The orbit of this great planet marks the boundary of our known system of planets. We have seen how the five great planets of antiquity have been increased in these modern days by the addition of two more, Uranus and Neptune, while the discovery of a multitude of small planets has given a further increase to the number of the sun’s family. We have still some other objects in our solar system to describe; some of them are excessively big; these are the comets. Some of them are exceedingly small; they are the shooting stars. We shall talk about comets and shooting stars in our next lecture.
LECTURE V.
COMETS AND SHOOTING STARS.
The Movements of a Comet—Encke’s Comet—The Great Comet of Halley—How the Telegraph is used for Comets—The Parabola—The Materials of a Comet—Meteors—What becomes of the Shooting Stars—Grand Meteors—The Great November Showers—Other Great Showers—Meteorites.
THE MOVEMENTS OF A COMET.
The planets are all massive globes, more or less flattened at the Poles; but now we have to talk about a multitude of objects of the most irregular shapes, and of the most flimsy description. We call them comets, and they exist in such numbers that an old astronomer has said “there were more comets in the sky than fishes in the sea,” though I think we cannot quite believe him. There is also another wide difference between planets and comets: planets move round in nearly circular ellipses, and not only do we know where a planet is to-night, but we know where it was a month ago, or a hundred years ago, or where it will be in a hundred years or a thousand years to come. All such movements are conducted with conspicuous regularity and order; but now we are to speak of bodies which generally come in upon us in the most uncertain and irregular fashion. They visit us we hardly know whence, except that it is from outer space, and they are adorned in a glittering raiment, almost spiritual in its texture. They are always changing their appearance in a baffling, but still very fascinating manner. If an artist tries to draw a comet, he will have hardly finished his picture of it in one charming robe before he finds it arrayed in another. The astronomer has also his complaints to make against the comets. I have told you how thoroughly we can rely on the movements of the planets, but comets often play sad pranks with our calculations. They sometimes take the astronomers by surprise, and blaze out with their long tails just when we do not expect them. Then by way of compensation they frequently disappoint us by not appearing when they have been most anxiously looked for.
After a voyage through space the comet at length begins to draw in towards the central parts of our system, and as it approaches the sun, its pace becomes gradually greater and greater; in fact, as the body sweeps round the sun the speed is sometimes 20,000 times faster than that of an express train. It is sometimes more than 1000 times as fast as the swiftest of rifle bullets, occasionally attaining the rate of 200 miles a second. The closer the comet goes to the sun, the faster it moves; and a case has been known in which a comet, after coming in for an incalculable duration of time towards the sun, has acquired a speed so tremendous, that in two hours it has whirled round the sun and has commenced to return to the depths of outer space. This terrific outburst of speed does not last long. A pace which near the sun is 20,000 times that of our express trains diminishes to 10,000 times, to fifty times, to ten times that pace; while in the outermost part of its path the comet seems to creep along so slowly that we might think it had been fatigued by its previous exertions.
Fig. 72.—How the Comet’s Tail is disposed.
We have so often seen a stream of sparks stretching out along the track of a sky-rocket, that we might naturally suppose the tail of a comet streamed out along its path in a somewhat similar manner. This would be quite wrong. You see from [Fig. 72] that the tail does not lie along the comet’s path, but is always directed outwards from the sun. If you will draw a line from the sun to the head of the comet and follow the direction of the line, it shows the way in which the tail is arranged. You will also notice how the tail of the comet seems to grow in length as it approaches the sun. When the comet is first seen, the tail is often a very insignificant affair, but it shoots out with enormous rapidity until it becomes many millions of miles long by the time the comet is whirling round the sun. Those glories soon begin to wane as the comet flies outward; the tail gradually vanishes, and the wanderer retreats again to the depths of space in the same undecorated condition as that in which it first approached.