Part 39 (1/2)
Only since it was adopted and enforced by Zollner in 1865,[1043] can it be regarded as permanently acquired to science. The rapid changes in the cloud-belts both of Jupiter and Saturn, he remarked, attest a high internal temperature. For we know that all atmospheric movements on the earth are sun-heat transformed into motion. But sun-heat at the distance of Jupiter possesses but 1/27, at that of Saturn 1/100 of its force here. The large amount of energy, then, obviously exerted in those remote firmaments must have some other source, to be found nowhere else than in their own active and all-pervading fires, not yet banked in with a thick solid crust.
The same acute investigator dwelt, in 1871,[1044] on the similarity between the modes of rotation of the great planets and of the sun, applying the same principles of explanation to each case. The fact of this similarity is undoubted. Ca.s.sini[1045] and Schroter both noticed that markings on Jupiter travelled quicker the nearer they were to his equator; and Ca.s.sini even hinted at their possible a.s.similation to sun-spots.[1046] It is now well ascertained that, as a rule (not without exceptions), equatorial spots give a period some 5-1/2 minutes shorter than those in lat.i.tudes of about 30. But, as Mr. Denning has pointed out,[1047] no single period will satisfy the observations either of different markings at the same epoch, or of the same markings at different epochs. Accelerations and r.e.t.a.r.dations, depending upon processes of growth or change, take place in very much the same kind of way as in solar maculae, inevitably suggesting similarity of origin.
The interesting query as to Jupiter's surface incandescence has been studied since Bond's time with the aid of all the appliances furnished to physical inquirers by modern inventiveness, yet without bringing to it a categorical reply. Zollner in 1865, Muller in 1893, estimated his albedo at 062 and 075 respectively, that of fresh-fallen snow being 078, and of white paper 070.[1048] But the disc of Jupiter is by no means purely white. The general ground is tinged with ochre; the polar zones are leaden or fawn coloured; large s.p.a.ces are at times stained or suffused with chocolate-browns and rosy hues. It is occasionally seen ruled from pole to pole with dusky bars, and is never wholly free from obscure markings. The reflection, then, by it, as a whole, of about 70 per cent. of the rays impinging upon it, might well suggest some original reinforcement.
Nevertheless, the spectroscope gives little countenance to the supposition of any considerable permanent light-emission. The spectrum of Jupiter, as examined by Huggins, 1862-64, and by Vogel, 1871-73, shows the familiar Fraunhofer rays belonging to reflected sunlight. But it also shows lines of native absorption. Some of these are identical with those produced by the action of our own atmosphere, especially one or more groups due to aqueous vapours; others are of unknown origin; and it is remarkable that one among the latter--a strong band in the red--agrees in position with a dark line in the spectra of some ruddy stars.[1049] There is, besides, a general absorption of blue rays, intensified--as Le Sueur observed at Melbourne in 1869[1050]--in the dusky markings, evidently through an increase of depth in the atmospheric strata traversed by the light proceeding from them.
All these observations, however (setting aside the stellar line as of doubtful significance), point to a cool planetary atmosphere. One spectrograph, it is true, taken by Dr. Henry Draper, September 27, 1879,[1051] seemed to attest the action of intrinsic light; but the peculiarity was referred by Dr. Vogel, with convincing clearness, to a flaw in the film.[1052] So far, then, native emissions from any part of Jupiter's diversified surface have not been detected; and, indeed, the blackness of the shadows cast by his satellites on his disc sufficiently proves that he sends out virtually none but reflected light.[1053] This conclusion, however, by no means invalidates that of his high internal temperature.
The curious phenomena attending Jovian satellite-transits may be explained, partly as effects of contrast, partly as due to temporary obscurations of the small discs projected on the large disc of Jupiter.
At their first entry upon its marginal parts, which are several times less luminous than those near the centre, they invariably show as bright spots, then usually vanish as the background gains l.u.s.tre, to reappear, after crossing the disc, thrown into relief, as before, against the dusky limb. But instances are not rare, more especially of the third and fourth satellites standing out, during the entire middle part of their course, in such inky darkness as to be mistaken for their own shadows.
The earliest witness of a ”black transit” was Ca.s.sini, September 2, 1665; Romer in 1677, and Maraldi in 1707 and 1713, made similar observations, which have been multiplied in recent years. In some cases the process of darkening has been visibly attended by the formation, or emergence into view, of spots on the transiting body, as noted by the two Bonds at Harvard, March 18, 1848.[1054] The third satellite was seen by Dawes, half dark, half bright, when crossing Jupiter's disc, August 21, 1867;[1055] one-third dark by Davidson of California, January 15, 1884, under the same circ.u.mstances;[1056] and unmistakably spotted, both on and off the planet, by Schroter, Secchi, Dawes, and La.s.sell.
The first satellite sometimes looks dusky, but never absolutely black, in travelling over the disc of Jupiter. The second appears uniformly white--a circ.u.mstance attributed by Dr. Spitta[1057] to its high albedo.
The singularly different aspects, even during successive transits, of the third and fourth satellites, are connected by Professor Holden[1058]
with the varied luminosity of the segments of the planetary surface they are projected upon, and W. H. Pickering inclines to the same opinion; but fluctuations in their own brightness[1059] may be a concurrent cause. Herschel concluded in 1797 that, like our moon, they always turn the same face towards their primary, and as regards the outer satellite, Engelmann's researches in 1871, and C. E. Burton's in 1873, made this almost certain; while both for the third and fourth Jovian moons it was completely a.s.sured by W. H. Pickering's and A. E. Dougla.s.s's observations at Arequipa in 1892,[1060] and at Flagstaff in 1894-95.[1061] Strangely enough, however, the interior members of the system have preserved a relatively swift rotation, notwithstanding the enormous checking influence upon it of Jove-raised tides.
All the satellites are stated, on good authority, to be more or less egg-shaped. On September 8, 1890, Barnard saw the first elongated and bisected by a bright equatorial belt, during one of its dark transits;[1062] and his observation, repeated August 3, 1891, was completely verified by Schaeberle and Campbell, who ascertained, moreover, that the longer axis of the prolate body was directed towards Jupiter's centre.[1063] The ellipticity of its companions was determined by Pickering and Dougla.s.s; indeed, that of No. 3 had long previously been noticed by Secchi.[1064] No. 3 also shows equatorial stripes, perceived in 1891 by Schaeberle and Campbell,[1065] and evident later to Pickering and Dougla.s.s;[1066] nor need we hesitate to admit as authentic their records of similar, though less conspicuous markings on the other satellites. A const.i.tution a.n.a.logous to that of Jupiter himself was thus unexpectedly suggested; and Vogel's detection of lines--or traces of lines--in their spectra, agreeing with absorption-rays derived from their primary, lends support to the conjecture that they possess gaseous envelopes similar to his.
The system of Jupiter, as it was discovered by Galileo, and investigated by Laplace, appeared in its outward aspect so symmetrical, and displayed in its inner mechanism such harmonious dynamical relations, that it might well have been deemed complete. Nevertheless, a new member has been added to it. Near midnight on September 9, 1892, Professor Barnard discerned with the Lick 36-inch ”a tiny speck of light,” closely following the planet.[1067] He instantly divined its nature, watched its hurried disappearance in the adjacent glare, and made sure of the reality of his discovery on the ensuing night. It was a delicate business throughout, the Liliputian luminary subsiding into invisibility before the slightest glint of Jovian light, and tarrying, only for brief intervals, far enough from the disc to admit of its exclusion by means of an occulting plate. The new satellite is estimated to be of the thirteenth stellar magnitude, and, if equally reflective of light with its next neighbour, Io (satellite No. 1), its diameter must be about one hundred miles. It revolves at a distance of 112,500 miles from Jupiter's centre, and of 68,000 from his bulging equatorial surface. Its period of 11h. 57m. 23s. is just two hours longer than Jupiter's period of rotation, so that Phobos still remains a unique example of a secondary body revolving faster than its primary rotates. Jupiter's innermost moon conforms in its motions strictly, indeed inevitably, to the plane of his equatorial protuberance, following, however, a sensibly elliptical path the major axis of which is in rapid revolution.[1068] Its very insignificance raises the suspicion that it may not prove solitary.
Possibly it belongs to a zone peopled by asteroidal satellites. More than fifteen thousand such small bodies could be furnished out of the materials of a single full-sized satellite spoiled in the making. But we must be content for the present to register the fact without seeking to penetrate the meaning of its existence. Very high and very fine telescopic power is needed for its perception. Outside the United States, it has been very little observed. The only instruments in this country successfully employed for its detection are, we believe, Dr.
Common's 5-foot reflector and Mr. Newall's 25-inch refractor.
In the course of his observations on Jupiter at Brussels in 1878, M.
Niesten was struck with a rosy cloud attached to a whitish zone beneath the dark southern equatorial band.[1069] Its size was enormous. At the distance of Jupiter, its measured dimensions of 13” by 3” implied a real extension in longitude of 30,000, in lat.i.tude of something short of 7,000 miles. The earliest record of its appearance seems to be by Professor Pritchett, director of the Morrison Observatory (U.S.), who figured and described it July 9, 1878.[1070] It was again delineated August 9, by Tempel at Florence.[1071] In the following year it attracted the wonder and attention of almost every possessor of a telescope. Its colour had by that time deepened into a full brick-red, and was set off by contrast with a white equatorial spot of unusual brilliancy. During three ensuing years these remarkable objects continued to offer a visible and striking ill.u.s.tration of the compound nature of the planet's rotation. The red spot completed a circuit in nine hours fifty-five minutes thirty-six seconds; the white spot in about five and a half minutes less. Their _relative_ motion was thus no less than 260 miles an hour, bringing them together in the same meridian at intervals of forty-four days ten hours forty-two minutes. Neither, however, preserved continuously the same uniform rate of travel. The period of each had lengthened by some seconds in 1883, while sudden displacements, a.s.sociated with the recovery of l.u.s.tre after recurrent fadings, were observed in the position of the white spot,[1072]
recalling the leap forward of a reviving sun-spot. Just the opposite effect attended the rekindling of the companion object. While semi-extinct, in 1882-84, it lost little motion; but a fresh access of r.e.t.a.r.dation was observed by Professor Young[1073] in connection with its brightening in 1886. This suggests very strongly that the red spot is _fed from below_. A s.h.i.+ning aureola of ”faculae,” described by Bredichin at Moscow, and by Lohse at Potsdam, as encircling it in September, 1879,[1074] was held to strengthen the solar a.n.a.logy.
The conspicuous visibility of this astonis.h.i.+ng object lasted three years. When the planet returned to opposition in 1882-83, it had faded so considerably that Ricc's uncertain glimpse of it at Palermo, May 31, 1883, was expected to be the last. It had, nevertheless, begun to recover in December, and presented to Mr. Denning in the beginning of 1886 much the same aspect as in October, 1882.[1075] Observed by him in an intermediate stage, February 25, 1885, when ”a mere skeleton of its former self,” it bore a striking likeness to an ”elliptical ring”
descried in the same lat.i.tude by Mr. Gledhill in 1869-70. This, indeed, might be called the preliminary sketch for the famous object brought to perfection ten years later, but which Mr. H. C. Russell of Sydney saw and drew still unfinished in June, 1876,[1076] before it had separated from its matrix, the dusky south tropical belt. In earlier times, too, a marking ”at once fixed and transient” had been repeatedly perceived attached to the southernmost of the central belts. It gave Ca.s.sini in 1665 a rotation-period of nine hours fifty-six minutes,[1077] reappeared and vanished eight times during the next forty-three years, and was last seen by Maraldi in 1713. It was, however, very much smaller than the recent object, and showed no unusual colour.[1078]
The a.s.siduous observations made on the ”Great Red Spot” by Mr. Denning at Bristol and by Professor Hough at Chicago afforded grounds only for negative conclusions as to its nature. It certainly did _not_ represent the outpourings of a Jovian volcano; it was in no sense attached to the Jovian soil--if the phrase have any application to that planet; it was _not_ a mere disclosure of a glowing ma.s.s elsewhere seethed over by rolling vapours. It was, indeed, certainly not self-luminous, a satellite projected upon it in transit having been seen to show as bright as upon the dusky equatorial bands. A fundamental objection to all three hypotheses is that the rotation of the spot was variable. It did not then ride at anchor, but floated free. Some held that its surface was depressed below the average cloud-level, and that the cavity was filled with vapours. Professor Wilson, on the other hand, observing with the 16-inch equatorial of the Goodsell Observatory in Minnesota, received a persistent impression of the object ”being at a higher level than the other markings.”[1079] A crucial experiment on this point was proposed by Mr. Stanley Williams in 1890.[1080] A dark spot moving faster along the same parallel was timed to overtake the red spot towards the end of July. A unique opportunity hence appeared to be at hand of determining the relative vertical depths of the two formations, one of which must inevitably, it was thought, pa.s.s above the other. No forecast included a third alternative, which was nevertheless adopted by the dark spot. It evaded the obstacle in its path by skirting round its southern edge.[1081] Nothing, then, was gained by the conjunction, beyond an additional proof of the singular repellent influence exerted by the red spot over the markings in its vicinity. It has, for example, gradually carved out a deep bay for its accommodation in the gray belt just north of it. The effect was not at first steadily present. A premonitory excavation was drawn by Schwabe at Dessau, September 5, 1831, and again by Trouvelot, Barnard, and Elvins in 1879; yet there was no sign of it in the following year. Its development can be traced in Dr. Boedd.i.c.ker's beautiful delineations of Jupiter, made with the Parsonstown 3-foot reflector, from 1881 to 1886.[1082] They record the belt as straight in 1881, but as strongly indented from January, 1883; and the cavity now promises to outlast the spot. So long as it survives, however, the forces at work in the spot can have lost little of their activity. For it must be remembered that the belt has a shorter rotation-period than the red spot, which, accordingly (as Mr. Elvins of Toronto has pointed out), b.r.e.a.s.t.s and diverts, by its interior energy, a current of flowing matter, ever ready to fill up its natural bed, and override the barrier of obstruction.
The famous spot was described by Keeler in 1889, as ”of a pale pink colour, slightly lighter in the middle. Its outline was a fairly true ellipse, framed in by bright white clouds.”[1083] The fading continuously in progress from 1887 was temporarily interrupted in 1891.
The revival, indeed, was brief. Professor Barnard wrote in August, 1892: ”The great red spot is still visible, but it has just pa.s.sed through a crisis that seemingly threatened its very existence. For the past month it has been all but impossible to catch the feeblest trace of the spot, though the ever-persistent bay in the equatorial belt close north of it, and which has been so intimately connected with the history of the red spot, has been as conspicuous as ever. It is now, however, possible to detect traces of the entire spot. An obscuring medium seems to have been pa.s.sing over it, and has now drifted somewhat preceding the spot.”[1084]
The object is now always inconspicuous, and often practically invisible, and may be said to float pa.s.sively in the environing medium.[1085] Yet there are sparks beneath the ashes. A rosy tinge faintly suffused it in April, 1900,[1086] and its absolute end may still be remote.
The extreme complexity of the planet's surface-movements has been strikingly evinced by Mr. Stanley Williams's detailed investigations. He enumerated in 1896[1087] nine princ.i.p.al currents, all flowing parallel to the equator, but unsymmetrically placed north and south of it, and showing scant signs of conformity to the solar rule of r.e.t.a.r.dation with increase of lat.i.tude. The linear rate of the planet's equatorial rotation was spectroscopically determined by Belopolsky and Deslandres in 1895. Both found it to fall short of the calculated speed, whence an enlargement, by self-refraction, of the apparent disc was inferred.[1088]
Jupiter was systematically photographed with the Lick 36-inch telescope during the oppositions of 1890, 1891, and 1892, the image thrown on the plates (after eightfold direct enlargement) being one inch in diameter.
Mr. Stanley Williams's measurements and discussion of the set for 1891 showed the high value of the materials thus collected, although much more minute details can be seen than can at present be photographed. The red spot shows as ”very distinctly annular” in several of these pictures.[1089] Recently, the planet has been portrayed by Deslandres with the 62-foot Meudon refractor.[1090] The extreme actinic feebleness of the equatorial bands was strikingly apparent on his plates.
In 1870, Mr. Ranyard[1091]--whose death, December 14, 1894, was a serious loss to astronomy--acting upon an earlier suggestion of Sir William Huggins, collected records of unusual appearances on the disc of Jupiter, with a view to investigate the question of their recurrence at regular intervals. He concluded that the development of the deeper tinges of colour, and of the equatorial ”port-hole” markings girdling the globe in regular alternations of bright and dusky, agreed, so far as could be ascertained, with epochs of sun-spot maximum. The further inquiries of Dr. Lohse at Bothkamp in 1873[1092] went to strengthen the coincidence, which had been antic.i.p.ated _a priori_ by Zollner in 1871.[1093] Moreover, separate and distinct evidence was alleged by Mr.
Denning in 1899 of decennial outbreaks of disturbance in north temperate regions.[1094] It may, indeed, be taken for granted that what Hahn terms the universal pulse of the solar system[1095] affects the vicissitudes of Jupiter; but the law of those vicissitudes is far from being so obviously subordinate to the rhythmical flow of central disturbance as are certain terrestrial phenomena. The great planet, being in fact himself a ”semi-sun,” may be regarded as an originator, no less than a recipient, of agitating influences, the combined effects of which may well appear insubordinate to any obvious law.
It is likely that Saturn is in a still earlier stage of planetary development than Jupiter. He is the lightest for his size of all the planets. In fact, he would float in water. And since his density is shown, by the amount of his equatorial bulging, to increase centrally,[1096] it follows that his superficial materials must be of a specific gravity so low as to be inconsistent, on any probable supposition, with the solid or liquid states. Moreover, the chief arguments in favour of the high temperature of Jupiter, apply, with increased force, to Saturn; so that it may be concluded, without much risk of error, that a large proportion of his bulky globe, 73,000 miles in diameter, is composed of heated vapours, kept in active and agitated circulation by the process of cooling.
His unique set of appendages has, since the middle of the last century, formed the subject of searching and fruitful inquiries, both theoretical and telescopic. The mechanical problem of the stability of Saturn's rings was left by Laplace in a very unsatisfactory condition.
Considering them as rotating solid bodies, he pointed out that they could not maintain their position unless their weight were in some way unsymmetrically distributed; but made no attempt to determine the kind or amount of irregularity needed to secure this end. Some observations by Herschel gave astronomers an excuse for taking for granted the fulfilment of the condition thus vaguely postulated; and the question remained in abeyance until once more brought prominently forward by the discovery of the dusky ring in 1850.