Popular Science Monthly/Volume 1/July 1872/Astro-Meteorology
By Prof. DANIEL KIRKWOOD, LL. D.
THE theory that shooting-stars, meteoric stones, and even comets, consist of matter, which has been expelled with enormous force from the solar surface, was proposed by Prof. Hackley, as long since as 1860. A similar hypothesis in regard to comets has also been advanced by Prof. William A. Norton. In the present paper, it is proposed to consider, first, the evidence derived from recent discoveries in favor of this theory; and, secondly, the indications afforded by observed phenomena in regard to the history of certain meteoric streams:
1. The observations of Zollner, Respighi, and others, have indicated the operation of stupendous eruptive forces beneath the solar surface. The rose-colored prominences, which Janssen and Lockyer have shown to be masses of incandescent hydrogen, are regarded by Prof. Respighi as phenomena of eruption. "They are the seat of movements of which no terrestrial phenomenon can afford any idea; masses of matter, the volume of which is many hundred times greater than that of the earth, completely changing their position and form in the space of a few minutes." The nature of this eruptive force is not understood. If caused, as we may reasonably assume, by chemical combinations among the solar elements, it was probably in active operation long before the sun had contracted to its present dimensions.
2. With an initial velocity of projection equal to 380 miles per second, the matter thrown off from the sun would be carried beyond the limits of the solar system, never to return. With velocities somewhat less, it would be transported to distances corresponding to those of the aphelia of the periodic comets.
3. In the explosion witnessed by Prof. Young on the 7th of September, 1871, the mean velocity between the altitudes of 100,000 and 200,000 miles was 166 miles per second. This indicates a velocity of about 200 miles per second at the lower elevation, and hence a considerably greater initial velocity. An equal force when the sun had but little more than twice its present diameter would have been sufficient to carry the projected matter beyond the orbit of Neptune.
4. This eruptive force, whatever be its nature, is probably common to the sun and the so-called fixed stars. If so, the dispersed fragments of ejected matter ought to be found in the spaces intervening between sidereal systems. Accordingly, the phenomena of comets and meteors have demonstrated the existence, in immense numbers, of extremely small cosmical bodies in the portions of space through which the solar system is moving. The origin of such masses, their collocations in groups, and their various physical characteristics, would seem to be satisfactorily accounted for by the theory under consideration.
5. According to Mr. Sorby, the microscopic structure of the aërolites he has examined points evidently to the fact that they have been at one time in a state of fusion from intense heat—a fact in striking harmony with this theory of their origin.
6. The velocity with which some meteoric bodies have entered the atmosphere has been greater than that which would have been acquired by simply falling toward the sun from any distance, however great. On the theory of their sidereal origin, this excess of velocity has been dependent on the primitive force of expulsion.
7. A striking argument in favor of this theory may be derived from the researches of the late Prof. Graham, considered in connection with those of Dr. Huggins and other eminent spectroscopists. Prof. Graham found large quantities of hydrogen confined in the pores or cavities of certain meteoric masses. Now, the spectroscope has shown that the sun's rose-colored prominences consist of immense volumes of incandescent hydrogen; that the same element exists in great abundance in many of the fixed stars, and even in certain nebulae; and that the star in the Northern Crown, whose sudden outburst in 1860 so astonished the Scientific world, afforded decided indications of its presence.
Dr. Weiss, of Vienna, regards comets as the original bodies by whose disintegration meteor-streams are gradually formed. In this respect his views differ somewhat from those of Schiaparelli. "Cosmical clouds," he remarks, "undoubtedly appear in the universe, but only of such density that in most cases they possess sufficient coherence to withstand the destructive operation of the sun's attraction, not only up to the boundaries of our solar system, but even within it. Such cosmical clouds will always appear to us as comets when they pass near enough to the earth to become visible. Approaching the sun, the comet undergoes great physical changes, which finally affect the stability of its structure: it can no longer hold together: parts of it take independent orbits around the sun, having great resemblance to the orbit of the parent comet. With periodical comets, this process is repeated at each successive approach to the sun. Gradually the products of disintegration are distributed along the comet's orbit, and if the earth's orbit cuts this, the phenomenon of shooting-stars is produced."
The characteristics of the different meteor-streams afford interesting indications in regard to their relative age, the composition and magnitude of their corpuscles, etc. etc. Thus, if we compare the streams of August 10th and November 14th, we shall find that the latter probably entered our system at a comparatively recent epoch. We have seen that at each return to perihelion the meteoric cluster is extended over a greater arc of its orbit. Now, Tuttle's comet and the August meteors undoubtedly constituted a single cluster previous to their entering the solar domain. It is evident, however, from the annual return of the shower during the last 90 years, that the ring is at present nearly if not quite continuous. That the meteoric mass had completed many revolutions before the ninth century of our era is manifest from the frequent showers observed between the years 811 and 841. At the same time, the long interval of 83 years between the last observed display in the ninth century, and the first in the tenth, indicates the existence of a wide chasm in the ring no more than 1,000 years since.
The fact that the meteors of the November stream are diffused around only a small portion of their path, seems to justify the conclusion that the transformation of their orbit occurred at a date comparatively recent. Leverrier has calculated that the meteoric cloud passed very near Uranus about a. d. 126. He regards it, therefore, as highly probable that this was its first approach to the centre of our system. It is proper to remark, however, that Newton's period of the meteors exceeds Oppolzer's period of the comet by twenty-seven days, and that each is liable to some uncertainty. But for the authority of the distinguished French astronomer, the writer would have fixed upon the year 43 b. c. as the probable epoch at which the cometary mass was thrown into its present orbit. Be this as it may, it undoubtedly suffered considerable perturbation about a. d. 126.
The question of the planetary disturbance of the meteor-streams is one of great interest. The November group has its perihelion at the orbit of the earth; its aphelion at that of Uranus. Both planets, therefore, at each encounter with the current not only appropriate a portion of the meteoric matter, but entirely change the orbits of a large number of meteors. In regard to the devastation produced by the earth in passing through the cluster, it is sufficient to state that, according to Weiss, the meteor orbits resulting from the disturbance will have all possible periods from 21 months to 390 years. It may be regarded, therefore, as an additional evidence of the recent introduction of this meteor-stream into the solar system that the comet of 1866, which constitutes a part of the cluster, has not been deflected from the meteoric orbit by either the earth or Uranus. It is, moreover, interesting to remark that the comet and Uranus will be in close proximity about the year 1983; perhaps so close as to throw the former into a new orbit.
As the comets 1862, III., and 1866, I., were doubtless more brilliant in ancient than in modern times, and as the former was conspicuously visible to the naked eye, it seems not improbable that they may have been formerly observed. The epochs of their ancient returns agree in several instances with those of comets of which the recorded observations are insufficient to determine their elements.
The writer as long since as 1861 suggested the probable disintegration of Biela's comet and the distribution of its matter around the orbit. The earth crosses the path of these cometary fragments about the 29th or 30th of November—a well-known aërolitic epoch. It is also worthy of notice that an extraordinary number of shooting-stars was observed by M. Heis, at Aix-la-Chapelle, on the 29th of November, 1850.
From the fact that the earth, about the 20th of April, very nearly crosses the orbit of the comet 1861, I., a connection between the latter and the meteors of that epoch has been suggested by some astronomers. The period of the comet is, according to Oppolzer, 415 years. The first recorded shower of the April meteors occurred in the year 687 b. c.; the last great display in 1803 a. d. The interval is equal to six periods of 415 years. It is evident, however, that, if these meteors and the first comet of 1861 originally constituted a single group, they must have entered the solar system at a very remote epoch. The writer has elsewhere given reasons for regarding 28 years as nearly the true meteoric period.
In its descending node, the orbit of Halley's comet is but 3,000,000 miles from that of the earth. Our planet passes, this point of nearest approach a little before the middle of May. Is it not probable that some of the meteoric stones of May 8th to 14th have been moving in nearly the same cometary orbit?
It has been pointed out by Dr. Weiss that the height at which the meteors of different rings appear and disappear depends, to some extent, on their respective velocities. The meteors of November 14th, for instance, move much more rapidly than those of August 10th, and are also observed at a greater altitude. Further observations of this interesting cluster can scarcely be expected till near the close of the present century.