Star-land: Being Talks With Young People About the Wonders of the Heavens

by Robert S. Ball

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.