The Story of the Heavens by Sir Robert Stawell Ball (fantasy books to read .txt) 📕

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.

It will occasionally happen that when the earth is in the act of crossing the track it encounters the bulk of the meteors. Through the shoal our globe then plunges, enveloped, of course, with the surrounding coat of air. Into this net the meteors dash in countless myriads, never again to emerge. In a few hours' time, the earth, moving at the rate of eighteen miles a second, has crossed the track and emerges on the other side, bearing with it the spoils of the encounter. Some few meteors, which have only narrowly escaped capture, will henceforth bear evidence of the fray by moving in slightly different orbits, but the remaining meteors of the shoal continue their journey without interruption; perhaps millions have been taken, but probably hundreds of millions have been left.

Such was the occurrence which astonished the world on the night between November 13th and 14th, 1866. We then plunged into the middle of the shoal. The night was fine; the moon was absent. The meteors were distinguished not only by their enormous multitude, but by their intrinsic magnificence. I shall never forget that night. On the memorable evening I was engaged in my usual duty at that time of observing nebulae with Lord Rosse's great reflecting telescope. I was of course aware that a shower of meteors had been predicted, but nothing that I had heard prepared me for the splendid spectacle so soon to be unfolded. It was about ten o'clock at night when an exclamation from an attendant by my side made me look up from the telescope, just in time to see a fine meteor dash across the sky. It was presently followed by another, and then again by more in twos and in threes, which showed that the prediction of a great shower was likely to be verified. At this time the Earl of Rosse (then Lord Oxmantown) joined me at the telescope, and, after a brief interval, we decided to cease our observations of the nebulae and ascend to the top of the wall of the great telescope (Fig. 7, p. 18), whence a clear view of the whole hemisphere of the heavens could be obtained. There, for the next two or three hours, we witnessed a spectacle which can never fade from my memory. The shooting stars gradually increased in number until sometimes several were seen at once. Sometimes they swept over our heads, sometimes to the right, sometimes to the left, but they all diverged from the east. As the night wore on, the constellation Leo ascended above the horizon, and then the remarkable character of the shower was disclosed. All the tracks of the meteors radiated from Leo. (_See_ Fig. 74, p. 368.) Sometimes a meteor appeared to come almost directly towards us, and then its path was so foreshortened that it had hardly any appreciable length, and looked like an ordinary fixed star swelling into brilliancy and then as rapidly vanishing. Occasionally luminous trains would linger on for many minutes after the meteor had flashed across, but the great majority of the trains in this shower were evanescent. It would be impossible to say how many thousands of meteors were seen, each one of which was bright enough to have elicited a note of admiration on any ordinary night.

The adjoining figure (Fig. 77) shows the remarkable manner in which the shooting stars of this shower diverged from a point. It is not to be supposed that all these objects were in view at the same moment. The observer of a shower is provided with a map of that part of the heavens in which the shooting stars appear. He then fixes his attention on one particular shooting star, and observes carefully its track with respect to the fixed stars in its vicinity. He then draws a line upon his map in the direction in which the shooting star moved. Repeating the same observation for several other shooting stars belonging to the shower, his map will hardly fail to show that their different tracks almost all tend from one point or region of the figure. There are, it is true, a few erratic ones, but the majority observe this law. It certainly looks, at first sight, as if all the shooting stars did actually dart from this point; but a little reflection will show that this is a case in which the real motion is different from the apparent. If there actually were a point from which these meteors diverged, then from different parts of the earth the point would be seen in different positions with respect to the fixed stars; but this is not the case. The radiant, as this point is called, is seen in the same part of the heavens from whatever station the shower is visible.

We are, therefore, led to accept the simple explanation afforded by the theory of perspective. Those who are acquainted with the principles of this science know that when a number of parallel lines in an object have to be represented in a drawing, they must all be made to pass through the same point in the plane of the picture. When we are looking at the shooting stars, we see the projections of their paths upon the surface of the heavens. From the fact that those paths pass through the same point, we are to infer that the shooting stars belonging to the same shower are moving in parallel lines.

We are now able to ascertain the actual direction in which the shooting stars are moving, because a line drawn from the eye of the observer to the radiant point must be parallel to that direction. Of course, it is not intended to convey the idea that throughout all space the shooting stars of one shower are moving in parallel lines; all we mean is that during the short time in which we see them the motion of each of the shooting stars is sensibly a straight line, and that all these straight lines are parallel.

In the year 1883 the great meteor shoal of the Leonids (for so this shower is called) attained its greatest distance from the sun, and then commenced to return. Each year the earth crossed the orbit of the meteors; but the shoal was not met with, and no noteworthy shower of stars was perceived. Every succeeding year found the meteors approaching the critical point, and the year 1899 brought the shoal to the earth's track. In that year a brilliant meteoric shower was expected, but the result fell far short of expectation. The shoal of meteors is of such enormous length that it takes more than a year for the mighty procession to pass through the critical portion of its orbit which lies across the track of the earth. We thus see that the meteors cannot escape the earth. It may be that when the shoal begins to reach this neighbourhood the earth will have just left this part of its path, and a year will have elapsed before the earth gets round again. Those meteors that have the good fortune to be in the front of the shoal will thus escape the net, but some of those behind will not be so fortunate, and the earth will again devour an incredible host. It has sometimes happened that casts into the shoal have been obtained in two consecutive years. If the earth happened to pass through the front part in one year, then the shoal is so long that the earth will have moved right round its orbit of 600,000,000 miles, and will again dash through the critical spot before the entire number have passed. History contains records of cases when, in two consecutive Novembers, brilliant showers of Leonids have been seen.

As the earth consumes such myriads of Leonids each thirty-three years, it follows that the total number must be decreasing. The splendour of the showers in future ages will, no doubt, be affected by this circumstance. They cannot be always so bright as they have been. It is also of interest to notice that the shape of the shoal is gradually changing. Each meteor of the shoal moves in its own ellipse round the sun, and is quite independent of the rest of these bodies. Each one has thus a special period of revolution which depends upon the length of the ellipse in which it happens to revolve. Two meteors will move around the sun in the same time if the lengths of their ellipses are exactly equal, but not otherwise. The lengths of these ellipses are many hundreds of millions of miles, and it is impossible that they can be all absolutely equal. In this may be detected the origin of a gradual change in the character of the shower. Suppose two meteors A and B be such that A travels completely round in thirty-three years, while B takes thirty-four years. If the two start together, then when A has finished the first round B will be a year behind; the next time B will be two years behind, and so on. The case is exactly parallel to that of a number of boys who start for a long race, in which they have to run several times round the course before the distance has been accomplished. At first they all start in a cluster, and perhaps for the first round or two they may remain in comparative proximity; gradually, however, the faster runners get ahead and the slower ones lag behind, so the cluster becomes elongated. As the race continues, the cluster becomes dispersed around the entire course, and perhaps the first boy will even overtake the last. Such seems the destiny of the November meteors in future ages. The cluster will in time come to be spread out around the whole of this mighty track, and no longer will a superb display have to be recorded every thirty-three years.

It was in connection with the shower of November meteors in 1866 that a very interesting and beautiful discovery in mathematical astronomy was made by Professor Adams. We have seen that the Leonids