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

by Robert S. Ball

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.

Fig. 71.—Orbits of Uranus and Neptune.

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.