CHAPTER XIX.

URANUS AND NEPTUNE.

Till the year 1781, Saturn was believed to be the outermost planet of the Solar System, and nobody suspected the fact of two great, lonely brother-planets wandering around the same sun at vast distances beyond,--Uranus, nine hundred millions of miles from Saturn; Neptune, nine hundred millions of miles from Uranus. No wonder they remained long undiscovered.

Uranus can sometimes be seen by the unaided eye as a dim star of the sixth magnitude. And when he was known for a planet, it was found that he _had_ been often so seen and noted. Again and again he had been taken for a fixed star, and as he moved on, disappearing from that particular spot, it was supposed that the star had vanished.

One night, March 13, 1781, when William Herschel was busily exploring the heavens with a powerful telescope, he noticed something which he took for a comet without a tail. He saw it was no mere point of light like the stars, but had a tiny round disk or face, which could be magnified. So he watched it carefully, and found in the course of a few nights that it moved--very slowly, certainly; but still it did move. Further watching and calculation made it clear that, though the newly-found heavenly body was at a very great distance from the sun, yet it was moving slowly in an orbit _round_ the sun. Then it was known to be a planet, and another member of the Solar System.

From that day the reputation of Herschel rapidly increased. King George III, who loved the sciences and patronized them, had the astronomer presented to him; charmed with the simple and modest account of his efforts and his labors, he secured him a life pension and a residence at Slough, in the neighborhood of Windsor Castle. His sister Caroline assisted him as secretary, copied all his observations, and made all his calculations; the king gave him the title and salary of assistant astronomer. Before long the observatory at Slough surpassed in celebrity the principal observatories of Europe; we may say that, in the whole world, this is the place where the most discoveries have been made.

Astronomers soon applied themselves to the observation of the new body. They supposed that this “comet” would describe, as usually happens, a very elongated ellipse, and that it would approach considerably to the sun at its perihelion. But all the calculations made on this supposition had to be constantly recommenced. They could never succeed in representing all its positions, although the star moved very slowly: the observations of one month would utterly upset the calculations of the preceding month.

Several months elapsed without a suspicion that a veritable planet was under observation, and it was not till after recognizing that all the imaginary orbits for the supposed comet were contradicted by the observations, and that it had probably a circular orbit much farther from the sun than Saturn, till then the frontier of the system, that astronomers came to consider it as a planet. Still, this was at first but a provisional consent.

[Illustration: PROMINENT ASTRONOMERS OF FORMER TIMES. Tycho Brahe, Hipparchos, Galilei, Newton, Kepler, Kant, Kopernikus, Laplace.]

It was, in fact, more difficult than we may think to increase without scruple the family of the sun. Indeed, for reasons of expediency this idea was opposed. Ancient ideas are tyrannical. Men had so long been accustomed to consider old Saturn as the guardian of the frontiers, that it required a rare boldness of spirit to decide on extending these frontiers and marking them by a new world.

William Herschel proposed the name of _Georgium Sidus_ “the Star of George,” just as Galileo had given the name of “Medicean stars” to the satellites of Jupiter discovered by him, and as Horace had said _Julium Sidus_. Others proposed the name of _Neptune_, in order to maintain the mythological character, and to give to the new body the trident of the English maritime power; others, _Uranus_, the most ancient deity of all, and the father of Saturn, to whom reparation was due for so many centuries of neglect. Lalande proposed the name of _Herschel_, to immortalize the name of its discoverer. These last two denominations prevailed. For a long time the planet bore the name of Herschel; but custom has since declared for the mythological appellation, and Jupiter, Saturn, and Uranus succeed each other in order of descent--son, father, and grandfather.

The discovery of Uranus extended the radius of the Solar System from 885,000,000 to 1,765,000,000 of miles.

The apparent brightness of this planet is that of a star of the sixth magnitude; observers whose sight is very piercing may succeed in recognizing it with the naked eye when they know where to look for it. Uranus moves slowly from west to east, and takes no less than eighty-four years to make the complete circuit of the sky. In its annual motion round the sun the earth passes between the sun and Uranus every 369 days--that is to say, once in a year and four days. It is at these times that this planet crosses the meridian at midnight. We can observe it in the evening sky for about six months of every year.

Everybody supposed that now, at least, the outermost member of all was discovered. But a very strange and remarkable thing happened.

Astronomers know with great exactness the paths of the planets in the heavens. They can tell, years beforehand, precisely what spot in space will be filled at any particular time by any particular planet. I am speaking now of their movements round the sun and in the Solar System--not of the movements of the whole family with the sun, about which little is yet known.

Each planet has its own particular pathway; its own particular distance from the sun, varying at each part of its pathway; its own particular speed in traveling round the sun, changing constantly from faster to slower or slower to faster, according to its distance from the sun, and according to the pull backwards or forwards of our neighboring planets in front or in rear. For as the orbits of all the planets are ovals, with the sun not in the middle, but somewhat to one side of the middle, it follows that all the planets, in the course of their years, are sometimes nearer to and sometimes farther from the sun.

The astronomers of the present day understand this well, and can describe with exactness the pathway of each planet. This knowledge does not come merely from watching one year how the planets travel, and remembering for another year, but is much more a matter of close and difficult calculation. Many things have to be considered, such as the planet’s distance from the sun, the sun’s power of attraction, the planet’s speed, the nearness and weight of other neighboring planets.

All these questions were gone into, and astronomers sketched out the pathway in the heavens which they expected Uranus to follow. He would move in such and such an orbit, at such and such distances from the sun, and at such and such rates of speed.

But Uranus would not keep to these rules. He quite discomfited the astronomers. Sometimes he went fast, when, according to their notions, he ought to have gone more slowly; and sometimes he went slowly, when they would have looked for him to go more fast, and the line of his orbit was quite outside the line of the orbit which they had laid down. He was altogether a perplexing acquaintance, and difficult to understand. However, astronomers felt sure of their rules and modes of calculation, often before tested, and not found to fail. They made a guess at an explanation. What if there were yet another planet beyond Uranus, disturbing his motions--now drawing him on, now dragging him back, now so far balancing the sun’s attraction by pulling in the opposite direction as to increase the distance of Uranus from the sun?

It might be so. But who could prove it? Hundreds of years might pass before any astronomer, in his star-gazing, should happen to light upon such a dim and distant world. Nay, the supposed planet might be, like Uranus, actually seen, and only be mistaken for a “variable star,” shining but to disappear.

There the matter seemed likely to rest. There the matter probably would have rested for a good while, had not two men set themselves to conquer the difficulty. One was a young Englishman, a student of Cambridge, John Couch Adams; the other, a young Frenchman, Urbain Jean Joseph Leverrier,--both being astronomers.

Each worked independently of the other, neither knowing of the other’s toil. The task which they had undertaken was no light one--that of reaching out into the unknown depths of space to find an unknown planet.

Each of these silent searchers into the sky-depths calculated what the orbit and speed of Uranus would be, without the presence of another disturbing planet beyond. Each examined what the amount of disturbance was, and considered the degree of attraction needful to produce that disturbance, together with the direction from which it had come. Each, in short, gradually worked his way through calculations far too deep and difficult for ordinary minds to grasp, till he had found just that spot in the heavens where a planet _ought_ to be, to cause, according to known laws, just such an effect upon Uranus as had been observed.

Adams finished his calculation first, and sent the result to two different observatories. Unfortunately, his report was not eagerly taken up. It was, in fact, hardly believed. Leverrier finished his calculation also, and sent the result to the Berlin Observatory. The planet was actually seen in England first, but the discovery was actually made known from Berlin first. The young Englishman had been beforehand, but the young Frenchman gained foremost honor.

It was on the 23d of September, 1846, that Leverrier’s letter reached the Berlin astronomers. The sky that night was clear, and we can easily understand with what anxiety Dr. Galle directed his telescope to the heavens. The instrument was pointed in accordance with Leverrier’s instructions. The field of view showed, as does every part of the heavens, a multitude of stars. One of these was really the planet. The new chart--prepared by the Berlin Academy of Sciences, upon which the place of every star, down even to those of the tenth magnitude, was engraved--was unrolled, and, star by star, the heavens were compared with the chart. As the process of identification went on, one object after another was found in the heavens as engraved on the chart, and was of course rejected. At length a star of the eighth magnitude--a brilliant object--was brought into review. The map was examined, but there was no star there. This object could not have been in its present place when the map was formed. The object was therefore a wanderer--a planet. Yet it is necessary to be excessively cautious in such a matter.

Many possibilities had to be guarded against. It was, for instance, possible that the object was really a star which, by some mischance, eluded the careful eye of the astronomer who constructed the map. It was even possible that the star might be one of the large class of variables which alternate in brightness, and it might have been conceivable that it was too faint to be seen when the chart was made. Even if neither of these explanations would answer, it was still necessary to show that the object was now moving, and moving with that particular velocity and in that particular direction which the theory of Leverrier indicated. The lapse of a single day was sufficient to dissipate all doubts. The next night the object was again observed. It had moved, and when its motion was measured it was found to accord precisely with what Leverrier had foretold. Indeed, as if no circumstance in the confirmation should be wanting, it was ascertained that the diameter of the planet, as measured by the micrometers at Berlin, was practically coincident with that anticipated by Leverrier.

The world speedily rang with the news of this splendid achievement. Instantly the name of Leverrier rose to a pinnacle hardly surpassed by that of any astronomer of any age or country. For a moment it seemed as if the French nation were to enjoy the undivided honor of this splendid triumph. But in the midst of the pæans of triumph with which the enthusiastic French nation hailed the discovery of Leverrier, there appeared a letter from Sir John Herschel in the _Athenæum_ for 3d October, 1846, in which he announced the researches made by Adams, and claimed for him a participation in the glory of the discovery. Subsequent inquiry has shown that this claim was a just one, and it is now universally admitted by all independent authorities.

This, however, was of slight comparative importance. The truly wonderful part of the matter was, that these two men could have so reasoned that, from the movements of one lately-discovered planet, they could point out the exact spot where a yet more distant planet ought to be, and that close to this very spot the planet was found. For when, both in England and in Germany, powerful telescopes were pointed in the direction named--_there the planet was_. No doubt about the matter. Not a star, but a real new planet in the far distance, wandering slowly round the sun.

Here we have the discovery of Neptune in its simple grandeur. This discovery is splendid, and of the highest order from a philosophical point of view, for it proves the security and the precision of the data of modern astronomy. Considered from the point of view of practical astronomy, it was but a simple exercise of calculation, and the most eminent astronomers saw in it nothing else! It was only after its verification, its public demonstration--it was only after the visual discovery of Neptune--that they had their eyes opened, and felt for a moment the dizziness of the infinite in view of the horizon revealed by the Neptunian perspective. The author of the calculation himself, the transcendent mathematician, did not even give himself the trouble to take a telescope and look at the sky to see whether a planet was really there! I even believe that he never saw it. For him, however, then and always, to the end of his life, astronomy was entirely inclosed in formulæ--the stars were but centers of force.

Very often I submitted to him the doubts of an anxious mind on the great problems of infinitude: I asked him if he thought that the other planets might be inhabited like ours, what might be especially the strange vital conditions of a world separated from the sun by the distance of Neptune, what might be the retinue of innumerable suns scattered in immensity, what astonishing colored lights the double stars should shed on the unknown planets which gravitate in these distant systems. His replies always showed me that these questions had no interest for him, and that, in his opinion, the essential knowledge of the universe consisted in equations, formulæ, and logarithmic series having for their object the mathematical theory of velocities and forces.

But it is not the less surprising that he had not the _curiosity_ to verify the position of his planet, which would have been easy, since it shows a planetary disk; and, besides, he might have had the aid of a chart, because he had only to ask for these charts from the Berlin Observatory, where they had just been finished and _published_. It is not the less surprising that Arago, who was more of a physicist than a mathematician, more of a naturalist than a calculator, and whose mind had so remarkable a synthetical character, had not himself directed one of the telescopes of the observatory towards this point of the sky, and that no other French astronomer had this idea. But what surprises us still more is to know that _nearly a year before_, in October, 1845, a young student of the University of Cambridge, Mr. Adams, had sought the solution of the _same_ problem, obtained the same results, and communicated these results to the director of the Greenwich Observatory, and that the astronomer to whom these results were confided had said nothing, and had not himself searched in the sky for the optical verification of his compatriot’s solution!

This was indeed a triumph of human intellect. Yet it is no matter for human pride, but rather of thankfulness to God, who gave to man this marvelous reasoning power. And the very delight we have in such a success may humble us in the recollection of the vast amount lying beyond of the utterly unknown. But while the rare mind of a Newton could meekly realize the littleness of all he knew, lesser minds are very apt to be puffed up with the thought of what the human intellect can accomplish.

Perhaps the chief feeling of lawful satisfaction in this particular discovery arises from the fact that it gives marked and strong proof of the truth of our present astronomical system and beliefs. Many mistakes may be made, and much has often to be unlearned. Nevertheless, if the general principles of modern astronomy were wrong, if the commonly-received facts were a delusion, such complete success could not have attended so delicate and difficult a calculation.

We do not know much about these two outer planets, owing to their enormous distance from us. Uranus is in size equal to seventy-four earths, and Neptune is in size equal to one hundred and five earths. Both these planets are formed of somewhat heavier materials than Saturn, being about as dense as water.

The size of the sun as seen from Uranus is about one three-hundred-and-ninetieth part of the size of the sun we see. To Neptune he shows a disk only one nine-hundredth part of the size of that visible to us--no disk at all, in fact, but only starlike brilliancy.

The Uranian year lasts about eighty-four of our years; and this, with a very sloping axis, must cause most long and dreary winters, the tiny sun being hidden from parts of the planet during half an earthly life-time.

Uranus has at least four moons, traveling in very different planes from the plane of the ecliptic. Once it was thought that he had eight, but astronomers have since searched in vain for the other four, believed for a while to exist. Neptune has one moon, and may possess others not yet discovered.

Sir Henry Holland, the celebrated physician, and a devoted student of astronomy, has left on record an incident of his life connected with the planet Neptune of singular interest. I give it here, and in his own words, because it is scarcely likely otherwise to fall under the notice of astronomical readers. After stating that his interest in astronomy had led him to take advantage of all opportunities of visiting foreign observatories, he says:

“Some of these opportunities, indeed, arising out of my visits to observatories both in Europe and America, have been remarkable enough to warrant a more particular mention of them. That which most strongly clings to my memory is an evening I passed with Encke and Galle in the observatory at Berlin, some ten or twelve days after the discovery of the planet Neptune on this very spot; and when every night’s observations of its motions had still an especial value in denoting the elements of its orbit. I had casually heard of the discovery at Bremen, and lost no time in hurrying on to Berlin. The night in question was one of floating clouds, gradually growing into cumuli; and hour after hour passed away without sight of the planet which had just come to our knowledge by so wonderful a method of predictive research. Frustrated in this main point, it was some compensation to stay and converse with Encke in his own observatory, one signalized by so many discoveries, the stillness and darkness of the place broken only by the solemn ticking of the astronomical clock, which, as the unfailing interpreter of the celestial times and motions, has a sort of living existence to the astronomer. Among other things discussed, while thus sitting together in a sort of tremulous impatience, was the name to be given to the new planet. Encke told me he had thought of ‘Vulcan,’ but deemed it right to remit the choice to Leverrier, then supposed to be the sole indicator of the planet and its place in the heavens; adding that he expected Leverrier’s answer by the first post. Not an hour had elapsed before a knock at the door of the observatory announced the letter expected. Encke read it aloud; and, coming to the passage where Leverrier proposed the name of ‘Neptune,’ exclaimed, ‘_So lass den Namen Neptun sein._’

It was a midnight scene not easily to be forgotten. A royal baptism, with its long array of titles, would ill compare with this simple naming of the remote and solitary planet thus wonderfully discovered. There is no place, indeed, where the grandeur and wild ambitions of the world are so thoroughly rebuked and dwarfed into littleness as in the astronomical observatory. As a practical illustration of this remark, I would add that my own knowledge of astronomers--those who have worked themselves with the telescope--has shown them to be generally men of tranquil temperament, and less disturbed than others by worldly affairs, or by the quarrels incident even to scientific research.”

The same reasoning which has been used in reference to Jupiter and his moons, and to Saturn and his moons, might perhaps be applied also to Uranus and Neptune and their moons.

We know too little yet of their condition to venture far in such speculations. Still, taking the matter as a whole, there seem many reasons to incline us to the idea that each of the four greater outside planets may be a kind of secondary half-cooled sun to his satellites, helping to make up to them for the small amount of light and heat which they can obtain from the far-off sun.