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of the Planet of War, and of the excep- hesitated to admit that the description tionally favorable conditions under which was a tolerably safe one. it could be observed in their latitude, There were, however, some who still the observers who have under their es- adhered to the view which Kepler had pecial charge the great telescope of the propounded in 1610. Thus the late AdWashington Observatory have scruti- miral Smyth, after describing the appearnised with special care the neighborhood ance which our earth and her companion of the planet which till lately was called moon must present to the inhabitants of "moonless Mars;" and their skill and Mars (if inhabitants he has), says: watchfulness have been rewarded by the " This appearance is not reciprocated; discovery of two moons attending on for though it is not at all improbable that that planet.
Mars may have a satellite revolving There are several circumstances which around him, it is probably very small, render the discovery of these moons in and close to his disc, so that it has hiththe first place, and in the second the ex- erto escaped our best telescopes; yet, istence of such bodies as attendants on being farther from the sun than the earth the small planet Mars, exceedingly inter- is, Mars—if at all habitable—would seem esting. These we propose briefly to in- to stand even more in need of a lumidicate.
nous auxiliary.” Galileo, after he had completed his This idea, in fact, that planets require largest telescope late in 1609, had to wait more moons the farther they lie from the for nearly a year before he had a favora- sun, and not only so, but that their reble opportunity for studying Mars. Thus quirements in this respect have been athe had already discovered the moons of tended to, and each planet carefully fitJupiter and the varying phases of Venus ted out with a suitable number of attendbefore he could study a planet from ants, is one which has found special fawhich he must have expected even more vor with many believers in other worlds interesting results. For on the one hand than ours. Whewell, for instance, who, Mars is seen under much more favorable although in his anonymously-written conditions than Venus, and on the other “Plurality of Worlds” he appeared as an it approaches us much more closely than opponent of the theory of other worlds, Jupiter. In the mean time, Kepler had had earlier, in his less known “Bridgehazarded the prediction that Mars has water Treatise," expressed opinions two moons—a suggestion which, in the strongly favoring that theory, reasons as light of the recent discovery, may be follows for the belief that satellites were called, like “the Pogram statter in mar- specially made to bless the planets with ble," "a pre-diction, cruel smart.” Gal- their useful light : “ Turning our attenileo saw no Martian moons, however, tion to the satellites of the other planets and could, indeed, barely recognise the of our system, there is one fact which gibbosity of Mars. From what is now immediately arrests our attention-the known, indeed, we perceive that one number of such attendant bodies apmight as hopefully try to read a newspa- pears to increase as we proceed to planper at the Faulhorn from the slopes of ets farther and farther from the sun. the Jungfrau, as attempt with such a Such at least is the general rule. Mertelescope as Galileo's to detect the mi- cury and Venus, the planets nearest the nute companions of the War Planet. sun, have no such attendants. The
Telescope after telescope was there- earth has one. Mars, indeed, who is after turned on Mars, until the great still farther removed, has none; nor four-feet mirrors of Sir W. Herschel and have the minor planets, Juno, Vesta, Mr. Lassell, and even the mightier six- Ceres, and Pallas” (when he wrote these feet mirror of Parsonstown, had taken only were known); so that the rule is part in the survey of the planet and its only approximately verified. But Jupineighborhood. But no satellites were ter, who is at five times the earth's disdiscovered ; insomuch that when Tenny- tance, has four satellites; and Saturn, son (in the first edition only of his po- who is again at a distance nearly twice
the snowy poles of moon- as great, has seven, besides that most less Mars,” few astronomers would have extraordinary phenomenon, his ring
(which for purposes of illumination is According to the method of viewing equivalent to many thousand satellites). such matters which is now generally in Of Uranus it is difficult to speak, for his favor among men of science, the considgreat distance renders it almost impossi- erations urged by Whewell will not be ble to observe the smaller circumstances regarded as of any weight. They would of his condition. It does not appear at not be so regarded even if the satellites all probable that he has a ring like Sat- of Jupiter, Saturn, and Uranus, or the arn; but he has at least five satellites rings which surround Saturn, really subwhich are visible to us" (four only are served the purpose which Whewell, now recognised) “ at the enormous dis- Brewster, Chalmers, Dick, Lardner, and tance of goo millions of miles; and we others have so complacently dwelt upon. believe that the astronomer will hardly But in reality, apart from the evidence deny that he" (Uranus, not the astrono- tending to show that none of these planmer) “ may possibly have thousands of ets can at present be inhabited, it is absmaller ones circulating about him. But solutely certain that moonlight on Jupileaving conjecture, and taking only the ter and Saturn must be far inserior to ascertained cases of Venus, the earth, moonlight on our earth despite the greatJupiter, and Saturn, we conceive that a er number of moons, while that received person of common understanding will be by Uranus from his four moons must be strongly impressed with the persuasion scarce superior to the light we receive that the satellites are placed in the sys- from Venus, Jupiter, Mars, and Saturn, tem with a view to compensate for the so faintly are the Uranian satellites illudiminished light of the sun at greater minated by a sun nineteen times more distances,” whence we may infer that remote than the sun we see. As for the in subsequently rejecting this opinion, in rings of Saturn, they act far more effecthis 'Plurality of Worlds,' Whewell showed ively to deprive the planet of sunlight himself a person of uncommon under- than to illuminate the Saturnian nights. standing
Despite the efforts made by Lardner to According to Whewell's earlier way of defend these appendages from the reflecviewing the satellites, however, the fact tions cast upon them in this respect by that Mars seemed to have no satellites Sir J. Herschel, it may be mathematically was to some degree a difficulty, but not demonstrated (and has been by the presan insuperable one. “The smaller plan- ent writer) that the rings cast wide zones ets, Juno, Vesta, Ceres, and Pallas,” he of the planet-zones many times exceed
"differ from the rest in so many ing the whole surface of our earth-into ways, and suggest so many conjectures total eclipse lasting several years in sucof reasons for such differences, that we cession. Even were it otherwise, howshould almost expect to find them excep- ever, no one, familiar with the evidence tions to such a rule. Mars is a more which nature multiplies around us, would obvious exception. Some persons might have been disposed to argue, from the conjecture from this case, that the ar- presumed fitness of the Jovian and Sarangement itself, like other useful ar- turnian arrangements as to satellites, rangements, has been brought about by that Mars has moons. If there is a meansome wider law, which we have not yet ing in the arrangements actually obdetected. But whether or not we enter- served which should have led astronotain such a guess (it can be nothing mers to believe in the existence of Marmore), we see in other parts of creation tian satellites—a view which certainly so many examples of apparent excep- the discovery of such satellites goes far tions to rules, which are afterwards found to confirm—the meaning is one which to be capable of explanation, or to be the laws of physics alone can be expectprovided for by particular contrivances, ed to interpret. that no one, familiar with such contem- That Mars should have definitely plations, will by one anomaly be driven come to be regarded by nearly all asfrom the persuasion that the end which tronomers as without satellites will readthe arrangements of the satellites seem ily be understood if we consider the nasuited to answer is really one of the ends ture of the evidence which had been obof their creation."
tained. When Jupiter is at his farthest
from us, but in opposition* (that is, on in diameter--a moon of Mars had a dithe side remote from the sun), all four of ameter so much less that the disc were his satellites, the least of which is rather reduced to one-800,000th part of such a less than our own moon, are quite easily moon's disc, it would be as readily visiseen in the smallest telescopes ever used in ble with one of the very powerful teleastronomical observation. Certainly they scopes above mentioned as is Jupiter's can then be all seen with a good telescope least moon with a one-inch telescope. one inch in aperture. At such times Jupi- This would be the case if the diameter ter lies at a distance of about 410 millions were reduced to one-895th part (for 895 of miles from us. Now Mars, when he times 895 is very nearly equal to 800,makes his nearest opposition approaches 000). So that, were it not for one con(as for instance in the present autumn), sideration now to be mentioned, it would lies at a distance from us of about 35 have seemed that astronomers might millions of miles, or less than Jupiter's safely have assumed that Mars has not in the proportion of about seven to a moon exceeding 2/4 miles in diameter. eighty-two, or at not much more than The consideration in question is this : a one-twelfth of Jupiter's distance. This satellite might travel very near to Mars, would cause a self-luminous body to ap- so that it would always be more or less pear about 140 times brighter at Mars's involved in the luminosity surrounding distance than at Jupiter's. But satellites his disc. The best telescope cannot get are not self-luminous. Their brightness rid of this luminosity; for, in fact, it is depends on sunlight, and the nearer they not an optical but a real light. It is, are to the sun the more brightly they in fact, our own air, which is lit up by necessarily shine. Mars is illuminated, the planet's rays for some distance all when nearest to the sun, with an amount round. Now a small satellite amidst of sunlight exceeding that which illu- this light, even though the planet itself mines Jupiter when farthest from the sun might be kept out of view, would be (these being the cases we are dealing much less readily viewed than a satellite with) in a proportion of more than fif- seen like one of Jupiter's at a great disteen to one. So that a satellite near tance from its primary. Yet, as it is Mars, as large as the least satellite of known that Jupiter's satellites can be Jupiter, would shine fifteen times 140 traced right up to the edge of the planet, times more brightly, or, in round num- we do not think so much importance bers, fully 2,000 times more brightly, should be attributed to this circumstance than one of those bodies which the ob- as is sometimes done. It should cerserver can readily see with a telescope tainly be possible to see a Martian satelonly one inch in aperture. But most lite two diameters of the planet, let us certainly it is not assuming too much to say, from the edge, if it shine with twice claim for the most powerful telescopes as much light as would make it visible with which Mars's neighborhood had on a perfectly dark sky. Let us, howbeen searched for satellites an illuminat- ever, say that the satellite ought to be ing power exceeding that of so minute four times instead of twice as bright. a telescope 400 times. This would have Then the diameter, instead of being 24 made such a moon as we have imagined miles in order that a satellite close to appear at least 800,000 times brighter Mars should just be visible in a very than the least of Jupiter's moons actual- powerful telescope, should be 472 miles. ly appears in a telescope one inch in Certainly we should expect that a satelaperture. If, then, instead of being so lite five miles in diameter would have large as this—that is, 2,000 miles or so been long since revealed under the
searching scrutiny to which the neigh
borhood of Mars has again and again * The reader must not understand us here
been subjected. to mean that it is when in opposition that Jupiter is farthest from us, for the reverse is Now it could not but be admitted that the case.
It is at his successive opposition a moon five miles or even ten miles in that he makes his nearest approach to the diameter would differ so much from any earth ; but he is nearer at some oppositions known moon that the difference must be than at others, and we are speaking above of those oppositions when his distance is regarded as rather one of kind than one greatest.
of degree. No such body had as yet
been heard of—at least no such body the rings of Saturn we have a system travelling as an independent moon. A formed of multitudes of tiny moons trav-. hundred years ago, indeed, men would elling so closely together as to appear hardly have been prepared to admit the from our distant station as continuous possibility of a body whose existence, if rings. In the ring of minor planets we demonstrated, would have overthrown have multitudes of tiny planets; but all their ideas as to the structure of the they are so widely strewn that each must solar system. They knew of suns, of be separately sought for with the teleplanets attending on one sun, and of scope and no signs of the ring as a moons attending on several planets, and whole can be seen in the heavens. Then they knew also of a ring-system accom- we have the rings of meteors, oval for panying one planet in its course round the most part in figure and often curithe sun.
Thus they were prepared to ously eccentric as well as extended ; recognise new suns, new planets, new sometimes complete rings, or nearly so, moons, and new rings. Sir W. Herschel like those which produce the August was nightly engaged in observing hun- displays of shooting-stars ; sometimes indreds of before unknown suns. He dis- complete, and at others known only by covered one new planet (Uranus), seve
of the ring,” one rich region ral new moons attending on Uranus and in the entire circuit. Saturn, and, as he thought, a pair of new But even with our actual knowledge rings attending on Uranus. But that of the diversity existing among the orany of the primary planets should be at- ders of bodies constituting the inaterial tended by a moon so small as not to ad- universe, we were scarcely prepared to mit of being fairly classed with the other hear of moons like those of Mars. It is known moons of the solar system would not the smallness of these bodies which have seemed to most of the astronomers is so surprising. There would have of the last century an idea as inadmissi- been nothing very remarkable in the exble as that an orbital region of the solar istence of even smaller moons attending system should be occupied by a number on any of the minor planets. Nor is it of very small planets instead of a single merely the enormous difference of diprimary planet. In recent times, how- mensions between the planet and its ever, men have become accustomed to moons; for in the case of Jupiter we recognise how small is our right to as- have a planet whose moons bear a very sert definitely the characteristics of suns, much smaller proportion to the mass of planets, moons, rings, and other such or their primary than our moon bears to the ders of bodies in the universe. We have earth; and, though the disproportion found that, besides such suns as our is nothing nearly so great as that beown, there are some so much larger that tween Mars and his moons, it would they must be regarded as forming a dis- still prepare us for recognizing any tinct class of giant suns; while others, degree almost of disproportion between again, are separated in kind, not merely a planet and its satellite. The strange in degree, from such suns as ours, be- circumstance in the actual case lies cause of their relative minuteness. We in the fact that Mars belongs to a have learned in like manner to distinguish known family of planets, viz., the terresthe planets into classes, recognising in trial family of which our earth is a leadthe giant planets Jupiter, Saturn, Uranus, ing member; and hitherto it had apand Neptune a family altogether distinct peared as if all moons attending on the from that of the terrestrial planets, the planets of one and the same class beearth and Venus, Mars and Mercury; longed themselves to one and the same while among the minor planets which class. The range of diversity of magnithrong in hundreds, perhaps in thousands, tude among the moons, for instance, atthe orbit region between Mars and Jupi- tending on the giant planets, though ter we find another family separated from considerable, is not such as to prevent the terrestrial planets as definitely by us from regarding these moons as all of their extreme minuteness as are the giant one class. Then, too, it seemed from planets by their enormous dimensions. the fact that our own moon is of the Among ring-systems, again, we had same class as those others, that, speaking learned to recognise many varieties. In generally, diversity of size is not to be
looked for to the same degree among The telescope which Professor Hall moons even attending on planets of dif- has been privileged to use may fairly be ferent classes, as among planets or among described as the finest refractor yet suns. Certainly there was nothing in mounted. Newall, in England, has a the past experience of astronomers to telescope 25 inches in aperture, which, suggest that a planet like Mars, belong- until the Washington telescope had been ing to the same class as our earth, might made, was the largest refractor in existhave a moon or moons belonging to an ence. The Washington instrument has altogether inferior class.
an aperture of 26 inches, making its illuIt was, then, with a sense of astonish- minating power between one-twelfth and ment, which would have been mingled one-thirteenth greater. But this telewith doubt but for the altogether unex- scope is also remarkable for the skill ceptionable source whence the informa- with which it has been made by Messrs. tion came, that astronomers heard of the Alvan Clark & Sons, of Cambridgeport, discovery of two Martian satellites with Mass. We know few more interesting the great telescope of the Washington histories in scientific biography than that Observatory.
which records the progress of Alvan The discoverer of the satellites, and Clark's labors in the construction of obthe telescope with which they were dis- ject-glasses — from the first small one covered, both promised well for the which he made (which fell from his truth of what some regarded at first as a hands and was destroyed within a few mere report.
moments of its completion) to the noble Professor Asaph Hall, who has long telescope which was mounted at Washbeen known as one of that band of skil- ington five years ago, after meeting satisful and original observers of which factorily all the tests applied to it by Mr. American astronomy has just reason to Clark and his two sons, who inherit his be proud, had during the last few years energy and skill. But in this place we made many observations showing that must be content with noting that all who besides scientific skill, he possesses a have ever used object-glasses construct
Some of his observations ed by the Clarks have found their optiwere such as must have taxed even the cal performance all but perfect; in fact, power of the noble instrument which has
as nearly perfect as can be obtained from lately been erected at Washington. For lenses made of a substance which cannot instance, the faintest of Saturn's satel- possibly be altogether free from defects, lites, the coy Hyperion, though discov- however carefully prepared. Those obered nearly thirty years ago, had been servers at Washington who have used very little observed, insomuch that the the great telescope systematically, agree true path of this small moon (a perfect in regarding with peculiar favor the pergiant, however, compared with the Mar- formance of the great compound lens tian satellites) had not been determined. which forms what is technically called In 1875, Professor Hall undertook the its object-glass. difficult task of closely observing this When, then, news came that Professor body; and now, at last, astronomers at Hall, using this powerful instrument, had least know where, at any hour, on any discovered two satellites of Mars, even night, Hyperion is to be looked for those who at first supposed the news to though the search would be to very lit- be a mere report, felt that the observer tle purpose with any save two or three and the telescope were alike worthy of of the most powerful telescopes in exist- being credited with a success of the kind. ence. Again, amongst other of his ob- But in reality there was no room for servations which required keen vision doubt from the beginning. The news and patient watchfulness, must be cited had been telegraphed to Leverrier by the re-determination of the period in the Smithsonian Institution of Washingwhich the planet Saturn turns on its axis. ton, and by Leverrier announced to EngThis he accomplished in the year 1876. lish and Continental observers. It was But, undoubtedly, the detection of the known that an arrangement had been Martian satellites must be regarded as a made by the oceanic telegraph companies far more noteworthy achievement than to forward such intelligence, and that either of these.
the news must of necessity have come