On a certain evening in March, 1781, Sir William Herschel, then gradually coming into notice as a practical astronomer, was engaged in looking at different fields of stars in the constellation Gemini when he lighted on one which at once attracted his special attention. Altering his eyepiece, and substituting a higher magnifying power he found the apparent size of the mysterious object enlarged, which conclusively proved that it was not a star; for it is a well-known optical property of all stars that whatever be the size of telescope employed on them, and however high the magnifying power no definite disc of light can be obtained when in focus. Herschel’s new find, therefore, was plainly not a star, and no idea having in those days come into men’s minds of there being any new planets awaiting discovery, he announced as a matter of course that he had found a new comet, so soon as he ascertained that the new body was in motion. The announcement was not made to the Royal Society till April 26, more than six weeks after the date of the actual discovery, an indication, by the way, of the dilatory circulation of news a hundred years ago. The supposed comet was observed by Maskelyne, the Astronomer Royal, four days after Herschel had first seen it, and Maskelyne seems to have at once got the idea into his head that he was looking at a planet and not at a comet. As soon as possible after the discovery of a new comet the practice of astronomers is to endeavour to determine what is the shape of the orbit which it is pursuing. All attempts to carry out this in the case of Herschel’s supposed new comet proved abortive, because it was found impossible to harmonise, except for a short period of time, the movements of the new body with the form of curve usually affected by most comets, namely, the parabola. It is true, as we shall see later on in speaking of comets, that a certain number of those bodies do revolve in the closed curve known as the ellipse, but it is usual to calculate the parabolic form first of all, because it is the easier to calculate; and to persevere with it until it plainly appears that the parabola will not fit in with the observed movements of the new object. This practice was carried out in the case of Herschel’s new body, and it was eventually found that not only was its orbit not parabolic; that not only was its orbit not an elongated ellipse of the kind affected by comets; but that it was nearly a circle, and as the body itself showed a defined disc the conclusion was inevitable: it was in real truth a new planet. It has not taken long to write this statement, and it will take still less time for the reader to read what has been written, but the result just mentioned occupied the attention of astronomers many months in working out, step by step, in such a way as to make sure that no mistake had been made.
When it was once clearly determined that Herschel had added a new planet to the list of known planets it became an interesting matter of inquiry to find out whether it had ever been seen before; and to settle the name it should bear. A little research soon showed that the new planet had been seen and recorded as a fixed star by various observers on 20 previous occasions, beginning as far back as Dec. 13, 1690, when Flamstead registered at Greenwich as a star. These various observations, spread over a period of 91 years, and all recorded by observers of skill and eminence materially helped astronomers in their efforts to calculate accurately the shape and nature of the new planet’s orbit. One observer, a Frenchman named Le Monnier, saw the planet no less than 12 times between 1750 and 1771, and if he had had (which it is known he had not) an orderly and methodical mind, the glory of this discovery would have been lost to England and obtained by France. Arago has left it on record that he was once shown one of these chance observations of Uranus, which had been recorded by Le Monnier on an old paper bag in which hair powder had been sold by a perfumer.
A long discussion took place on the question of a name for the new planet. Bode’s suggestion of “Uranus” is now in universal use, but it is within the recollection of many persons living that this planet bore sometimes the name of the “Georgium Sidus” and sometimes the name of “Herschel.” The former designation was proposed by Herschel himself in compliment to his sovereign and patron George III. of England; whilst a French astronomer suggested the latter name. However, neither of these appellations was acceptable to the astronomers of the Continent, who declared in favour of a mythological name, though it was a long time before they agreed to accept Bode’s “Uranus.” The symbol commonly used to represent the planet is formed of Herschel’s initial with a little circle added below, though the Germans employ something else, “made in Germany,” to quote a too familiar phrase.
The visible disc of Uranus is so small that none but telescopes of the very largest size can make anything of it. A few sentences therefore will dispose of this part of the subject. The disc is usually bluish in tinge, and most observers who look at it consider it uniformly bright, but there is satisfactory testimony to the effect that under the most favourable circumstances of instrument and atmosphere two or more belts, not unlike the belts of Jupiter, may be traced. From the position in which these belts have been seen it is inferred that the satellites of Uranus (presently to be mentioned) are unusually much inclined to the planet’s equator, and revolve in a retrograde direction, contrary to what is the ordinary rule of the planets and satellites. It is assumed as the basis of these ideas, (and by analogy it is reasonable to do this) that the belts are practically parallel to the planet’s equator, and at right angles to the planet’s axis of rotation. To speak of the planet’s axis of rotation is, in one sense, another assumption, because available observations can scarcely be said to enable us to demonstrate that the planet does rotate on its axis, yet we can have no moral doubt about it. Taylor has suggested grounds for the opinion that “there can be very little doubt that Uranus is to a very large extent self-luminous, and that we do not see it wholly by reflected light.” To this Gore adds the idea that there is “strong evidence in favour of the existence of intrinsic heat in the planet.”
Uranus is attended by several satellites. It was once thought that there were eight, of which six were due to Sir W. Herschel, the other two being of modern discovery. Astronomers are, however, now agreed that no more than four satellites can justly be recognised as known to exist, and they are so minute in size that only the very largest telescopes will show them; and therefore our knowledge of them is extremely limited. Sir W. Herschel’s idea that he had seen six satellites appears to have resulted from his having on some occasions mistaken some very small stars for satellites. Two only of his six are thought to have been real satellites. The other two recognised satellites were found both in 1847, one by Lassell, and the other by O. Struve.
Uranus revolves round the Sun in rather more than 84 years, at a mean distance of 1781 millions of miles. Its apparent diameter, seen from the Earth, does not vary much from 3½″ which corresponds to about 31,000 miles. It has been calculated that the light received from the Sun by Uranus would be about the amount furnished by 300 full Moons seen by us on the Earth, though another authority increases this to 1670 full Moons. From Uranus Saturn can be seen, and perhaps Jupiter, both as inferior planets, just as we see Venus and Mercury; but all the other inner planets, including Mars and the Earth, would be hopelessly lost to view, because perpetually too close to the Sun. Possibly, however, they might, on rare occasions, be seen in transit across the Sun’s disc. Neptune, of course, would be visible and be the only superior planet. The Sun itself would appear to an observer on Uranus as a very bright star, with a disc of 1¾′ of arc in diameter.
CHAPTER XII.
NEPTUNE.
We now come to the best known planet of the solar system, reckoning outwards from the Sun, and though this planet itself, as an object to look at, has no particular interest for the general public, yet the history of its discovery is a matter of extreme interest. Moreover, it is very closely mixed up with the history of the planet Uranus, which has just been described. After Uranus had become fully recognised as a regular member of the solar system, a French astronomer named Alexis Bouvard set himself the task of exhaustively considering the movements of Uranus with a view of determining its orbit with the utmost possible exactness. His available materials ranged themselves in two groups:—the modern observations between 1781 and 1820, and the early observations of Flamsteed, Bradley, Mayer, and Le Monnier, extending from 1690 to 1771. Bouvard found in substance that he could frame an orbit which would fit in with each group of observations, but that he could not obtain an orbit which would reconcile both sets of observations during the 130 years over which they jointly extended. He therefore rashly came to the conclusion that the earlier observations, having been made when methods and instruments were alike relatively imperfect, were probably inaccurate or otherwise untrustworthy, and had better be rejected. This seemed for awhile to solve the difficulty, and results which he published in 1821 represented with all reasonable accuracy the then movements of the planet. A very few years, however, sufficed to reveal discordances between observation and theory, so marked and regular as to make it perfectly clear that it was not Bouvard’s work which was faulty but that Uranus itself had gone astray through the operation of definite but as yet unknown causes. What these causes were could only be a matter of surmise based upon the evident fact that there was some source of disturbance which was evidently throwing Uranus out of its proper place and regular course. First one and then another astronomer gave attention and thought to the matter, and eventually the conclusion was arrived at that there existed, more remote from the Sun than Uranus, an undiscovered planet which was able to make its influence felt by deranging the movements of Uranus in its ordinary journey round the Sun every 84 years. This conclusion on the part of astronomers becoming known, a young Cambridge student, then at St. John’s College, John Couch Adams by name, resolved, in July 1841, to take up the subject, though it was not until 1843 that he actually did so. The problem to be solved was to suggest the precise place in the sky at a given time of an imaginary planet massive enough to push, or pull, out of its normal place the planet Uranus, which was evidently being pushed at one time and pulled at another. It would also be part of the problem to predict the distance from the Sun of the planet thus imagined to exist. Adams worked patiently and silently at this very profound and difficult problem for 1¾ years when he found himself able to forward to Airy, who had become Astronomer Royal after being a Cambridge Professor, some provisional elements of an imaginary planet of a size, at a distance, and in a position to meet the circumstances. It is greatly to be regretted, on more grounds than one, that Airy did nothing but pigeon-hole Adams’s papers. Had he done what might have been, and probably was, expected, that is, had he made them public, or better still had he made telescopic use of them, a long and unpleasant international controversy would have been avoided, and Adams would not have been robbed in part of the well-deserved fruits of his protracted labours.
We must now turn to consider something that was happening in France. In the summer of 1845, just before Adams had finished his work, and one and a half years after he commenced it a young Frenchman, who afterwards rose to great eminence, U. J. J. Le Verrier, turned his attention to the movements of Uranus with a view of ascertaining the cause of their recognised irregularity. In November 1845 he made public the conclusion that those irregularities did not exclusively depend upon Jupiter or Saturn. He followed this up in June 1846 by a second memoir to prove that an unknown exterior planet was the cause of all the trouble, and he assigned evidence as to its position very much as Adams had done 8 months previously. Airy on receiving a copy of Le Verrier’s memoir seems so far, at last, to have been roused that he took the trouble to compare Le Verrier’s conclusions with those of Adams so long in his possession neglected. Finding that a remarkably close accord existed between the conclusions of the two men, he came to realise that both must be of value, and he wrote a fortnight later to suggest to Professor Challis the desirability of his instituting a search for the suspected planet. Challis began within two days, but was handicapped by not having in his possession any map of the stars in the neighbourhood suggested as the locale of the planet. He lost no time however in making such a map, but, of course, the doing so caused an appreciable delay, and it was not until September 29, 1846, that he found an object which excited his suspicions and eventually proved to be the planet sought for. It was subsequently ascertained that the planet had been recorded as a star on August 4 and 12, and that the star of August 12 was missing from the zone observed on July 30. The discovery of the planet was therefore just missed on August 12 because the results of each evening’s work were not adequately compared with what had gone before.
Meanwhile things had not been standing still in France. In August 1846, Le Verrier published a third memoir intended to develope information respecting the probable position of the planet in the heavens. In September 23 a summary of this third memoir was received by Encke at Berlin, accompanied by the request that he would cooperate instrumentally in the search for it. Encke at once directed two of his assistants named D’Arrest and Galle to do this, and they were fortunately well circumstanced for the task. Unlike Challis, who, as we have seen, could do nothing until he had made a map for himself, the Berlin observers had one ready to hand, which by good chance had just been published by the Berlin Academy for the part of the heavens which both Adams and Le Verrier assigned as the probable locality in which the anxiously desired planet would be found. Galle called out the visible stars one by one whilst D’Arrest checked them by the map, and suddenly he came upon an unmarked object which at the moment looked like an 8th magnitude star. The following night showed that the suspicious object was in motion, and it was soon ascertained to be the trans-Uranian planet which was being searched for. The discovery when announced excited the liveliest interest all over the world. It did more; it created a bitter feeling of resentment on the part of French astronomers that the laurels claimed by them should have been also claimed in an equal share by a young and unknown Englishman, and accordingly the old cry of “perfide Albion” arose on all sides. I have been particular in stating the various dates which belong to this narrative, in order to make as clear as possible the facts of the case. This is even now necessary, because though the astronomers of England and Germany are willing to give Adams and Le Verrier each their fair share of this great discovery, the same impartial spirit is not to be found in France, for nothing is more common, even in the present day, in looking at French books of astronomy, than to find Adams’s name either glossed over or absolutely suppressed altogether when the planet Neptune is under discussion.