Next to a grand meteor such as that we have just described, the most striking display in connection with shooting stars is what is known as a shower. These phenomena have attracted a great deal of attention within the last century, and they have abundantly rewarded the labour devoted to them by affording some of the most interesting astronomical discoveries of modern times.
The showers of shooting stars do not occur very frequently. No doubt the quickened perception of those who especially attend to meteors will detect a shower when others see only a few straggling shooting stars; but, speaking generally, we may say that the present generation can hardly have witnessed more than two or three such occurrences. I have myself seen two great showers, one of which, in November, 1866, has impressed itself on my memory as a glorious spectacle.
To commence the history of the November meteors it is necessary to look back for nearly a thousand years. On the 12th of October, in the year 902, occurred the death of a Moorish king, and in connection with this event an old chronicler relates how "that night there were seen, as it were lances, an infinite number of stars, which scattered themselves like rain to right and left, and that year was called the Year of the Stars."
No one now believes that the heavens intended to commemorate the death of the king by that display. The record is, however, of considerable importance, for it indicates the year 902 as one in which a great shower of shooting stars occurred. It was with the greatest interest astronomers perceived that this was the first recorded instance of that periodical shower, the last of whose regular returns were seen in 1799, 1833, and 1866. Further diligent literary research has revealed here and there records of startling appearances in the heavens, which fit in with successive returns of the November meteors. From the first instance, in 902, to the present day there have been twenty-nine visits of the shower; and it is not unlikely that these may have all been seen in some parts of the earth. Sometimes they may have been witnessed by savages, who had neither the inclination nor the means to place on record an apparition which to them was a source of terror. Sometimes, however, these showers were observed by civilised communities. Their nature was not understood, but the records were made; and in some cases, at all events, these records have withstood the corrosion of time, and have now been brought together to illustrate this curious subject. We have altogether historical notices of twelve of these showers, collected mainly by the industry of Professor H.A. Newton whose labours have contributed so much to the advancement of our knowledge of shooting stars.
Let us imagine a swarm of small objects roaming through space. Think of a shoal of herrings in the ocean, extending over many square miles, and containing countless myriads of individuals; or think of those enormous flocks of wild pigeons in the United States of which Audubon has told us. The shoal of shooting stars is perhaps much more numerous than the herrings or the pigeons. The shooting stars are, however, not very close together; they are, on an average, probably some few miles apart. The actual bulk of the shoal is therefore prodigious; and its dimensions are to be measured by hundreds of thousands of miles.
Fig. 76.—The Orbit of a Shoal of Meteors.
The meteors cannot choose their own track, like the shoal of herrings, for they are compelled to follow the route which is prescribed to them by the sun. Each one pursues its own ellipse in complete independence of its neighbours, and accomplishes its journey, thousands of millions of miles in length, every thirty-three years. We cannot observe the meteors during the greater part of their flight. There are countless myriads of these bodies at this very moment coursing round their path. We never see them till the earth catches them. Every thirty-three years the earth makes a haul of these meteors just as successfully as the fisherman among the herrings, and in much the same way, for while the fisherman spreads his net in which the fishes meet their doom, so the earth has an atmosphere wherein the meteors perish. We are told that there is no fear of the herrings becoming exhausted, for those the fishermen catch are as nothing compared to the profusion in which they abound in ocean. We may say the same with regard to the meteors. They exist in such myriads, that though the earth swallows up millions every thirty-three years, plenty are left for future showers. The diagram (Fig. 76) will explain the way in which the earth makes her captures. We there see the orbit in which our globe moves around the sun, as well as the elliptic path of the meteors, though it should be remarked that it is not convenient to draw the figure exactly to scale, so that the path of the meteors is relatively much larger than here represented. Once each year the earth completes its revolution, and between the 13th and the 16th of November crosses the track in which the meteors move. It will usually happen that the great shoal is not at this point when the earth is passing. There are, however, some stragglers all along the path, and the earth generally catches a few of these at this date. They dart into our atmosphere as shooting stars, and form what we usually speak of as the November meteors.