A Popular History of Astronomy During the Nineteenth Century Part 38

CHAPTER VIII

_PLANETS AND SATELLITES_--(_continued_)

"The a.n.a.logy between Mars and the earth is perhaps by far the greatest in the whole solar system." So Herschel wrote in 1783,[965] and so we may safely say to-day, after six score further years of scrutiny. The circ.u.mstance lends a particular interest to inquiries into the physical habitudes of our exterior planetary neighbour.

Fontana first caught glimpses, at Naples in 1636 and 1638,[966] of dusky stains on the ruddy disc of Mars. They were next seen by Hooke and Ca.s.sini in 1666, and this time with sufficient distinctness to serve as indexes to the planet's rotation, determined by the latter as taking place in a period of twenty-four hours forty minutes.[967] Increased confidence was given to this result through Maraldi's precise verification of it in 1719.[968] Among the spots observed by him, he distinguished two as stable in position, though variable in size. They were of a peculiar character, showing as bright patches round the poles, and had already been noticed during sixty years back. A current conjecture of their snowy nature obtained validity when Herschel connected their fluctuations in extent with the progress of the Martian seasons. The inference of frozen precipitations could scarcely be resisted when once it was clearly perceived that the shining polar zones did actually by turns diminish and grow with the alternations of summer and winter in the corresponding hemisphere.

This, it may be said, was the opening of our acquaintance with the state of things prevailing on the surface of Mars. It was accompanied by a steady a.s.sertion, on Herschel's part, of permanence in the dark markings, notwithstanding partial obscurations by clouds and vapours floating in a "considerable but moderate atmosphere." Hence the presumed inhabitants of the planet were inferred to "probably enjoy a situation in many respects similar to ours."[969]

Schroter, on the other hand, went altogether wide of the truth as regards Mars. He held that the surface visible to us is a mere sh.e.l.l of drifting cloud, deriving a certain amount of apparent stability from the influence on evaporation and condensation of subjacent but unseen areographical features;[970] and his opinion prevailed with his contemporaries. It was, however, rejected by Kunowsky in 1822, and finally overthrown by Beer and Madler's careful studies during five consecutive oppositions, 1830-39. They identified at each the same dark spots, frequently blurred with mists, especially when the local winter prevailed, but fundamentally unchanged.[971] In 1862 Lockyer established a "marvellous agreement" with Beer and Madler's results of 1830, leaving no doubt as to the complete fixity of the main features, amid "daily, nay, hourly," variations of detail through transits of clouds.[972] On seventeen nights of the same opposition, F. Kaiser of Leyden obtained drawings in which nearly all the markings noted in 1830 at Berlin reappeared, besides spots frequently seen respectively by Arago in 1813, by Herschel in 1783, and one sketched by Huygens in 1672 with a writing-pen in his diary.[973] From these data the Leyden observer arrived at a period of rotation of 24h. 37m. 2262s., being just one second shorter than that deduced, exclusively from their own observations, by Beer and Madler. The exactness of this result was practically confirmed by the inquiries of Professor Bakhuyzen of Leyden.[974] Using for a middle term of comparison the disinterred observations of Schroter, with those of Huygens at one, and of Schiaparelli at the other end of an interval of 220 years, he was enabled to show, with something like certainty, that the time of rotation (24h. 37m. 22735s.) ascribed to Mars by Mr. Proctor[975] in reliance on a drawing executed by Hooke in 1666, was too long by _nearly one-tenth of a second_. The minuteness of the correction indicates the nicety of care employed. Nor employed vainly; for, owing to the comparative antiquity of the records available in this case, an almost infinitesimal error becomes so multiplied by frequent repet.i.tion as to produce palpable discrepancies in the positions of the markings at distant dates. Hence Bakhuyzen's period of 24h. 37m. 2266s. is undoubtedly of a precision unapproached as regards any other heavenly body save the earth itself.

Two facts bearing on the state of things at the surface of Mars were, then, fully acquired to science in or before the year 1862. The first was that of the seasonal fluctuations of the polar spots; the second, that of the general permanence of certain dark gray or greenish patches, perceived with the telescope as standing out from the deep yellow ground of the disc. That these varieties of tint correspond to the real diversities of a terraqueous globe, the "ripe cornfield"[976] sections representing land, the dusky spots and streaks, oceans and straits, has long been the prevalent opinion. Sir J. Herschel in 1830 led the way in ascribing the redness of the planet's light to an inherent peculiarity of soil.[977] Previously it had been a.s.similated to our sunset glows rather than to our red sandstone formations--set down, that is, to an atmospheric stoppage of blue rays. But the extensive Martian atmosphere, implicitly believed in on the strength of some erroneous observations by Ca.s.sini and Romer in the seventeenth century, vanished before the sharp occultation of a small star in Leo, witnessed by Sir James South in 1822;[978] and Dawes's observation in 1865,[979] that the ruddy tinge is deepest near the central parts of the disc, certified its non-atmospheric origin. The absolute whiteness of the polar snow-caps was alleged in support of the same inference by Sir William Huggins in 1867.[980]

All recent operations tend to show that the atmosphere of Mars is much thinner than our own. This was to have been expected _a priori_, since the same proportionate ma.s.s of air would on his smaller globe form a relatively spa.r.s.e covering.[981] Besides, gravity there possesses less than four-tenths its force here, so that this spa.r.s.er covering would weigh less, and be less condensed, than if it enveloped the earth.

Atmospheric pressure would accordingly be of about two and a quarter, instead of fifteen terrestrial pounds per square inch. This corresponds with what the telescope shows us. It is extremely doubtful whether any features of the earth's actual surface could be distinguished by a planetary spectator, however well provided with optical a.s.sistance.

Professor Langley's inquiries[982] led him to conclude that fully twice as much light is absorbed by our air as had previously been supposed--say 40 per cent. of vertical rays in a clear sky. Of the sixty reaching the earth, less than a quarter would be reflected even from white sandstone; and this quarter would again pay heavy toll in escaping back to s.p.a.ce. Thus not more than perhaps ten or twelve out of the original hundred sent by the sun would, under the most favourable circ.u.mstances, and from the very centre of the earth's disc, reach the eye of a Martian or lunar observer. The light by which he views our world is, there is little doubt, light reflected from the various strata of our atmosphere, cloud or mist-laden or serene, as the case may be, with an occasional snow-mountain figuring as a permanent white spot.

This consideration at once shows us how much more tenuous the Martian air must be, since it admits of topographical delineations of the Martian globe. The clouds, too, that form in it seem in general to be rather of the nature of ground-mists than of heavy c.u.mulus.[983]

Occasionally, indeed, durable and extensive strata become visible.

During the latter half of October, 1894, for instance, a region as large as Europe remained apparently cloud-covered. Yet most recent observers are unable to detect the traces of aqueous absorption in the Martian spectrum noted by Huggins in 1867[984] and by Vogel in 1873.[985]

Campbell vainly looked for them,[986] visually in 1894, spectrographically in 1896; Keeler was equally unsuccessful;[987]

Jewell[988] holds that they could, with present appliances, only be perceived if the atmosphere of Mars were much richer in water-vapour than that of the earth. There can be little doubt, however, that its supply is about the minimum adequate to the needs of a _living_, and perhaps a life-nuturing planet.

The climate of Mars seems to be unexpectedly mild. Its _theoretical_ mean temperature, taking into account both distance from the sun and albedo, is 34 C. below freezing.[989] Yet its polar snows are both less extensive and less permanent than those on the earth. The southern white hood, noticed by Schiaparelli in 1877 to have survived the summer only as a small lateral patch, melted completely in 1894. Moreover, Mr. W. H.

Pickering observed with astonishment the disappearance, in the course of thirty-three days of June and July, 1892, of 1,600,000 square miles of southern snow.[990] Curiously enough, the initial stage of shrinkage in the white calotte was marked by its division into two unequal parts, as if in obedience to the mysterious principle of duplication governing so many Martian phenomena.[991] Changes of the hues a.s.sociated respectively with land and water accompanied in lower lat.i.tudes, and were thought to be occasioned by floods ensuing upon this rapid antarctic thaw. It is true that scarcity of moisture would account for the scantiness and transitoriness of snowy deposits easily liquefied because thinly spread.

But we might expect to see the whole wintry hemisphere, at any rate, frost-bound, since the sun radiates less than half as much heat on Mars as on the earth. Water seems, nevertheless, to remain, as a rule, uncongealed everywhere outside the polar regions. We are at a loss to imagine by what beneficent arrangement the rigorous conditions naturally to be looked for can be modified into a climate which might be found tolerable by creatures const.i.tuted like ourselves.

Martian topography may be said to form nowadays a separate sub-department of descriptive astronomy. The amount of detail become legible by close scrutiny on a little disc which, once in fifteen years, attains a maximum of about 1/5000 the area of the full moon, must excite surprise and might provoke incredulity. Spurious discoveries, however, have little chance of holding their own where there are so many compet.i.tors quite as ready to dispute as to confirm.

The first really good map of Mars was constructed in 1869 by Proctor from drawings by Dawes. Kaiser of Leyden followed in 1872 with a representation founded upon data of his own providing in 1862-64; and Terby, in his valuable _Areographie_, presented to the Brussels Academy in 1873[992] a careful discussion of all important observations from the time of Fontana downwards, thus virtually adding to knowledge by summarising and digesting it. The memorable opposition of September 5, 1877, marked a fresh epoch in the study of Mars. While executing a trigonometrical survey (the first attempted) of the disc, then of the unusual size of 25" across, G. V. Schiaparelli, director of the Milan Observatory, detected a novel and curious feature. What had been taken for Martian continents were found to be, in point of fact, agglomerations of islands, separated from each other by a network of so-called "ca.n.a.ls" (more properly _channels_).[993] These are obviously extensions of the "seas," originating and terminating in them, and sharing their gray-green hue, but running sometimes to a length of three or four thousand miles in a straight line, and preserving throughout a nearly uniform breadth of about sixty miles. Further inquiries have fully substantiated the discovery made at the Brera Observatory. The "ca.n.a.ls" of Mars are an actually existent and permanent phenomenon. An examination of the drawings in his possession showed M. Terby that they had been seen, though not distinctively recognised, by Dawes, Secchi, and Holden; several were independently traced out by Burton at the opposition of 1879; all were recovered by Schiaparelli himself in 1879 and 1881-82; and their indefinite multiplication resulted from Lovell's observations in 1894 and 1896.

When the planet culminated at midnight, and was therefore in opposition, December 26, 1881, its distance was greater, and its apparent diameter less than in 1877, in the proportion of sixteen to twenty-five. Its atmosphere was, however, more transparent, and ours of less impediment to northern observers, the object of scrutiny standing considerably higher in northern skies. Never before, at any rate, had the true aspect of Mars come out so clearly as at Milan, with the 8-3/4-inch Merz refractor of the observatory, between December, 1881, and February, 1882. The ca.n.a.ls were all again there, but this time they were--in as many as twenty cases--_seen in duplicate_. That is to say, a twin-ca.n.a.l ran parallel to the original one at an interval of 200 to 400 miles.[994]

We are here brought face to face with an apparently insoluble enigma.

Schiaparelli regards the "germination" of his ca.n.a.ls as a periodical phenomenon depending on the Martian seasons. It is, a.s.suredly, not an illusory one, since it was plainly apparent, during the opposition of 1886, to MM. Perrotin and Thollon at Nice,[995] and to the former, using the new 30-inch refractor of that observatory, in 1888; Mr. A. Stanley Williams, with the help of only a 6-1/2-inch reflector, distinctly perceived in 1890 seven of the duplicate objects noted at Milan,[996]

and the Lick observations, both of 1890 and of 1892, together with the drawings made at Flagstaff and Mexico during the last favourable oppositions of the nineteenth century, brought unequivocal confirmation to the accuracy of Schiaparelli's impressions.[997] Various conjectures have been hazarded in explanation of this bizarre appearance. The difficulty of conceiving a physical reality corresponding to it has suggested recourse to an optical rationale. Proctor regarded it as an effect of diffraction;[998] Stanislas Meunier, of oblique reflection from overlying mist-banks;[999] Flammarion considers it possible that companion-ca.n.a.ls might, under special circ.u.mstances, be evoked by refraction as a kind of mirage.[1000] But none of these speculations are really admissible, when all the facts are taken into account. The view that the ca.n.a.ls of Mars are vast rifts due to the cooling of the globe, is recommended by the circ.u.mstance that they tend to follow great circles; nevertheless, it would break down if, as Schiaparelli holds, the fluctuations in their visibility depend upon actual obliterations and re-emergencies. Fantastic though the theory of their artificial origin appear, it is held by serious astronomers. Its vogue is largely due to Mr. Lowell's ingenious advocacy. He considers the Martian globe to be everywhere intersected by an elaborate system of irrigation-works, rendered necessary by a perennial water-famine, relieved periodically by the melting of the polar snows. Nor does he admit the existence of oceans, or lakes. What have been taken for such are really tracts covered with vegetation, the bright areas intermixed with them representing sandy deserts. And it is noteworthy in this connection that Professor Barnard obtained in 1894,[1001] with the great Lick refractor, "suggestive and impressive views" disclosing details of light and shade on the gray-green patches so intricate and minute as almost to preclude the supposition of their aqueous nature.

The closeness of the terrestrial a.n.a.logy has thus of late been much impaired. Even if the surface of Mars be composed of land and water, their distribution must be of a completely original type. The interlacing everywhere of continents with arms of the sea (if that be the correct interpretation of the visual effects) implies that their levels scarcely differ;[1002] and Schiaparelli carries most observers with him in holding that their outlines are not absolutely constant, encroachments of dusky upon bright tints suggesting extensive inundations.[1003] The late N. E. Green's observations at Madeira in 1877 indicated, on the other hand, a rugged south polar region. The contour of the snow-cap not only appeared indented, as if by valleys and promontories, but brilliant points were discerned outside the white area, attributed to isolated snow-peaks.[1004] Still more elevated, if similarly explained, must be the "ice island" first seen in a comparatively low lat.i.tude by Dawes in January, 1865.

On August 4, 1892, Mars stood opposite to the sun at a distance of only 34,865,000 miles from the earth. In point of vicinity, then, its situation was scarcely less favourable than in 1877. The low alt.i.tude of the planet, however, practically neutralised this advantage for northern observers, and public expectation, which had been raised to the highest pitch by the announcements of sensation-mongers, was somewhat disappointed at the "meagreness" of the news authentically received from Mars. Valuable series of observations were, nevertheless, made at Lick and Arequipa; and they unite in testifying to the genuine prevalence of surface-variability, especially in certain regions of intermediate tint, and perhaps of the "crude consistence" of "boggy Syrtes, neither sea, nor good dry land." Professor Holden insisted on the "enormous difficulties in the way of completely explaining the recorded phenomena by terrestrial a.n.a.logies";[1005] Mr. W. H. Pickering spoke of "conspicuous and startling changes." They, however, merely overlaid, and partially disguised, a general stability. Among the novelties detected by Mr. Pickering were a number of "lakes," or "oases" (in Lowell's phraseology), under the aspect of black dots at the junctions of two or more ca.n.a.ls;[1006] and he, no less than the Lick astronomers and M.

Perrotin at Nice,[1007] observed brilliant clouds projecting beyond the terminator, or above the limb, while carried round by the planet's rotation. They seemed to float at an alt.i.tude of at least twenty miles, or about four times the height of terrestrial cirrus; but this was not wonderful, considering the low power of gravity acting upon them. Great capital was made in the journalistic interest out of these imaginary signals from intelligent Martians, desirous of opening communications with (to them) problematical terrestrial beings. Similar effects had, however, been seen before by Mr. k.n.o.bel in 1873, by M. Terby in 1888, and at the Lick Observatory in 1890; and they were discerned again with particular distinctness by Professor Hussey at Lick, August 27, 1896.[1008]

The first photograph of Mars was taken by Gould at Cordoba in 1879.

Little real service in planetary delineation has, it is true, been so far rendered by the art, yet one achievement must be recorded to its credit. A set of photographs obtained by Mr. W. H. Pickering on Wilson's Peak, California, April 9, 1890, showed the southern polar cap of Mars as of moderate dimensions, but with a large dim adjacent area.

Twenty-four hours later, on a corresponding set, the dim area was brilliantly white. The polar cap had become enlarged in the interim, apparently through a wide-spreading snow-fall, by the annexation of a territory equal to that of the United States. The season was towards the close of winter in Mars. Never until then had the process of glacial extension been actually (it might be said) superintended in that distant globe.

Mars was gratuitously supplied with a pair of satellites long before he was found actually to possess them. Kepler interpreted Galileo's anagram of the "triple" Saturn in this sense; they were perceived by Micromegas on his long voyage through s.p.a.ce; and the Laputan astronomers had even arrived at a knowledge, curiously accurate under the circ.u.mstances, of their distances and periods. But terrestrial observers could see nothing of them until the night of August 11, 1877. The planet was then within one month of its second nearest approach to the earth during the last century; and in 1845 the Washington 26-inch refractor was not in existence.[1009] Professor Asaph Hall, accordingly, determined to turn the conjecture to account for an exhaustive inquiry into the surroundings of Mars. Keeping his glaring disc just outside the field of view, a minute attendant speck of light was "glimpsed" August 11. Bad weather, however, intervened, and it was not until the 16th that it was ascertained to be what it appeared--a satellite. On the following evening a second, still nearer to the primary, was discovered, which, by the bewildering rapidity of its pa.s.sages. .h.i.ther and thither, produced at first the effect of quite a crowd of little moons.[1010]

Both these delicate objects have since been repeatedly observed, both in Europe and America, even with comparatively small instruments. At the opposition of 1884, indeed, the distance of the planet was too great to permit of the detection of both elsewhere than at Washington. But the Lick equatoreal showed them, July 18, 1888, when their brightness was only 012 its amount at the time of their discovery; so that they can now be followed for a considerable time before and after the least favourable oppositions.

The names chosen for them were taken from the Iliad, where "Deimos" and "Phobos" (Fear and Panic) are represented as the companions in battle of Ares. In several respects, they are interesting and remarkable bodies.

As to size, they may be said to stand midway between meteorites and satellites. From careful photometric measures executed at Harvard in 1877 and 1879, Professor Pickering concluded their diameters to be respectively six and seven miles.[1011] This is on the a.s.sumption that they reflect the same proportion of the light incident upon them that their primary does. But it may very well be that they are less reflective, in which case they would be more extensive. The albedo of Mars is put by Muller at 027; his surface, in other words, returns 27 per cent. of the rays striking it. If we put the albedo of his satellites equal to that of our moon, 017, their diameters will be increased from 6 and 7 to 7-1/2 and 9 miles, Phobos, the inner one, being the larger. Mr. Lowell, however, formed a considerably larger estimate of their dimensions.[1012] It is interesting to note that Deimos, according to Professor Pickering's very distinct perception, does not share the reddish tint of Mars.

Deimos completes its nearly circular revolutions in thirty hours eighteen minutes, at a distance from the surface of its ruling body of 12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours thirty-nine minutes twenty-two seconds, at a distance of only 3,760 miles. This is the only known instance of a satellite circulating faster than its primary rotates, and is a circ.u.mstance of some importance as regards theories of planetary development. To a Martian spectator the curious effect would ensue of a celestial object, seemingly exempt from the general motion of the sphere, rising in the west, setting in the east, and culminating twice, or even thrice a day; which, moreover, in lat.i.tudes above 69 north or south, would be permanently and altogether hidden by the intervening curvature of the globe.

The detection of new members of the solar system has come to be one of the most ordinary of astronomical events. Since 1846 no single year has pa.s.sed without bringing its tribute of asteroidal discovery. In the last of the seventies alone, a full score of miniature planets were distinguished from the thronging stars amid which they seem to move; 1875 brought seventeen such recognitions; their number touched a minimum of one in 1881; it rose in 1882, and again in 1886, to eleven; dropped to six in 1889, and sprang up with the aid of photography to twenty-seven in 1892. That high level has since, on an average, been maintained; and on January 1, 1902, nearly 500 asteroids were recognised as revolving between the orbits of Mars and Jupiter. Of these, considerably more than one hundred are claimed by one investigator alone--Dr. Max Wolf of Heidelburg; M. Charlois of Nice comes second with 102; while among the earlier observers Palisa of Vienna contributed 86, and C. H. F. Peters of Clinton (N. Y.), whose varied and useful career terminated July 19, 1890, 52 to the grand total. The construction by Chacornac and his successors at Paris, and more recently by Peters at Clinton, of ecliptical charts showing all stars down to the thirteenth and fourteenth magnitudes respectively, rendered the picking out of moving objects above that brightness a mere question of time and diligence. Both, however, are vastly economised by the photographic method. Tedious comparisons of the sky with charts are no longer needed for the identification of unrecorded, because simulated stars. Planetary bodies declare themselves by appearing upon the plate, not in circular, but in linear form. Their motion converts their images into trails, long or short according to the time of exposure. The first asteroid (No. 323) thus detected was by Max Wolf, December 22, 1891.[1014] Eighteen others were similarly discovered in 1892, by the same skilful operator; and ten more through Charlois's adoption at Nice of the novel plan now in exclusive use for picking up errant light-specks. Far more onerous than the task of their discovery is that of keeping them in view once discovered--of tracking out their paths, ixing their places, and calculating the disturbing effects upon them of the mighty Jovian ma.s.s. These complex operations have come to be centralised at Berlin under the superintendence of Professor Tietjen, and their results are given to the public through the medium of the _Berliner Astronomisches Jahrbuch_.

The _cui bono?_ however, began to be agitated. Was it worth while to maintain a staff of astronomers for the sole purpose of keeping hold over the ident.i.ty of the innumerable component particles of a cosmical ring? The prospect, indeed, of all but a select few of the asteroids being thrown back by their contemptuous captors into the sea of s.p.a.ce seemed so imminent that Professor Watson provided by will against the dereliction of the twenty-two discovered by himself. But the fortunes of the whole family improved through the distinction obtained by one of them. On August 14, 1898, the trail of a rapidly-moving, star-like object of the eleventh magnitude imprinted itself on a plate exposed by Herr Witt at the Urania Observatory, Berlin. Its originator proved to be unique among asteroids. "Eros" is, in sober fact,

'one of those mysterious stars Which hide themselves between the Earth and Mars,'

divined or imagined by Sh.e.l.ley.[1015] True, several of its congeners invade the Martian sphere at intervals; but the proper habitat of Eros is within that limit, although its excursions transcend it. In other words, its mean distance from the sun is about 135, as compared with the Martian distance of 141 million miles. Further, its...o...b..t being so fortunately circ.u.mstanced as to bring it once in sixty-seven years within some 15 millions of miles of the earth, it is of extraordinary value to celestial surveyors. The calculation of its movements was much facilitated by detections, through a retrospective search,[1016] of many of its linear images among the star-dots on the Harvard plates.[1017]

The little body--which can scarcely be more than twenty miles in diameter--shows peculiarities of behaviour as well as of position. Dr.

von Oppolzer, in February, 1901,[1018] announced it to be extensively and rapidly variable. Once in 2 hours 38 minutes it lost about three-fourths of its light,[1019] but these fluctuations quickly diminished in range, and in the beginning of May ceased altogether.[1020] Evidently, then, they depend upon the situation of the asteroid relatively to ourselves; and, so far, events lent countenance to M. Andre's eclipse hypothesis, since mutual occultations of the supposed planetary twins could only take place when the plane of their revolutions pa.s.sed through the earth, and this condition would be transitory. Yet the recognition in Eros of an "Algol asteroid" seems on other grounds inadmissible;[1021] nor until the phenomenon is conspicuously renewed--as it probably will be at the opposition of 1903--can there be much hope of finding its appropriate rationale.

The crowd of orbits disclosed by asteroidal detections invites attentive study. D'Arrest remarked in 1851,[1022] when only thirteen minor planets were known, that supposing their paths to be represented by solid hoops, not one of the thirteen could be lifted from its place without bringing the others with it. The complexity of interwoven tracks thus ill.u.s.trated has grown almost in the numerical proportion of discovery. Yet no two actually intersect, because no two lie exactly in the same plane, so that the chances of collision are at present _nil_. There is only one case, indeed, in which it seems to be eventually possible. M. Lespiault has pointed out that the curves traversed by "Fides" and "Maa" approach so closely that a time may arrive when the bodies in question will either coalesce or unite to form a binary system.[1023]

The maze threaded by the 500 asteroids contrasts singularly with the harmoniously ordered and rhythmically separated orbits of the larger planets. Yet the seeming confusion is not without a plan.

The established rules of our system are far from being totally disregarded by its minor members. The orbit of Pallas, with its inclination of 34 42', touches the limit of departure from the ecliptic level; the average obliquity of the asteroidal paths is somewhat less than that of the sun's equator;[1024] their mean eccentricity is below that of the curve traced out by Mercury, and all without exception are pursued in the planetary direction--from west to east.

The zone in which these small bodies travel is about three times as wide as the interval separating the earth from the sun. It extends perilously near to Jupiter, and dovetails into the sphere of Mars.

Their distribution is very unequal. They are most densely congregated about the place where a single planet ought, by Bode's Law, to revolve; it may indeed be said that only stragglers from the main body are found more than fifty million miles within or without a mean distance from the sun 28 times that of the earth. Significant gaps, too, occur where some force prohibitive of their presence would seem to be at work. The probable nature of that force was suggested by the late Professor Kirkwood, first in 1866, when the number of known asteroids was only eighty-eight, and again with more confidence in 1876, from the study of a list then run up to 172.[1025] It appears that these bare s.p.a.ces are found just where a revolving body would have a period connected by a simple relation with that of Jupiter. It would perform two or three circuits to his one, five to his two, nine to his five, and so on.

Kirkwood's inference was that the gaps in question were cleared of asteroids by the attractive influence of Jupiter. For disturbances recurring time after time--owing to commensurability of periods--nearly at the same part of the orbit, would have acc.u.mulated until the shape of that orbit was notably changed. The body thus displaced would have come in contact with other cosmical particles of the same family with itself--then, it may be a.s.sumed, more evenly scattered than now--would have coalesced with them, and permanently left its original track. In this way the regions of maximum perturbation would gradually have become denuded of their occupants.

We can scarcely doubt that this law of commensurability has largely influenced the present distribution of the asteroids. But its effects must have been produced while they were still in an unformed, perhaps a nebular condition. In a system giving room for considerable modification through disturbance, the recurrence of conjunctions with a dominating ma.s.s at the same orbital point need not involve instability.[1026] On the whole, the correspondence of facts with Kirkwood's hypothesis has not been impaired by their more copious collection.[1027] Some chasms of secondary importance have indeed been bridged; but the princ.i.p.al stand out more conspicuously through the denser scattering of orbits near their margins. Nor is it doubtful that the influence of Jupiter in some way produced them. M. de Freycinet's study of the problem they present[1028] has, however, led him to the conclusion that they existed _ab origine_, thus testifying rather to the preventive than to the perturbing power of the giant planet.

The existence, too, of numerous asteroidal pairs travelling in approximately coincident tracks, must date from a remote antiquity. They result, Professor Kirkwood[1029] believed, from the divellent action of Jupiter upon embryo pigmy planets, just as comets moving in pursuit of one another are a consequence of the sundering influence of the sun.

Leverrier fixed, in 1853,[1030] one-fourth of the earth's ma.s.s as the outside limit for the combined ma.s.ses of all the bodies circulating between Mars and Jupiter; but it is far from probable that this maximum is at all nearly approached. M. Berberich[1031] held that the moon would more than outweigh the whole of them, a million of the lesser bodies shining like stars of the twelfth magnitude being needed, according to his judgment, to const.i.tute her ma.s.s. And M. Niesten estimated that the whole of the 216 asteroids discovered up to August, 1880, amounted in _volume_ to only 1/4000th of our globe,[1032] and we may safely add--since they are tolerably certain to be lighter, bulk for bulk, than the earth--that their proportionate _ma.s.s_ is smaller still. A fairly concordant result was published in 1895 by Mr. B. M. Roszel.[1033] He found that the lunar globe probably contains forty times, the terrestrial globe 3,240 times the quant.i.ty of matter parcelled out among the first 311 minor planets. The actual size of a few of them may now be said to be known. Professor Pickering, from determinations of light-intensity, a.s.signed to Vesta a diameter of 319 miles, to Pallas 167, to Juno 94, down to twelve and fourteen for the smaller members of the group.[1034] An albedo equal to that of Mars was a.s.sumed as the basis of the calculation. Moreover, Professor G. Muller[1035] of Potsdam examined photometrically the phases of seven among them, of which four--namely, Vesta, Iris, Ma.s.salia, and Amphitrite--were found to conform precisely to the behaviour of Mars as regards light-change from position, while Ceres, Pallas, and Irene varied after the manner of the moon and Mercury. The first group were hence inferred to resemble Mars in physical const.i.tution, nature of atmosphere, and reflective capacity; the second to be moon-like bodies.

Finally, Professor Barnard, directly measuring with the Yerkes refractor the minute discs presented by the original quartette, obtained the following authentic data concerning them:[1036] Diameter of Ceres, 477 miles, albedo = 018; diameter of Pallas, 304 miles, albedo = 023; diameter of Vesta, 239 miles, albedo = 074; diameter of Juno, 120 miles, albedo = 045. Thus, the rank of premier asteroid proves to belong to Ceres, and to have been erroneously a.s.signed to Vesta in consequence of its deceptive brilliancy. What kind of surface this indicates, it is hard to say. The dazzling whiteness of snow can hardly be attributed to bare rock; yet the dynamical theory of gases--as Dr.

Johnstone Stoney pointed out in 1867[1037]--prohibits the supposition that bodies of insignificant gravitative power can possess aerial envelopes. Even our moon, it is calculated, could not permanently hold back the particles of oxygen, nitrogen, or water-gas from escaping into infinite s.p.a.ce; still less, a globe one thousand times smaller. Vogel's suspicion of an air-line in the spectrum of Vesta[1038] has, accordingly, not been confirmed.

Crossing the zone of asteroids on our journey outward from the sun, we meet with a group of bodies widely different from the "inferior" or terrestrial planets. Their gigantic size, low specific gravity, and rapid rotation, obviously from the first threw the "superior" planets into a cla.s.s apart; and modern research has added qualities still more significant of a dissimilar physical const.i.tution. Jupiter, a huge globe 86,000 miles in diameter, stands pre-eminent among them. He is, however, only _primus inter pares_; all the wider inferences regarding his condition may be extended, with little risk of error, to his fellows; and inferences in his case rest on surer grounds than in the case of the others, from the advantages offered for telescopic scrutiny by his comparative nearness.

Now the characteristic modern discovery concerning Jupiter is that he is a body midway between the solar and terrestrial stages of cosmical existence--a decaying sun or a developing earth, as we choose to put it--whose vast unexpended stores of internal heat are mainly, if not solely, efficient in producing the interior agitations betrayed by the changing features of his visible disc. This view, impressed upon modern readers by Mr. Proctor's popular works, was antic.i.p.ated in the last century. Buffon wrote in his _epoques de la Nature_ (1778):[1039]--"La surface de Jupiter est, comme l'on sait, sujette a des changemens sensibles, qui semblent indiquer que cette grosse planete est encore dans un etat d'inconstance et de bouillonnement."

Primitive incandescence, attendant, in his fantastic view, on planetary origin by cometary impacts with the sun, combined, he concluded, with vast bulk to bring about this result. Jupiter has not yet had time to cool. Kant thought similarly in 1785;[1040] but the idea did not commend itself to the astronomers of the time, and dropped out of sight until Mr. Nasmyth arrived at it afresh in 1853.[1041] Even still, however, terrestrial a.n.a.logies held their ground. The dark belts running parallel to the equator, first seen at Naples in 1630, continued to be a.s.sociated--as Herschel had a.s.sociated them in 1781--with Jovian trade-winds, in raising which the deficient power of the sun was supposed to be compensated by added swiftness of rotation. But opinion was not permitted to halt here.

In 1860 G. P. Bond of Cambridge (U.S.) derived some remarkable indications from experiments on the light of Jupiter.[1042] They showed that fourteen times more of the photographic rays striking it are reflected by the planet than by our moon, and that, unlike the moon, which sends its densest rays from the margin, Jupiter is brightest near the centre. But the most perplexing part of his results was that Jupiter actually seemed to give out more light than he received. Bond, however, rightly considered his data too uncertain for the support of so bold an a.s.sumption as that of original luminosity, and, even if the presence of native light were proved, thought that it might emanate from auroral clouds of the terrestrial kind. The conception of a sun-like planet was still a remote, and seemed an extravagant one.