URANUS AND NEPTUNE

Hitherto we have been dealing with bodies which, from time immemorial, have been known to man as planets. There must have been a period when one by one the various members of our system known to the ancients were discriminated from the fixed stars by unknown but patient and skilful observers; but, from the dawn of historical astronomy, up to the night of March 13, 1781, there had been no addition to the number of those five primary planets the story of whose discovery is lost in the mists of antiquity.

It may be questioned whether any one man, Kepler and Newton being possible exceptions, has ever done so much for the science of astronomy as was accomplished by Sir William Herschel. Certainly no single observer has ever done so much, or, which is almost more important than the actual amount of his achievement, has so completely revolutionized methods and ideas in observing.

A Hanoverian by birth, and a member of the band of the Hanoverian Guards, Herschel, after tasting the discomforts of war in the shape of a night spent in a ditch on the field of Hastenbeck, where that egregious general the Duke of Cumberland was beaten by the French, concluded that he was not designed by Nature for martial distinction, and abruptly solved the problem of his immediate destiny by recourse to the simple and unheroic expedient of desertion. He came to England, got employment after a time as organist of the Octagon Chapel at Bath, and was rapidly rising into notice as a musician, when the force of his genius, combined with a discovery which came certainly unsought, but was grasped as only a great man can grasp the gifts of Fortune, again changed the direction of his life, and gave him to the science of astronomy.

He had for several years employed his spare time in assiduous observation; and, finding that opticians' prices were higher than he could well afford, had begun to make Newtonian reflectors for himself, and had finally succeeded in constructing one of 6½ inches aperture, and of high optical quality. With this instrument, on the night of March 13, 1781, he was engaged in the execution of a plan which he had formed of searching the heavens for double stars, with a view to measuring their distance from the earth by seeing whether the apparent distance of the members of the double from one another varied in any degree in the course of the earth's journey round the sun. He was working through the stars in the constellation Gemini, when his attention was fixed by one which presented a different appearance from the others which had passed his scrutiny.

In a good telescope a fixed star shows only a very small disc, which indeed should be but a point of light; and the finer the instrument the smaller the disc. The disc of this object, however, was unmistakably larger than those of the fixed stars in its neighbourhood—unmistakably, that is, to an observer of such skill as Herschel, though those who have seen Uranus under ordinary powers will find their respect considerably increased for the skill which at once discriminated the tiny greenish disc from that of a fixed star. Subsequent observation revealed to Herschel that he was right in supposing that this body was not a star, for it proved to be in motion relatively to the stars among which it was seen. But, in spite of poetic authority, astronomical discoveries do not happen quite so dramatically as the sonnet 'On First looking into Chapman's Homer' suggests.

is a noble simile, were it only true to the facts. But new planets do not swim around promiscuously in this fashion; and in the case of Uranus, which more nearly realizes the thought of Keats than any other in the history of astronomy, the 'watcher of the skies' felt probably more puzzlement than anything else. Herschel was far from realizing that he had found a new planet. When unmistakable evidence was forthcoming that the newly discovered body was not a fixed star, he merely felt confirmed in the first conjecture which had been suggested by the size of its disc—namely, that he had discovered a new comet; and it was as a new comet that Uranus was first announced to the astronomical world.

It quickly became evident, however, that the new discovery moved in no cometary orbit, but in one which marked it out as a regular member of the solar system. A search was then instituted for earlier observations of the planet, and it was found to have been observed and mistaken for a fixed star on twenty previous occasions! One astronomer, Lemonnier, had actually observed it no fewer than twelve times, several of them within a few weeks of one another, and, had he but reduced and compared his observations, could scarcely have failed to have anticipated Herschel's discovery. But perhaps an astronomer who, like Lemonnier, noted some of his observations on a paper-bag which had formerly contained hair-powder, and whose astronomical papers have been described as 'the image of Chaos,' scarcely deserved the honour of such a discovery!

When it became known that this new addition to our knowledge of the solar system had been made by the self-taught astronomer at Bath, Herschel was summoned to Court by George III., and enabled to devote himself entirely to his favourite study by the bestowal of the not very magnificent pension of £200 a year, probably the best investment that has ever been made in the interests of astronomical science. In gratitude to the penurious monarch who had bestowed on him this meagre competence, Herschel wished to call his planet the Georgium Sidus—the Georgian Star, and this title, shortened in some instances to the Georgian, is still to be found in some ancient volumes on astronomy. The astronomers of the Continent, however, did not feel in the least inclined to elevate Farmer George to the skies before his due time, and for awhile the name of Herschel was given to the new planet, which still bears as its symbol the first letter of its discoverer's name with a globe attached to the cross-bar Uranus. Finally, the name Uranus ('a' short) prevailed, and has for long been in universal use.

Uranus revolves round the sun at a distance from him of about 1,780,000,000 miles, in an orbit which takes eighty-four of our years to complete. Barnard gives his diameter at 34,900 miles, and if this measure be correct, he is the third largest planet of the system. Other measures give a somewhat smaller diameter, and place Neptune above him in point of size.

Subsequent observers have been able to see but little more than Herschel saw upon the diminutive disc to which even so large a body is reduced at so vast a distance. When near opposition, Uranus can readily be seen with the naked eye as a star of about the sixth magnitude, and there is no difficulty in picking him up with the finder of an ordinary telescope by means of an almanac and a good star map, nor in raising a small disc by the application of a moderately high power, say 200 and upwards. (Herschel was using 227 at the time of his discovery.) But small telescopes do little more than give their owners the satisfaction of seeing, pretty much as Herschel saw it, the object on which his eye was the first to light. Nor have even the largest instruments done very much more. Rings, similar to those of Saturn, were once suspected, but have long since been disposed of, and most of the observations of spots and belts have been gravely questioned. The Lick observers in 1890 and 1891 describe the belts as 'the merest shades on the planet's surface.'

The spectrum of Uranus is marked by peculiarities which distinguish it from that of the other planets. It is crossed by six dark absorption-bands, which indicate at all events that the medium through which the sunlight which it reflects to us has passed is of a constitution markedly different from that of our own atmosphere. It was at first thought that the spectrum gave evidence of the planet's self-luminosity; but this has not proved to be the case, though doubtless Uranus, like Jupiter and Saturn, is in the condition of a semi-sun. Like the other members of the group of large exterior planets, his density is small, being only ? greater than that of water.

Six years after his great discovery, Herschel, with the 40-foot telescope of 4 feet in aperture which he had now built, discovered two satellites, and believed himself to have discovered four more. Later observations have shown that, in the case of the four, small stars near the planet had been mistaken for satellites. Subsequently two more were discovered, one by Lassell, and one by Otto Struve, making the number of the Uranian retinue up to four, so far as our present knowledge goes. These four satellites, known as Ariel, Umbriel, Oberon, and Titania, are distinguished by the fact that their orbits are almost perpendicular to the plane of the orbit of Uranus, and that the motions of all of them are retrograde. Titania and Oberon, the two discovered by Herschel, are the easiest objects; but although they are said to have been seen with a 4·3-inch refractor, this is a feat which no ordinary observer need hope to emulate. An 8-inch is a more likely instrument for such a task, and a 12-inch more likely still; the average observer will probably find the latter none too big. Accordingly, they are quite beyond the range of such observation as we are contemplating. The rotation period of Uranus is not known.

In a few years after the discovery of Uranus, it became apparent that by no possible ingenuity could his places as determined by present observation be satisfactorily combined with those determined by the twenty observations available, as already mentioned, from the period before he was recognised as a planet. Either the old observations were bad, or else the new planet was wandering from the track which it had formerly followed. It appeared to Bouvard, who was constructing the tables for the motions of Uranus, the simplest course to reject the old observations as probably erroneous, and to confine himself to the modern ones. Accordingly this course was pursued, and his tables were published in 1821, but only for it to be found that in a few years they also began to prove unsatisfactory; discrepancies began to appear and to increase, and it quickly became apparent that an attempt must be made to discover the cause of them.

Bouvard himself appears to have believed in the existence of a planet exterior to Uranus whose attraction was producing these disturbances, but he died in 1843 before any progress had been made with the solution of the enigma. In 1834 Hussey approached Airy, the Astronomer Royal, with the suggestion that he might sweep for the supposed exterior planet if some mathematician would help him as to the most likely region to investigate. Airy, however, returned a sufficiently discouraging answer, and Hussey apparently was deterred by it from carrying out a search which might very possibly have been rewarded by success. Bessel, the great German mathematician, had marked the problem for his own, and would doubtless have succeeded in solving it, but shortly after he had begun the gathering of material for his researches, he was seized with the illness which ultimately proved fatal to him.

The question was thus practically untouched when in 1841, John Couch Adams, then an undergraduate of St. John's College, Cambridge, jotted down a memorandum in which he indicated his resolve to attack it and attempt the discovery of the perturbing planet, 'as soon as possible after taking my degree.' The half-sheet of notepaper on which the memorandum was made is still extant, and forms part of the volume of manuscripts on the subject preserved in the library of St. John's College.

On October 21, 1845, Adams, who had taken his degree (Senior Wrangler) in 1843, communicated to Airy the results of his sixth and final attempt at the solution of the problem, and furnished him with the elements and mass of the perturbing planet, and an indication of its approximate place in the heavens. Airy, whose record in the matter reads very strangely, was little more inclined to give encouragement to Adams than to Hussey. He replied by propounding to the young investigator a question which he considered 'a question of vast importance, an experimentum crucis,' which Adams seemingly considered of so little moment, that strangely enough he never troubled to answer it. Then the matter dropped out of sight, though, had the planet been sought for when Adams's results were first communicated to the Astronomer Royal, it would have been found within 3½ lunar diameters of the place assigned to it.

Meanwhile, in France, another and better-known mathematician had taken up the subject, and in three memoirs presented to the French Academy of Sciences in 1846, Leverrier furnished data concerning the new planet which agreed in very remarkable fashion with those furnished by Adams to Airy. The coincidence shook Airy's scepticism, and he asked Dr. Challis, director of the Cambridge Observatory, to begin a search for the planet with the large Northumberland equatorial. Challis, who had no complete charts of the region to be searched, began to make observations for the construction of a chart which would enable him to detect the planet by means of its motion. It is more than likely that had he adopted Hussey's suggestion of simply sweeping in the vicinity of the spot indicated, he would have been successful, for the Northumberland telescope was of 11 inches aperture, and would have borne powers sufficient to distinguish readily the disc of Neptune from the fixed stars around it. However, Challis chose the more thorough, but longer method of charting; and even to that he did not devote undivided attention. 'Some wretched comet,' says Proctor, 'which he thought it his more important duty to watch, prevented him from making the reductions which would have shown him that the exterior planet had twice been recorded in his notes of observations.'

Indeed, a certain fatality seems to have hung over the attempts made in Britain to realize Adams's discovery. In 1845, the Rev. W. R. Dawes, one of the keenest and most skilful of amateur observers, was so much impressed by some of Adams's letters to the Astronomer Royal that he wrote to Lassell, asking him to search for the planet. When Dawes's letter arrived, Lassell was suffering from a sprained ankle, and laid the letter aside till he should be able to resume work. In the meantime the letter was burned by an officious servant-maid, and Lassell lost the opportunity of a discovery which would have crowned the fine work which he accomplished as an amateur observer.

A very different fate had attended Leverrier's calculations. On September 23, 1846, a letter from Leverrier was received at the Berlin Observatory, asking that search should be made for the planet in the position which his inquiries had pointed out. The same night Galle made the search, and within a degree of the spot indicated an object was found with a measurable disc of between two and three seconds diameter. As it was not laid down on Bremiker's star-chart of the region, it was clearly not a star, and by next night its planetary nature was made manifest. The promptitude with which Leverrier's results were acted upon by Encke and Galle is in strong contrast to the sluggishness which characterized the British official astronomers, who, indeed, can scarcely be said to have come out of the business with much credit.

A somewhat undignified controversy ensued. The French astronomers, very naturally, were eager to claim all the laurels for their brilliant countryman, and were indignant when a claim was put in on behalf of a young Englishman whose name had never previously been heard of. Airy, however, displayed more vigour in this petty squabble than in the search for Neptune, and presented such evidence in support of his fellow-countryman's right to recognition that it was impossible to deny him the honour which, but for official slackness, would have fallen to him as the actual as well as the potential discoverer of the new planet. Adams himself took no part in the strife; spoke, indeed, no words on the matter, except to praise the abilities of Leverrier, and gave no sign of the annoyance which most men in like circumstances would have displayed.

Galle suggested that the new planet should be called Janus; but the name of the two-faced god was felt to be rather too pointedly suitable at the moment, and that of Neptune was finally preferred. Neptune is about 32,900 miles in diameter, his distance from the sun is 2,792,000,000 miles, and he occupies 165 years in the circuit of his gigantic orbit. The spectroscopic evidence, such as it is, seems to point to a condition somewhat similar to that of Uranus.

Neptune had only been discovered seventeen days when Lassell found him to be attended by one satellite. First seen on October 10, 1846, it was not till the following July that the existence of this body was verified by Lassell himself and also by Otto Struve and Bond of Harvard. From the fact that it is visible at such an enormous distance, it is evident that this satellite must be of considerable size—probably at least equal to our own moon.

Small instruments can make nothing of Neptune beyond, perhaps, distinguishing the fact that, whatever the tiny disc may be, it is not that of a star. His satellite is an object reserved for the very finest instruments alone.

Should Neptune have any inhabitants, their sky must be somewhat barren of planets. Jupiter's greatest elongation from the sun would be about 10°, and he would be seen under somewhat less favourable conditions than those under which we see Mercury; while the planets between Jupiter and the sun would be perpetually invisible. Saturn and Uranus, however, would be fairly conspicuous, the latter being better seen than from the earth.

Suspicions have been entertained of the existence of another planet beyond Neptune, and photographic searches have been made, but hitherto without success. So far as our present knowledge goes, Neptune is the utmost sentinel of the regular army of the solar system.