Part 15

You must use a very good telescope to see the satellites of Uranus. They are four in number, bearing the names of Ariel, Umbriel, Titania, and Oberon. The innermost of these, Ariel, completes a journey round the planet in two days and a half; Oberon, the most distant, requires thirteen days and a half. A planet is always tending to pull its satellite down, and the satellite is kept from falling by the speed with which it revolves. The heavier the planet, the faster must its satellites go round. Thus, to take an illustration from our own moon, we know that, if the earth were to be made four times heavier than it is, the moon would have to spin round twice as fast as it does, in order to remain in the same orbit. The speed with which the satellites of Uranus revolve accordingly affords a measure of the mass of the planet. Were Uranus heavier than he is, his satellites would revolve more quickly than they do; were he lighter, the satellites would take a longer period to go round.

Uranus also seems to be greatly swollen by clouds, in the same manner as are both Jupiter and Saturn; in fact, if our earth was as big as Uranus, it would weigh four or five times as much as Uranus does. Hence we are certain that Uranus must consist of materials less dense on the whole than are those of which our earth is made.

There is another singular circumstance connected with the moons of Uranus. I have told you how every body revolving round another by gravitation will describe an ellipse; but, of course, there are many different kinds of this curve, and some may be nearly circles. There is nothing whatever to prevent a satellite from revolving around its primary in an exact circle if it be started properly; that is, in the right direction and with the right speed. All the four satellites of this planet seem to revolve in circles so perfect that we can make an accurate picture of this system with a pair of compasses. It is further to be noticed that the four circles seem to lie exactly in the same plane. The orbits of the other great planets and of their satellites lie in planes inclined at angles of less than 35° to the ecliptic, the plane in which the earth moves. Here again the satellites of Uranus are exceptional. The plane in which they are contained stands up almost squarely to the plane in which the motion of the planet is performed. The moons of Uranus seem to have got a twist, from some accidental circumstance for which we are not able to account.

THE DISCOVERY OF NEPTUNE.

The boundaries of the solar system had been much extended by the discovery of Uranus, but they were destined to receive still further enlargement by the detection of another vast planet, revolving far outside Uranus, the orbit of which forms, according to our present knowledge, the outline of the planetary system.

I have here to describe one of the greatest discoveries that have ever been made. It is not the magnificence of the outermost planet itself that I refer to, though, indeed, it is bigger than Uranus. I am rather thinking of the _way_ in which the discovery was made. I do not mean any disrespect to Herschel when I say that the discovery of Uranus was chiefly a stroke of good fortune; but I may be permitted to describe it in this manner by way of emphasizing as strongly as I can how utterly different was the train of ideas which led to the discovery of Neptune. Herschel merely looked at one star after another till suddenly he dropped on the planet, having beforehand not the slightest notion that any such planet was likely to exist. But Neptune was shown to exist before it was ever seen, and, in fact, the man that first saw the planet, and knew it to be a planet, was not the discoverer. This is rather a difficult subject; and it would take you years of hard study to be able to understand the train of reasoning by which Neptune was found. I shall, however, make an attempt to explain this matter sufficiently to give at least some idea of the kind of problem that had to be solved.

You will remember that law of Kepler which tells us that every planet moves round the sun in an ellipse. If the planet be uninterfered with in any way and guided only by the attraction of the sun, it will forever continue to describe precisely the same ellipse without the slightest alteration. It was ascertained that the path which Uranus followed was not always regular. The early observations of the planet, when it was mistaken for a star, have here been of the utmost service. They have indicated the ellipse which Uranus described the last time it went round, and our modern observations have taught us the path which the planet is at present describing. These two ellipses are slightly different, and the consequence is that, supposing we take the observations of Uranus made 100 years ago, and calculate from them where Uranus ought to be now, we find that the planet is a little astray. Astronomers are not accustomed to be wrong in such calculations, and when discrepancies arise, the first thing to be done is to see what has caused them. It is certain that the position in which Uranus is found this very night, for example, is not what it would have been had the sun alone been guiding the planet. Perhaps you will think that it is impossible for reliable computations to be made about such matters; but I assure you they can, and the very fact that the motion of Uranus appeared to be irregular made it interesting to try and find out the cause of the disturbance.

I have already explained, when speaking about Mars (p. 187), that there is an attraction between every two bodies, but in the group of planets to which the earth belongs the sun’s attraction is so much stronger than any other force that all the movements are guided mainly by it. Nevertheless it is true that not only does the sun pull our earth and all the other planets as well, but all the planets, including the earth, are pulling one another. In fact, there is an incessant struggle going on in the family party. Fortunately the sun is so much more powerful than any other member, that he keeps them all pretty well in order; and unless you look very carefully you will not see the effects of the little struggles that are going on between every pair of the system. Our earth itself is pulled and swayed to and fro by the actions of its brothers and sisters. It is dragged perhaps a thousand or two thousand miles this way by Jupiter, or it gets a good tug in the other direction by Venus. Mars and Saturn also do their little best to force the earth away from its strict path. However, our earth does not suffer much from these irregularities. It pursues its route fairly enough, just as a coach from London to Brighton will get safely to its destination notwithstanding the fact that it has to swerve a little from its path whenever it meets other vehicles on the way, or when the coachman wishes to avoid a piece of the road on which stones have been freshly laid down.

The track followed by Uranus was found to be somewhat irregular, like that of every other planet. Jupiter gave it a pull, and so did Saturn, and at first it was thought that the irregularities which were perceived could be explained by the action of these planets, so big and so well known. Here is a question for calculation; it involves a very long and a very hard piece of work, but it is possible to estimate how far each of the other planets is capable of dragging Uranus from its path. Is it not remarkable that by working out long calculations we should be able to find what one planet hundreds of millions of miles away was able to do to another planet still further off, and not only for to-day or yesterday, but for past time extending over more than a century? If, however, you will listen to me a little longer, I think I shall give you a proof that these sums could be worked out correctly.

When the calculations had been made which showed how much the known planets could disturb Uranus, it was found that there were still some deviations of the planet that remained unexplained. They were not large; they only amounted to showing that the body was just a little astray from the spot where the calculations indicated it should be. The rest of astronomy was so perfect, and the law of attraction prevailed so universally, that it was thought the law of attraction must provide some way of explaining the behavior of Uranus. He could not have left his track of his own accord; therefore there must be some agency at work upon him of which we did not know. What could this unknown source of disturbance be? Every such trouble had hitherto been found to be a consequence of the attraction of gravitation; therefore there must be some unknown body pulling at Uranus which no one had ever seen. Where could it be? How was it to be discovered? Such were the questions that were asked, and they were answered in a most satisfactory manner.

First of all, what sort of body could it be that was pulling Uranus? It is obvious that none of the stars would be competent to produce so great an effect; they are all so far off that they have nothing whatever to say to any of the domestic matters in our little solar system, which is simply a group by itself. It would be more reasonable to suppose that there must be yet another planet which nobody had ever recognized, but which affected Uranus so as to account for his truant behavior. To begin to search for this planet with telescopes without some guidance would be futile; in fact, astronomers had been scanning the heavens for planets for nearly fifty years, and though several had been discovered, they all belonged to the zone of little planets, and none of them were big enough to pull Uranus about appreciably. Of course, if all the stars could be blotted out of the sky, so that nothing but planets were left, then, by sweeping the telescope over the heavens, every planet that exists might be speedily found. The difficulty is that the planets, which are either small or very distant, look so like the stars that it is impossible to recognize them among the millions of glittering points in the sky. It was, however, hoped that the unknown planet would be large enough to be visible in the telescope, if only we knew exactly where to point it.

Two illustrious astronomers, Adams of Cambridge, and Leverrier of Paris, both separately undertook an astonishing piece of calculation. They tried to find out the position of the unknown planet from the mere fact that it deranged Uranus in a particular way. I dare say many of those who are reading this book have learned simple equations in algebra, and they have worked such questions as to find the length of a pole, half of which is in mud, a quarter in water, and ten feet above the water. Those who know this much can perhaps realize the problem that had to be solved in trying to discover the unknown planet. So difficult a question as this had to be solved in a way that your masters would hardly allow you to use when working your sums in algebra. I do not think they would let you make a series of guesses. Let us try 20 feet, for instance, as the length of the pole; that will make 10 feet in the mud, 5 feet in the water, and 5 feet outside. This will not do; it is not enough; we must try again; and after another guess or two, we see that a pole 40 feet long will exactly answer. We do not use this method of guessing in algebra, because solving the simple equation is a much better method. Adams and Leverrier found that to discover the unknown planet was a question so very difficult, that they were obliged to use a sort of guessing, but very intelligent guessing, I need hardly assure you. They proceeded in this way (Fig. 71). They would draw a circle outside the path of Uranus, and then suppose that a planet was revolving in that circle. Its effect upon Uranus would then be calculated, and it would be found whether the observed irregularities could be in this manner accounted for. The first planet they tried was not the right one; then they began again with another, until at last, after many trials and much very hard work, they saw that there might be a planet in a particular path far outside Uranus, such that if this planet were of the right weight and moving with the right speed, then it would pull Uranus exactly in the way that astronomers had observed it to be pulled. They found at last that there could be little doubt about the matter; for this unknown body would account for all the facts. Then, indeed, they had solved their equation; they had found the unknown.

The two great astronomers had thus discovered a planet, but as yet it was only a planet on paper. Those who could judge of the subject had no doubt that the planet was really in the sky; but just as you like to prove that you have found the correct answer to your sum, so people were naturally anxious to prove the truth of this wonderful sum that Adams and Leverrier had worked out. This was to be done by actually seeing the planet of which the astronomers had asserted the existence. Leverrier calculated that the new planet in a certain night would be in a particular position on the sky. Accordingly he wrote to Dr. Galle, of the observatory at Berlin, requesting him on the evening in question to point his telescope to the very spot indicated, and there he would see a planet which human eyes had never before beheld. Of course, Dr. Galle was only too delighted to undertake so marvellous a commission. The evening was fine; the telescope was opened; it was directed towards the heavens; and there, in the very spot which the calculations of Leverrier had indicated, shone the beautiful little planet. At Cambridge arrangements had also been made to search for the new member of the solar system, in accordance with Professor Adams’ calculations. There also the planet that had given all this trouble to Uranus was brought to light. At first it looked like a star, as all such planets do; but that it was not a star was speedily proved, by the two tests which are sure indications of a planet. First the body was so moving that its position with respect to the adjacent stars was constantly changing. Then, when a strong magnifying power was placed on the telescope, the little object was seen, not to be a mere starlike point, but to expand into the little disk which shows us we are not looking at a distant sun, but at a world like our own.

Was not this truly a great discovery? Have we not shown you how entitled the calculations of astronomers are to our respect, when we find that they actually discovered the existence of a majestic planet before the telescope had revealed it? See also the greatly increased interest that belongs to Herschel’s discovery of Uranus. We can hardly imagine anything that would have given more gratification to this old astronomer than to think that his Uranus should have given rise to a discovery even more splendid than his own. He died, however, more than twenty years before this achievement.

The authorities who decide on such matters christened the new planet Neptune; and this body wanders round on the outskirts of our solar system, requiring for each journey a period of no less than 165 years. The circle thus described has a radius thirty times as great as that of the earth’s track.

Neptune is altogether invisible to the unaided eye, but it is sufficiently bright to have been occasionally recorded as a star. Indeed, nearly fifty years before it was actually discovered to be a planet it had been included by the astronomer Lalande in a list of stars he was observing. A curious circumstance was afterwards brought to light. When reference was made to the books in which Lalande’s observations were written, it was found that he had observed this object twice, namely, on May 8 and May 10, 1785. Of course, if the object had indeed been a star its position on the two days would have been the same, but being a planet it had moved. When Lalande, on looking over his papers, saw that the places of this supposed star were different on the two nights, he concluded that he must have made a mistake on the first night, and accordingly treated the object as if the place on the 10th was the right one. Just think how narrowly Lalande missed making a discovery! Unhappily for his renown, he took it for granted that one or both of his observations were erroneous, and so they must have been if the object had been a star. But they were both right; it was the planet which had moved in the interval.

As Neptune is half as far again from the earth as Uranus, we can hardly expect to learn much about the actual nature of the planet. We do know that it has four times the diameter of the earth, so that it exceeds the earth in the same proportion that the earth is larger than the moon.

Like the other great planets, Neptune is also enveloped with copious clouds; in fact, it only weighs one-fifth part as much as it would do if it were made of materials as substantial as are those of the earth. Like our earth, Neptune is attended by one moon, which revolves round the planet in a little more than six days.

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.

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.

When a comet appears, it is always a matter of interest to see whether it is an entirely new object, or whether it may not be only another return of a comet which has paid us one or more previous visits. The question then arises as to how they are to be identified. Here we see a wide contrast between unsubstantial bodies like comets and the weighty and stately planets. Sketches of the various planets or of the face of the sun, though they might show slight differences from time to time, are still always sufficiently characteristic, just as a photographic portrait will identify the individual, even though the lapse of years will bring some changes in his appearance. But the drawing of a comet is almost useless for identification. You might as well try to identify a cloud or a puff of smoke by making a picture of it. Make a drawing of a comet at one appearance, and sketch particularly the ample tail with which it is provided. The next time the comet comes round it may very possibly have two tails, or possibly no tail at all. We are therefore unable to place any reliance on the comet’s personal appearance in our efforts to identify it. The highway which it follows through the sky affords the only means of recognition; for the comet, if undisturbed by other objects, will never change its actual orbit. But even this method of identification often fails, for it not unfrequently happens that during its erratic movements the comet gets into fearful trouble with other heavenly bodies. In such cases the poor comet is sometimes driven so completely out of its road that it has to make for itself an entirely new path, and our efforts to identify it are plunged in confusion. It has happened that a second comet or even a third will be found in nearly the same track, but whether these are wholly different, or whether they are merely parts of the same original object, it is often impossible to determine.