The Story of the Heavens, Sir Robert Stawell Ball [snow like ashes series TXT] 📗

must move in an elliptic path, and that they return every thirty-three years, but the telescope cannot follow them during their wanderings. All that we know by observation is the date of their occurrence, the point of the heavens from which they radiate, and the great return every thirty-three years. Putting these various facts together, it is possible to determine the ellipse in which the meteors move--not exactly: the facts do not go so far--they only tell us that the ellipse must be one of five possible orbits. These five possible orbits are--firstly, the immense ellipse in which we now know the meteorites do revolve, and for which they require the whole thirty-three years to complete a revolution; secondly, a nearly circular orbit, very little larger than the earth's path, which the meteors would traverse in a few days more than a year; another similar orbit, in which the time would be a few days short of the year; and two other small elliptical orbits lying inside the earth's orbit. It was clearly demonstrated by Professor Newton, of New Haven, U.S.A., that the observed facts would be explained if the meteors moved in any one of these paths, but that they could not be explained by any other hypothesis. It remained to see which of these orbits was the true one. Professor Newton himself made the suggestion of a possible method of solving the problem. The test he proposed was one of some difficulty, for it involved certain intricate calculations in the theory of perturbations. Fortunately, however, Professor Adams undertook the inquiry, and by his successful labours the path of the Leonids has been completely ascertained.

When the ancient records of the appearance of great Leonid showers were examined, it was found that the date of their occurrence undergoes a gradual and continuous change, which Professor Newton fixed at one day in seventy years. It follows as a necessary consequence that the point where the path of the meteors crosses the earth's track is not fixed, but that at each successive return they cross at a point about half a degree further on in the direction in which the earth is travelling. It follows that the orbit in which the meteors are revolving is undergoing change; the path they follow in one revolution varies slightly from that pursued in the next. As, however, these changes proceed in the same direction, they may gradually attain considerable dimensions; and the amount of change which is produced in the path of the meteors in the lapse of centuries may be estimated by the two ellipses shown in Fig. 78. The continuous line represents the orbit in A.D. 126; the dotted line represents it at present.

This unmistakable change in the orbit is one that astronomers attribute to what we have already spoken of as perturbation. It is certain that the elliptic motion of these bodies is due to the sun, and that if they were only acted on by the sun the ellipse would remain absolutely unaltered. We see, then, in this gradual change of the ellipse the influence of the attractions of the planets. It was shown that if the meteors moved in the large orbit, this shifting of the path must be due to the attraction of the planets Jupiter, Saturn, Uranus, and the Earth; while if the meteors followed one of the smaller orbits, the planets that would be near enough and massive enough to act sensibly on them would be the Earth, Venus, and Jupiter. Here, then, we see how the question may be answered by calculation. It is difficult, but it is possible, to calculate what the attraction of the planets would be capable of producing for each of the five different suppositions as to the orbit. This is what Adams did. He found that if the meteors moved in the great orbit, then the attraction of Jupiter would account for two-thirds of the observed change, while the remaining third was due to the influence of Saturn, supplemented by a small addition on account of Uranus. In this way the calculation showed that the large orbit was a possible one. Professor Adams also computed the amount of displacement in the path that could be produced if the meteors revolved in any of the four smaller ellipses. This investigation was one of an arduous character, but the results amply repaid the labour. It was shown that with the smaller ellipses it would be impossible to obtain a displacement even one-half of that which was observed. These four orbits must, therefore, be rejected. Thus the demonstration was complete that it is in the large path that the meteors revolve.

The movements in each revolution are guided by Kepler's laws. When at the part of its path most distant from the sun the velocity of a meteor is at its lowest, being then but little more than a mile a second; as it draws in, the speed gradually increases, until, when the meteor crosses the earth's track, its velocity is no less than twenty-six miles a second. The earth is moving very nearly in the opposite direction at the rate of eighteen miles a second, so that, if the meteor happen to strike the earth's atmosphere, it does so with the enormous velocity of nearly forty-four miles a second. If a collision is escaped, then the meteor resumes its onward journey with gradually declining velocity, and by the time it has completed its circuit a period of thirty-three years and a quarter will have elapsed.

The innumerable meteors which form the Leonids are arranged in an enormous stream, of a breadth very small in comparison with its length. If we represent the orbit by an ellipse whose length is seven feet, then the meteor stream will be represented by a thread of the finest sewing-silk, about a foot and a half or two feet long, creeping along the orbit.[34] The size of this stream may be estimated from the consideration that even its width cannot be less than 100,000 miles. Its length may be estimated from the circumstance that, although its velocity is about twenty-six miles a second, yet the stream takes about two years to pass the point where its orbit crosses the earth's track. On the memorable night between the 13th and 14th of November, 1866, the earth plunged into this stream near its head, and did not emerge on the other side until five hours later. During that time it happened that the hemisphere of the earth which was in front contained the continents of Europe, Asia, and Africa, and consequently it was in the Old World that the great shower was seen. On that day twelvemonth, when the earth had regained the same spot, the shoal had not entirely passed, and the earth made another plunge. This time the American continent was in the van, and consequently it was there that the shower of 1867 was seen. Even in the following year the great shoal had not entirely passed, and since then a few stragglers along the route have been encountered at each annual transit of the earth across this meteoric highway.

The diagram is also designed to indicate a remarkable speculation which was put forward on the high authority of Le Verrier, with the view of explaining how the shoal came to be introduced into the solar system. The orbit in which the meteors revolve does not intersect the paths of Jupiter, Saturn, or Mars, but it does intersect the orbit of Uranus. It must sometimes happen that Uranus is passing through this point of its path just as the shoal arrives there. Le Verrier has demonstrated that such an event took place in the year A.D. 126, but that it has not happened since. We thus seem to have a clue to a very wonderful history by which the meteors are shown to have come into our system in the year named. The expectations or a repetition of the great shower in 1899 which had been widely entertained, and on good grounds, were not realised. Hardly more than a few meteors of the ordinary type were observed.

Assuming that the orbit of the August meteors was a parabola, Schiaparelli computed the dimensions and position in space of this orbit, and when he had worked this out, he noticed that the orbit corresponded in every particular with the orbit of a fine comet which had appeared in the summer of 1862. This could not be a mere matter of accident. The plane in which the comet moved coincided exactly with that in which the meteors moved; so did the directions of the axes of their orbits, while the direction of the motion is the same, and the shortest distance from the sun to the orbit is also in the two cases identical. This proved to demonstration that there must be some profound physical connection between comets and swarms of meteors. And a further proof of this was shortly afterwards furnished, when Le Verrier had computed the orbit of the November meteors, for this was at once noticed to be precisely the same as the orbit of a comet which had passed its perihelion in January, 1866, and for which the period of revolution had been found to be thirty-three years and two months.

Among the Leonids we see occasionally fireballs brighter than Venus, and even half the apparent size of the moon, bursting out with lightning-like flashes, and leaving streaks which last from a minute to an hour or more. But the great majority are only as bright as stars of the second, third, or fourth magnitude. As the amount of light given by a meteor depends on its mass and velocity, we can form some idea as to the actual weight of one of these meteors, and it appears that most of them do not weigh nearly as much as a quarter of an ounce; indeed, it is probable that many do not weigh a single grain. But we have seen that a comet in all probability is nothing but a very loose swarm of small particles surrounded by gas of very slight density, and we have also seen that the material of a comet must by degrees be more or less dissipated through space. We have still to tell a wonderful story of the breaking up of a comet and what appears to have become of the particles thereof.

A copious meteoric shower took place on the night of the 27th November, 1872. On this occasion the shooting stars diverged from a radiant point in the constellation of Andromeda. As a spectacle, it was unquestionably inferior to the magnificent display of 1866, but it is difficult to say which of the two showers has been of greater scientific importance.

It surely is a remarkable coincidence that the earth should encounter the Andromedes (for so this shower is called) at the very moment when it is crossing the track of Biela's comet. We have observed the direction from which the Andromedes come when they plunge into the atmosphere; we can ascertain also the direction in which Biela's comet is moving when it passes the earth's track, and we find that the direction in which the comet moves and the direction in which the meteors move are identical. This is, in itself, a strong and almost overwhelming presumption that the comet and the shooting stars are connected; but it is not all. We have observations of this swarm dating back to the eighteenth century, and we find that the date of its appearance has changed from the 6th or 7th of December to the end of November in perfect accordance with the retrograde motion of the crossing-point of the earth's orbit and the orbit of Biela's comet. This comet was observed in 1772, and again in 1805-6, before its periodic return every seven years was discovered. It