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 ⅕ 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.'