Curiosities of the Sky, Garrett Putman Serviss [free novel reading sites txt] 📗

perihelion every five and a half years; yet it had never been seen before and, although often searched for, has never been seen since. Laplace and Leverrier proved mathematically that in 1767 it had approached so close to Jupiter as to be involved among the orbits of his satellites. What its track had been before is not known, but on that occasion the giant planet seized the interloper, threw it into a short elliptic orbit and sent it, like an arrested vagrant, to receive sentence at the bar of the sun. On this journey it passed within less than 1,500,000 miles of the earth. The form of orbit which Jupiter had impressed required, as we have said, its return in about five and a half years; but soon after 1770 it had the misfortune a second time to encounter Jupiter at close range, and he, as if dissatisfied with the leniency of the sun, or indignant at the stranger's familiarity, seized the comet and hurled it out of the system, or at any rate so far away that it has never since been able to rejoin the family circle that basks in the immediate rays of the solar hearth. Nor is this the only instance in which Jupiter has dealt summarily with small comets that have approached him with too little deference.

The function which Jupiter so conspicuously fulfills as master of the hounds to the sun is worth considering a little more in detail. To change the figure, imagine the sun in its voyage through space to be like a majestic battleship surrounded by its scouts. Small vessels (the comets, as they are overhauled by the squadron, are taken in charge by the scouts, with Jupiter for their chief, and are forced to accompany the fleet, but not all are impressed. If a strange comet undertakes to run across Jupiter's bows the latter brings it to, and makes prize of it by throwing it into a relatively small ellipse with the sun for its focus. Thenceforth, unless, as happened to the unhappy comet of Lexell, it encounters Jupiter again in such a way as to be diverted by him into a more distant orbit, it can never get away. About thirty comets are now known to have thus been captured by the great planet, and they are called ``Jupiter's Comet Family.'' But, on the other hand, if a wandering comet crosses the wake of the chief planetary scout the latter simply drives it away by accelerating its motion and compels it to steer off into open space. The transformation of comets into meteors will be considered in the next chapter, but here, in passing, mention may be made of the strange fate of one member of Jupiter's family, Biela's comet, which, having become over bold in its advances to its captor, was, after a few revolutions in is impressed orbit, torn to pieces and turned into a flock of meteors.

And now let us return to the mystery of comets' tails. That we are fully justified in speaking of the tails of comets as mysterious is proved by the declaration of Sir John Herschel, who averred, in so many words, that ``there is some profound secret and mystery of nature concerned in this phenomenon,'' and this profound secret and mystery has not yet been altogether cleared up. Nevertheless, the all-explaining hypothesis of Arrhenius offers us once more a certain amount of aid. Comets' tails, Arrhenius assures us, are but another result of the pressure of light. The reader will recall the applications of this theory to the Zodiacal Light and the Aurora. In the form in which we now have to deal with it, the supposition is made that as a comet approaches the sun eruptions of vapor, due to the solar heat, occur in its nucleus. These are naturally most active on the side which is directly exposed to the sun, whence the appearance of the immense glowing envelopes that surround the nucleus on the sunward side. Among the particles of hydro-carbon, and perhaps solid carbon in the state of fine dust, which are thus set free there will be many whose size is within the critical limit which enables the light-waves from the sun to drive them away. Clouds of such particles, then, will stream off behind the advancing comet, producing the appearance of a tail. This accounts for the fact that the tails of comets are always directed away from the sun, and it also explains the varying forms of the tails and the extraordinary changes that they undergo. The speed of the particles driven before the light-waves must depend upon their size and weight, the lightest of a given size traveling the most swiftly. By accretion certain particles might grow, thus losing velocity and producing 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