Curiosities of the Sky - Garrett Serviss (books to read in your 20s .TXT) 📗

the appearance of bunches in the tail, such as have been observed. The hypothesis also falls in with the researches of Bredichin, who has divided the tails of comets into three principal classes - viz.: (1) Those which appear as long, straight rays; (2) Those which have the form of curved plumes or scimitars; (3) Those which are short, brushy, and curved sharply backward along the comet's path. In the first type he calculates the repulsive force at from twelve to fifteen times the force of gravity; in the second at from two to four times; and in the third at about one and a half times. The straight tails he ascribes to hydrogen because the hydrogen atom is the lightest known; the sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. It will be seen that, if the force driving off the tails is that which Arrhenius assumes it to be, the forms of those appendages would accord with those that Bredichin's theory calls for. At the same time we have an explanation of the multiple tails with which some comets have adorned themselves. The comet of 1744, for instance, had at one time no less than seven tails spread in a wide curved brush behind it. Donati's comet of 1858 also had at least two tails, the principal one sword-shaped and the other long, narrow, and as straight as a rule. According to Bredichin, the straight tail must have been composed of hydrogen, and the other of some form of hydro-carbon whose atoms are heavier than those of hydrogen, and, consequently, when swept away by the storm of light-waves, followed a curvature depending upon the resultant of the forces operating upon them. The seven tails of the comet of 1744 presented a kind of diagram graphically exhibiting its complex composition, and, if we knew a little more about the constituents of a comet, we might be able to say from the amount of curvature of the different tails just what were the seven substances of which that comet consisted.

If these theories seem to the reader fantastic, at any rate they are no more fantastic than the phenomena that they seek to explain.

Meteors, Fire-Balls, and Meteorites

One of the most terrorizing spectacles with which the heavens have ever caused the hearts of men to quake occurred on the night of November 13, 1833. On that night North America, which faced the storm, was under a continual rain of fire from about ten o'clock in the evening until daybreak.

The fragments of a comet had struck the earth.

But the meaning of what had happened was not discovered until long afterward. To the astronomers who, with astonishment not less than that of other people, watched the wonderful scene, it was an unparalleled 'shower of meteors.' They did not then suspect that those meteors had once formed the head of a comet. Light dawned when, a year later, Prof. Denison Olmsted, of Yale College, demonstrated that the meteors had all moved in parallel orbits around the sun, and that these orbits intersected that of the earth at the point where our planet happened to be on the memorable night of November 13th. Professor Olmsted even went so far as to suggest that the cloud of meteors that had encountered the earth might form a diffuse comet; but full recognition of the fact that they were cometary débris came later, as the result of further investigation. The key to the secret was plainly displayed in the spectacle itself, and was noticed without being understood by thousands of the terror-stricken beholders. It was an umbrella of fire that had opened overhead and covered the heavens; in other words, the meteors all radiated from a particular point in the constellation Leo, and, being countless as the snowflakes in a winter tempest, they ribbed the sky with fiery streaks. Professor Olmsted showed that the radiation of the meteors from a fixed point was an effect of perspective, and in itself a proof that they were moving in parallel paths when they encountered the earth. The fact was noted that there had been a similar, but incomparably less brilliant, display of meteors on the same day of November, 1832, and it was rightly concluded that these had belonged to the same stream, although the true relationship of the phenomena was not immediately apprehended. Olmsted ascribed to the meteors a revolution about the sun once in every six months, bringing them to the intersection of their orbit with that of the earth every November 13th; but later investigators found that the real period was about thirty-three and one-quarter years, so that the great displays were due three times in a century, and their return was confidently predicted for the year 1866. The appearance of the meteors in 1832, a year before the great display, was ascribed to the great length of the stream which they formed in space - so great that they required more than two years to cross the earth's orbit. In 1832 the earth had encountered a relatively rare part of the stream, but in 1833, on returning to the crossing-place, it found there the richest part of the stream pouring across its orbit. This explanation also proved to be correct, and the predicted return in 1866 was duly witnessed, although the display was much less brilliant than in 1833. It was followed by another in 1867.

In the mean time Olmsted's idea of a cometary relationship of the meteors was demonstrated to be correct by the researches of Schiaparelli and others, who showed that not only the November meteors, but those of August, which are seen more or less abundantly every year, traveled in the tracks of well-known comets, and had undoubtedly an identical origin with those comets. In other words the comets and the meteor-swarms were both remnants of original masses which had probably been split up by the action of the sun, or of some planet to which they had made close approaches. The annual periodicity of the August meteors was ascribed to the fact that the separation had taken place so long ago that the meteors had become distributed all around the orbit, in consequence of which the earth encountered some of them every year when it arrived at the crossing-point. Then Leverrier showed that the original comet associated with the November meteors was probably brought into the system by the influence of the planet Uranus in the year 126 of the Christian era. Afterward Alexander Herschel identified the tracks of no less than seventy-six meteor-swarms (most of them inconspicuous) with those of comets. The still more recent researches of Mr W. F. Denning make it probable that there are no meteors which do not belong to a flock or system probably formed by the disintegration of a cometary mass; even the apparently sporadic ones which shoot across the sky, 'lost souls in the night,' being members of flocks which have become so widely scattered that the earth sometimes takes weeks to pass through the region of space where their paths lie.

The November meteors should have exhibited another pair of spectacles in 1899 and 1900, and their failure to do so caused at first much disappointment, until it was made plain that a good reason existed for their absence. It was found that after their last appearance, in 1867, they had been disturbed in their movements by the planets Jupiter and Saturn, whose attractions had so shifted the position of their orbit that it no longer intersected that of the earth, as it did before. Whether another planetary interference will sometime bring the principal mass of the November meteors back to the former point of intersection with the earth's orbit is a question for the future to decide. It would seem that there may be several parallel streams of the November meteors, and that some of them, like those of August, are distributed entirely around the orbit, so that every mid-November we see a few of them.

We come now to a very remarkable example of the disintegration of a comet and the formation of a meteor-stream. In 1826 Biela, of Josephstadt, Austria, discovered a comet to which his name was given. Calculation showed that it had an orbital period of about six and a half years, belonging to Jupiter's 'family.' On one of its returns, in 1846, it astonished its watchers by suddenly splitting in two. The two comets thus formed out of one separated to a distance of about one hundred and sixty thousand miles, and then raced side by side, sometimes with a curious ligature connecting them, like Siamese twins, until they disappeared together in interplanetary space. In 1852 they came back, still nearly side by side, but now the distance between them had increased to a million and a quarter of miles. After that, at every recurrence of their period, astronomers looked for them in vain, until 1872, when an amazing thing happened. On the night of November 28th, when the earth was crossing the plane of the orbit of the missing comet, a brilliant shower of meteors burst from the northern sky, traveling nearly in the track which the comet should have pursued. The astronomers were electrified. Klinkerfues, of Göttingen, telegraphed to Pogson, of Madras: 'Biela touched earth; search near Theta Centauri.' Pogson searched in the place indicated and saw a cometary mass retreating into the southern heavens, where it was soon swallowed from sight!

Since then the Biela meteors have been among the recognized periodic spectacles of the sky, and few if any doubt that they represent a portion of the missing comet whose disintegration began with the separation into two parts in 1846. The comet itself has never since been seen. The first display of these meteors, sometimes called the 'Andromedes,' because they radiate from the constellation Andromeda, was remarkable for the great brilliancy of many of the fire-balls that shot among the shower of smaller sparks, some of which were described as equaling the full moon in size. None of them is known to have reached the earth, but during the display of the same meteors in 1885 a meteoric mass fell at Mazapil in Northern Mexico (it is now in the Museum at Vienna), which many have thought may actually be a piece of the original comet of Biela. This brings us to the second branch of our subject.

More rare than meteors or falling stars, and more startling, except that they never appear in showers, are the huge balls of fire which occasionally dart through the sky, lighting up the landscapes beneath with their glare, leaving trains of sparks behind them, often producing peals of thunder when they explode, and in many cases falling upon the earth and burying themselves from a few inches to several feet in the soil, from which, more than once, they have been picked up while yet hot and fuming. These balls are sometimes called bolides. They are not really round in shape, although they often look so while traversing the sky, but their forms are fragmentary, and occasionally fantastic. It has been supposed that their origin is different from that of the true meteors; it has even been conjectured that they may have originated from the giant volcanoes of the moon or have been shot out from the sun during some of the tremendous explosions that accompany the formation of eruptive prominences. By the same reasoning some of them might be supposed to have come from some distant star. Others have conjectured that they are wanderers in space, of unknown origin, which the earth encounters as it journeys on, and Lord Kelvin made a suggestion which has become classic because of its imaginative reach - viz., that the first germs of life may have been brought to the earth by one of these