half is visible at any one moment, since our globe prevents the other half from being seen. If, for a moment, we imagine ourselves left in space, our globe having vanished from under our feet, we should then see the whole Galaxy forming a continuous belt in the heavens, at the centre of which we should apparently be situated.

While only one-half of the galactic belt can be seen at once from any point on the Earth, yet, according to the position of the observer, a larger or smaller portion of the whole can be seen at different times. In high northern or southern latitudes but little more than half can be seen even by continuous observations; but as we approach the equatorial regions, more and more of it becomes visible, until the whole may be seen at different hours and seasons. In the latitudes of the northern states, about two-thirds of the Galaxy is visible, the rest remaining hidden below the horizon; but from the southern states very nearly the whole can be seen. The half of the Milky-way visible at any one time from any latitude on the Earth never entirely sets below the horizon, although in some places it may be so near the horizon as to be rendered invisible by vapors. In the latitude of Cambridge, when in its lowest position, the summit of its arc is still about 12° or 15° above the northern horizon. The great circle of the celestial sphere, occupied by the galactic belt, is inclined at an angle of about 63° to the celestial equator, and intersects this great circle on one side in the constellation Monoceros in 6h. 47m., and on the opposite side in the constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so that its northern pole is situated in the constellation Coma Berenices in R. A. 12h. 47m., declination N. 27°, and the southern in the constellation Cetus in R. A. 0h. 47m., declination S. 27°.

According to the seasons and to the hours of the night at which it is observed, the galactic arch presents different inclinations in the sky. Owing to its inclination to the equator of the celestial sphere, its opposite parts exhibit opposite inclinations when they pass the meridian of a place. That part of the Galaxy which is represented on Plate XIII., and which intersects the celestial equator in the constellation Aquila, is inclined to the left or towards the east, when it is on the meridian; while the opposite part, situated in Monoceros, is inclined to the right, or towards the west, when it reaches the meridian. The former passes the meridian in the evening in the summer and autumn months; the latter, in the winter and spring months.

[PLATE XIII.--PART OF THE MILKY WAY.

From a study made during the years 1874, 1875 and 1876]

By beginning at its northernmost part, represented at the upper part of Plate XIII., situated in "the chair" of the constellation Cassiopeia, and descending southwardly, and continuing in the same direction until the whole circle is completed, the course of the Milky-way through the constellations may be briefly described as follows: From Cassiopeia's chair, the Galaxy, forming two streams, descends south, passing partly through Lacerta on the left, and Cepheus on the right; at this last point it approaches nearest to the polar star. Then it enters Cygnus, where it becomes very complicated and bright, and where several large cloudy masses are seen terminating its left branch, which passes to the right, near the bright star Deneb, the leader of this constellation. Below Deneb, the Galaxy is apparently disconnected and separated from the northern part by a narrow, irregular dark gap. From this rupture, the Milky-way divides into two great streams separated by an irregular dark rift. An immense branch extends to the right, which, after having formed an important luminous mass between the stars _γ_ and _β_, continues its southward progress through parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it gradually terminates a few degrees south of the equator. The main stream on the left, after having formed a bright mass around _ε_ Cygni, passes through Vulpecula and then Aquila, where it crosses the equinoctial just below the star _η_ after having involved in its nebulosity the bright star Altair, the leader of Aquila. In the southern hemisphere the Galaxy becomes very complicated and forms a succession of very bright, irregular masses, the upper one being in Scutum Sobieskii, while the others are respectively situated in Sagittarius and in Scorpio; the last, just a little above our horizon, being always considerably dimmed by vapors. From Scutum Sobieskii, the Galaxy expands considerably on the right, and sends a branch into Scorpio, in which the fiery red star Antares is somewhat involved.