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ed in the same proportion. And therefore (roughly) any given portion of the surface of Uranus—say a hundred miles square near the middle of his visible disc —sends to us but about one-sixteenth part of the light which an equal and similarlyplaced portion of the surface of Saturn would send to us. Now every astronomer knows how difficult it is, even with very powerful telescopes, to study the physical ifeatures of Saturn. A telescope of moderate power will show us his ring-system and some of his satellites; but to study the belts which mark his surface, the aspect of his polar regions, and in particular those delicate tints which characterize various portions of his disc, requires a telescope of great power. It will be understood, there•fore, that in the case of Uranus, which receives so much less light from the sun and is so much farther from us, even the best telescopes yet made by man must fail to reveal any features of interest. We may add also that Uranus is a much smaller planet than Saturn, though far larger than the combined volume of all the four planets, Mars, Venus, the Earth, and Mercury. If Saturn (without his' rings) and Uranus were both visible together in the same telescopic field (a circumstance which may from time to time happen) the'Herschelian planet would appear so small and faint that it might readily be taken for one of Saturn's moons, the ringed planet sending us altogether some sixty times as much light as Uranus.
But what the telescope had hitherto failed to accomplish, has just been achieved by means of that wonderful ally of the telescope, the spectroscope, in the able hands of the eminent astronomer and physicist, Dr. Huggins. News has been received about the constitution of the atmosphere of Uranus, and news so strange (apart from the strangeness of the mere fact that any information could be gained at all respecting a vaporous envelope so far away) as to lead us to speculate somewhat curiously respecting the conditions under which the Uranians, if there are any, have their being.
Before describing the results of Dr. Huggins's late study of the planet, it may be well to give a brief account of what is known or may be surmised respecting Uranus. The question has been raised whether Uranus was known to the astronomers of old times. There is nothing New Series.— Vol. XIV, No. 6.
altogether improbable in the supposition that in countries where the skies are unusually clear, the planet might have been detected by its motions. Even in ourlatitudes Uranus can be quite readily seen on clear and moonless nights, when favorably situated. He shines at such times as a star of about the fifth magnitude—that is somewhat more brightly than the faintest stars visible to the naked eye. In the clear skies of more southerly latitudes he would appear a sufficiently conspicuous object, though, of course, it would be wholly impossible for even the most keen-sighted observer to recognise any difference between the aspect of the planet and that of a star of equal brightness. The steadiness of the light of Saturn causes this planet to present a very marked contrast with the first magnitude stars whose lustre nearly equals his own. But although the stars of the lower orders of magnitude scintillate like the leading orbs, their scintillations are not equally distinguishable by the unaided eye. Nor is it unlikely that if Uranus were carefully watched (without telescopic aid) he would appear to scintillate slightly. Uranus would only be recognizable as a planet by his movements. There seems little reason for doubting, however, that even the motions of so faint a star might have been recognized by some of the ancient astronomers, whose chief occupation consisted in the actual study of the star groups. We might thus understand the Burmese tradition that there are eight planets, the sun, the moon, Mercury, Venus, Jupiter, and Saturn, and another named Rahu which is invisible. If Uranus was actually discovered by ancient astronomers, it seems far from unlikely that the planet was only discovered to be lost again, and perhaps within a very short time. For if anything positive had been learned respecting the revolution of this distant orb, the same tradition which recorded the discovery of the planet would probably have recorded the nature of its apparent motions.
Be this as it may, we need by no means accept the opinion of Buchanan, that if the Burmese tradition relates to Uranus, Sir William Herschel must be " stripped of his honors." The rediscovery of a lost planet, especially of one which had remained concealed for so many centuries, must be regarded as at least as interesting 46
as the discovery of a planet altogether unknown. Nor was there any circumstance in the actual discovery of Uranus, which would lose its interest, even though we accepted quite certainly the conclusion that the Herschelian planet was no other than old R&hu.*
Let us turn to Herschel's own narrative of his detection of Uranus. It is in many respects very instructive.
In the first place, we must note the nature of the work he was engaged upon. He had conceived the idea of measuring the distances of the stars, or at least of the nearer stars, by noting whether as the earth circles around the sun the relative positions of stars lying very close to each other seem to vary in any degree. To this end he was searching the heavens for those objects which we now call double stars, most of which were in his day supposed to be not in reality pairs of stars— that is, not physically associated together —but seen near together only because lying nearly in the same direction. The brighter star of a pair was in fact supposed to lie very much nearer than the fainter; and it was because, being so much nearer, the brighter star should be much more affected (seemingly) by the earth's motion around the sun, that Herschel hoped to learn much by studying the aspect of these unequal double stars at different seasons of the year. He hoped yet more from the study of such bright orbs as are surrounded by several very faint stars. It was a case of this kind that he was dealing with, when accident led him to the discovery of Uranus. "On Tuesday, the 13th of March (1781)," he writes, "between ten and eleven in the evening, while I was examining the small stars in the neighborhood of Eta in Gemini, I perceived one that appeared visibly larger
* It is, after all, at least as likely that Rahu— assuming there really was a planet known under this name—might have been Vesta, the brightest of the small planets which circle between Mars and Jupiter, as the distant and slow-moving Uranus. For although Vesta is not nearly so bright as Uranus, shining indeed only as a star of the seventh magnitude, yet she can at times be seen without telescopic aid by persons of extremely good sight; and her movements are far more rapid than those of Uranus. In the high tableands of those eastern countries, where some place the birth of astronomy, keen-sighted observers might quite readily have discovered her planetary nature, whereas the slow movements of Uranus would probably have escaped their notice.
than the rest. Being struck with its uncommon magnitude, I compared it to Eta and the small stars in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet. I was then engaged in a series of observations (which I hope soon to have the opportunity of laying before the Royal Society) requiring very high powers, and I had ready at hand the several magnifiers of 227, 660, 932,* 1,536, 2,010, etc., all of which I have successfully used on that occasion. The power I had on when I first saw the (supposed) comet was 227. From experience I knew that the diameters of the fixed stars are not proportionally magnified with higher powers, as those of the planets are; therefore I now put on the powers of 460 • and 932, and found the diameter of the comet, increased in proportion to the power, as it ought to be on a supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared' hazy and ill-defined with these great powers, while the stars presented that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were wellfounded."
There are three points to be specially noted in this account. Firstly, the astronomer was engaged in a process of systematic survey of the celestial depths—so that the discovery of the new orb cannot be properly regarded as accidental, although Herschel was not at the time on the look-out for as yet unknown planets. Secondly, the instruments he was employing were of his own construction and device, and probably none others in existence in his day would have led him to the discovery that the strange orb was not a fixed star. And, thirdly, without the experience he had acquired in the study of the heavens he would not have been able to apply the test which, as we have seen, he found so decisive. The fact that the stars are not magnified by increased telescopic power to the same extent as planets or comets, is, as Professor Pritchard has justly remarked, "an important result of the undulatory theory of light, and was unsuspected in Sir William Herschel's day." So that whether we consider the work Herschel was engaged upon, the instruments he used, or the experience he had acquired, we recognize the fact that he alone of the astronomers of his time was capable of discovering Uranus otherwise than by a fortunate accident. Others might have lighted on the discovery—indeed, we shall presently see that the wonder rather is that Uranus had not been
* for many years a recognized member of the solar system—but there was none but Herschel who could within a few minutes of his first view of the planet have pronounced confidently that the strange orb (whatever it might be) was not a fixed star.
I do not propose to enter here, at length, into the series of researches by which it was finally demonstrated that the newly-discovered body was not a comet but a planet, travelling on a nearly circular path around the sun, at about twice Saturn's distance from that orb. With this part of the work Herschel had very little to do. To use Professor Pritchard's words, having ascertained the apparent size, position, and motion of the stranger, "Herschel very properly consigned it to the care of those professional astronomers
1 who possessed fixed instruments of precision in properly constituted observatories —to Dr. Maskelyne, for instance, who was then the Astronomer-Royal at Greenwich, and to Lalande, who presided over the observatory in Paris." As the newlydiscovered body travelled onwards upon its apparent path, astronomers gradually acquired the means for determining what its real path might be. At first they were misled by erroneous measures of the stranger's apparent size, which suggested that the supposed comet had in the course of the first month after its discovery approached to within half its original distance. At length, setting aside all these measures, and considering only the movements of the stranger, Professor Saron was led to the belief that it was no comet, but a member of the solar system. It was eventually proved, chiefly by the labors of Lexell, Lalande, and the great mathematician Laplace, that this theory fully explained all the observed motions of the newly-discovered body; and before long (so complete is the mastery which the Newtonian system gives astronomers over the motions of the heavenly bodies) all the
circumstances of the new planet's real motions became very accurately known. It was now possible, not only to predict the future movements of the stranger, but to calculate his motions during former years. This last process was quickly applied to the planet, with the object of determining whether among the records of observations made on stars, any might be detected which related in reality to the newly-discovered body. The result will appear at first sight somewhat surprising. The new planet had actually been observed np less than nineteen times before that night when Herschel first showed that it was not a fixed star, and those observations were made by astronomers no less eminent than Flamstead, Bradley, Mayer, and Lemonnier. Flamstead had seen the planet five several times, each time cataloguing it as a star of the sixth magnitude, so that five such stars had to be dismissed from Flamstead's lists. But the case of Lemonnier was even more singular; for he had actually observed the planet no less than twelve times, several of his observations having been made within the space of a few weeks. "M. Arago naturally comments," says Professor Pritchard, "on the want of system displayed by Lemonnier in 1769 ; had he but reduced and arranged his observations in a properly-constructed register, his name instead of Herschel's would have been attached for all time to one of the starry host. But Lemonnier was not a man of order ; his astronomical papers are said to have been a very picture of chaos ;. and M. Bouvard, to whom we have long been indebted for the best tables of the new planet, narrates that he had seen one of Lemonnier's observations of this very star written on a paper bag which hail contained hair powder!"
In our days, when fresh planets are being discovered and named in the course of each year that passes, it may appear strange that much difficulty was found in assigning a suitable name to the stranger. But we must remember that for ages the planetary system had been supposed to comprise no other primary members than those known to the ancients. The discovery of Uranus was an altogether novel and unlooked-for circumstance. It was not supposed that fresh discoveries of like nature would be made, still less that a planet would hereafter be discovered under circumstances far more interesting even than those which attended the discovery of Uranus. Accordingly a mighty work was made before Uranus was fitted with a name. Lalande proposed the name of the discoverer, and the new planet was indeed long known on the Continent by the name of Herschel. The symbol of the planet, the initial letter of Herschel's name with a small globe attached to the cross-stroke, still reminds us of the honor which Continental astronomers generously, proposed to render to their fellow-worker in England.* Lichtenberg proposed the name of Astrasa, the goddess of justice—for this "exquisite reason," that since justice had failed to establish her reign upon earth, she might be supposed to have removed herself as far as possible from our unworthy planet. Poinsinet suggested that Cybele would be a suitable name; for since Saturn and Jupiter, to whom the gods owed their origin, had long held their seat in the heavens, it was time to find a place for Cybele. "the great mother of the gods." Had the supposed Greek representative of Cybele—Rhaja—been selected for the honor, the name of the planet would have approached somewhat nearly in sound, and perhaps in signification, to the old name Rahu. But neither Astraea nor Cybele were regarded as of sufficient dignity and importance among the ancient deities to supply a name for the new planet.f Prosperin proposed Neptune as a suitable ■name, because Saturn would thus have the eldest of his sons on one side of him, and his second son on the other. Bode at length suggested the name of Uranus, the most ancient of the deities; and as Saturn, the Father of Jupiter, travels on a wider
* There is a certain incongruity, accordingly, among the symbols of the primary planets. Mercury is symbolized by his caduccns, Venus by her looking-glass (I suppose), Mars by his spear and shield, Jupiter by his throne, Saturn by his sickle; and again, when we pass to the symbols assigned to the planets discovered in the present century, we find Neptune symbolized by his trident, Vesta by her altar, Ceres by her sickle, Minerva by a sword, and Juno by a star-tipped sceptre. Uranus alone is represented by a symbol which has no relation to his position among the deities of mythology.
f Both these names are found among the asteroids, the fifth of these bodies (in order of discovery) being called Astraea, the eighty-ninth being named alter the great mother of gods and goddesses.
orbit than Jupiter, so it was judged fitting that an even wider orbit than Saturn's should be adjudged to Jupiter's grandfather. In accepting the name of Uranus for the new planet, astronomers seemed to assert a belief that no planet would be found to travel on a yet wider path ; and accordingly when a more distant planet was discovered, the suggestion of Prosperin had to be reconsidered ; but it was too late to change the accepted nomenclature, and accordingly the younger brother of Jupiter has had assigned to him a planet circling outside the paths of that assigned to their father and grandfather. It may be noted, also, that a more appropriate name for the new planet would have been Ccelus, since all the other planets have received the Uatin names of the deities.
Herschel himself proposed another name. As Galileo had called th| satellites of Jupiter the Medicean planets, while French astronomers proposed to call the spots on the sun the Bourbonian stars, so Herschel, grateful for the kindness which he had received at the hands of George III., proposed that the new planet should be called Georgium Sidus. On account of the interest attaching to all Herschel's remarks respecting his discovery, I quote in full the letter in which he submitted this proposition to Sir Joseph Banks, then the President of the Royal Society. "By the observations of the most eminent astronomers in Europe," he remarks, "it appears that the new star, which' I had the honor of pointing out to them in March, 1781, is a primary planet of our solar system. A body so nearly related to us by its similar condition and situation in the unbounded expanse of the starry heavens, must often be the subject of the conversation, not only of astronomers, but of every lover of science in general. This consideration, then, makes it necessary to give it a name, whereby it may be distinguished from the rest of the planets and fixed stars. In the fabulous ages of ancient times, the appellations of Mercury, Venus, Mars, Jupiter, and Saturn were given to the planets, as being their principal heroes and divinities. In the present more philosophical era, it would be hardly allowable to have recourse to the same method, and call on Juno, Pallas, Apollo, or Minerva, for a name to our new planet. The first consideration in any particular event or remarkable incident seems to be its chronology; if, in any future age it should be aked when this last-found planet was discovered, it would be very satisfactory to say, 'In the reign of George III.' As a philosopher, then, the name of Georgium Sidus presents itself to me as an appellation which will conveniently convey the information of the time and country where and when it was brought to view. But as a subject of the best of kings, who is the liberal protector of every art and science; as a native of the country from whence this illustrious family was called to the British throne ; as a member of that society which flourishes by the distinguished liberality of its royal patron; and last of all, as a person now more immediately under the protection of this excellent monarch, and owing everything to his unlimited bounty, I cannot but wish to take this opportunity of expressing my gratitude by giving the name of Georgium Sidus—
"' Georgium sidus —jam nunc assucsce vocari,'—
to a star which, with respect to us, first began to shine under his auspicious reign." Herschel concludes by remarking that, by addressing this letter to the President of the Royal Society, he takes the most effectual method of communicating the proposed name to the literati of Europe, which he hopes "they will receive with pleasure."
Herschel's proposition found little favor, however, among Continental astronomers. Indeed it is somewhat singular that for some time two names came into general use—one in Great Britain and the other on the Continent, neither being the name eventually adopted for the planet. In books published in England for more than a quarter of a century after the discovery of Uranus we find the planet called either the Georgium Sidus, or the Georgian. For a shorter season the planet was called on the Continent either the Herschelian planet, or simply Herschel. Many years elapsed before the present usage was definitely established.
In considering Herschel's telescopic study of the planet, we must remember that, owing to the enormous length of time occupied by Uranus in circling round his orbit, the astronomer labors under a difficulty distinct in character from the
difficulties which have already been considered. As Jupiter and Saturn circle on their wide orbits, they exhibit to us—the former in the course of eleven years, the latter in the course of twenty-nine and a half years—all those varying presentations which correspond to the seasons of these planets. Jupiter, indeed, owing to the uprightness of his axis (with reference to his path), presents but slight changes. But Saturn's globe is at one time bowed towards us so that a large portion of his north polar regions can be seen, and anon (fifteen years later) is so bowed, that a large portion of his southern polar regions can be seen; while between these epochs we see the globe of Saturn so posed that both poles are on the edge of his disc, and then only does the shape of his disc indicate truly the compression or polar flattening of the planet.
But, although similar changes occur in the case of Uranus, they occupy no less than eighty-four years in running through their cycle, or forty-two years in completing a half cycle—during which, necessarily, all possible presentations of the planet are exhibited. Now it is commonly recognized among telescopists that the observing time of an astronomer's life—that is, the period during which he retains not merely his full skill, but the energy neces sary for difficult researches — continue* but about twenty-five years at the outside. So that few astronomers can hope to study Uranus in all its presentations, as they can study Mars or Jupiter or Saturn.
When we add to this circumstance the extreme faintness of Uranus, we cannot wonder that Herschel should have been unable to speak very confidently on many points of interest. His measures of the planet's globe were sufficiently satisfactory, and, combined with modern researches, show that Uranus has a diameter exceeding the earth's rather less than four and a half times. Thus the surface of Uranus exceeds that of our globe about twenty times, and his bulk is more than eighty times as great as the earth's. His volume, in fact, exceeds the combined volume of Mercury, Venus, the Earth, and Mars, almost exactly forty times. But Sir W. Herschel was unable to measure the disc of Uranus in such a way as to determine whether the planet is compressed in the same marked degree as Jupiter