A Popular History of Astronomy During the Nineteenth Century Part 32

[Footnote 770: _Ibid._, p. 334.]

[Footnote 771: _Comptes Rendus_, t. xcii., p. 812.]

[Footnote 772: _Observatory_, vol. v., p. 205.]

[Footnote 773: _Transits of Venus_, p. 89 (1st ed.).]

[Footnote 774: _Am. Jour. of Sc._, vol. xx., p. 393.]

[Footnote 775: _Month. Not._, vol. xvii., p. 219.]

[Footnote 776: _Mem. Roy. Astr. Soc._, vol. xlvi., p. 163.]

[Footnote 777: _Astr. Nach._, No. 1,897.]

[Footnote 778: Hilfiker, _Bern Mittheilungen_, 1878, p. 109.]

[Footnote 779: _Annals of the Cape Observatory_, vols. vi., vii.]

[Footnote 780: _Rapport sur l'etat de l'Observatoire de Paris pour l'Annee 1900_, p. 7.]

[Footnote 781: _Observatory_, vol. xxiii., p. 311; Newcomb, _Astr.

Jour._, No. 480.]

[Footnote 782: _Comptes Rendus_, t. xciii., p. 569.]

[Footnote 783: _Ibid._, t. xcii., p. 481.]

[Footnote 784: _Bull. de l'Acad._, t. vi., p. 842.]

[Footnote 785: _Month. Not._, vol. xlviii., p. 201.]

[Footnote 786: _Astr. Jour._, No. 182.]

[Footnote 787: _Astr. Nach._, No. 3,066.]

[Footnote 788: _Comptes Rendus_, t. xcii., p. 375; _Am. Jour. of Sc._, vol. xxii., p. 375.]

[Footnote 789: _Month. Not._, vol. x.x.xv., p. 401.]

[Footnote 790: _Am. Jour. of Sc._, vol. xviii., p. 393.]

[Footnote 791: _Nature_, vol. x.x.xiv., p. 170; _Astron. Papers of the American Ephemeris_, vol. ii., p. 113.]

[Footnote 792: _Comptes Rendus_, t. cxii., p. 549.]

[Footnote 793: _Astr. Journ._, Nos. 169, 170]

[Footnote 794: _The Solar Parallax and its Related Constants_, Washington, 1891.]

[Footnote 795: _Astr. and Astrophysics_, vol. xiii., p. 626.]

CHAPTER VII

_PLANETS AND SATELLITES_

Johann Hieronymus Schroter was the Herschel of Germany. He did not, it is true, possess the more brilliant gifts of his rival. Herschel's piercing discernment, comprehensive intelligence, and inventive splendour were wanting to him. He was, nevertheless, the founder of descriptive astronomy in Germany, as Herschel was in England.

Born at Erfurt in 1745, he prosecuted legal studies at Gottingen, and there imbibed from Kastner a life-long devotion to science. From the law, however, he got the means of living, and, what was to the full as precious to him, the means of observing. Entering the sphere of Hanoverian officialism in 1788, he settled a few years later at Lilienthal, near Bremen, as "Oberamtmann," or chief magistrate. Here he built a small observatory, enriched in 1785 with a seven-foot reflector by Herschel, then one of the most powerful instruments to be found anywhere out of England. It was soon surpa.s.sed, through his exertions, by the first-fruits of native industry in that branch. Schrader of Kiel transferred his workshops to Lilienthal in 1792, and constructed there, under the superintendence and at the cost of the astronomical Oberamtmann, a thirteen-foot reflector, declared by Lalande to be the finest telescope in existence, and one twenty-seven feet in focal length, probably as inferior to its predecessor in real efficiency as it was superior in size.

Thus, with instruments of gradually increasing power, Schroter studied during thirty-four years the topography of the moon and planets. The field was then almost untrodden; he had but few and casual predecessors, and has since had no equal in the sustained and concentrated patience of his hourly watchings. Both their prolixity and their enthusiasm are faithfully reflected in his various treatises. Yet the one may be pardoned for the sake of the other, especially when it is remembered that he struck out a substantially new line, and that one of the main lines of future advance. Moreover, his infectious zeal communicated itself; he set the example of observing when there was scarcely an observer in Germany; and under his roof Harding and Bessel received their training as practical astronomers.

But he was reserved to see evil days. Early in 1813 the French under Vandamme occupied Bremen. On the night of April 20, the Vale of Lilies was, by their wanton destructiveness, laid waste with fire; the Government offices were destroyed, and with them the chief part of Schroter's property, including the whole stock of his books and writings. There was worse behind. A few days later, his observatory, which had escaped the conflagration, was broken into, pillaged, and ruined. His life was wrecked with it. He survived the catastrophe three years without the means to repair, or the power to forget it, and gradually sank from disappointment into decay, terminated by death, August 29, 1816. He had, indeed, done all the work he was capable of; and though not of the first quality, it was far from contemptible. He laid the foundation of the _comparative_ study of the moon's surface, and the descriptive particulars of the planets laboriously collected by him const.i.tuted a store of more or less reliable information hardly added to during the ensuing half century. They rested, it is true, under some shadow of doubt; but the most recent observations have tended on several points to rehabilitate the discredited authority of the Lilienthal astronomer. We may now briefly resume, and pursue in its further progress, the course of his studies, taking the planets in the order of their distances from the sun.

In April, 1792, Schroter saw reason to conclude, from the gradual degradation of light on its partially illuminated disc, that Mercury possesses a tolerably dense atmosphere.[796] During the transit of May 7, 1799, he was, moreover, struck with the appearance of a ring of softened luminosity encircling the planet to an apparent height of three seconds, or about a quarter of its own diameter.[797] Although a "mere thought" in texture, it remained persistently visible both with the seven-foot and the thirteen-foot reflectors, armed with powers up to 288. It had a well-marked grayish boundary, and reminded him, though indefinitely fainter, of the penumbra of a sun-spot. A similar appendage had been noticed by De Plantade at Montpellier, November 11, 1736, and again in 1786 and 1789 by Prosperin and Flaugergues; but Herschel, on November 9, 1802, saw the preceding limb of the planet projected on the sun cut the luminous solar clouds with the most perfect sharpness.[798]

The presence, however, of a "halo" was unmistakable in 1832, when Professor Moll, of Utrecht, described it as a "nebulous ring of a darker tinge approaching to the violet colour."[799] Again, to Huggins and Stone, November 5, 1868, it showed as lucid and most distinct. No change in the colour of the gla.s.ses used, or the powers applied, could get rid of it, and it lasted throughout the transit.[800] It was next seen by Christie and Dunkin at Greenwich, May 6, 1878,[801] and with much precision of detail by Trouvelot at Cambridge (U.S.).[802] Professor Holden, on the other hand, noted at Hastings-on-Hudson the total absence of all anomalous appearances.[803] Nor could any vestige of them be perceived by Barnard at Lick on November 10, 1894.[804] Various effects of irradiation and diffraction were, however, observed by Lowell and W.

H. Pickering at Flagstaff;[805] and Davidson was favoured at San Francisco with glimpses of the historic aureola,[806] as well as of a central whitish spot, which often accompanies it. That both are somehow of optical production can scarcely be doubted.

Nothing can be learned from them regarding the planet's physical condition. Airy showed that refraction in a Mercurian atmosphere could not possibly originate the noted aureola, which must accordingly be set down as "strictly an ocular nervous phenomenon."[807] It is the less easy to escape from this conclusion that we find the virtually airless moon capable of exhibiting a like appendage. Professor Stephen Alexander, of the United States Survey, with two other observers, perceived, during the eclipse of the sun of July 18, 1860, the advancing lunar limb to be bordered with a bright band;[808] and photographic effects of the same kind appear in pictures of transits of Venus and partial solar eclipses.

The spectroscope affords little information as to the const.i.tution of Mercury. Its light is of course that of the sun reflected, and its spectrum is consequently a faint echo of the Fraunhofer spectrum. Dr. H.

C. Vogel, who first examined it in April, 1871, _suspected_ traces of the action of an atmosphere like ours,[809] but, it would seem, on slight grounds. It is, however, certainly very poor in blue rays. More definite conclusions were, in 1874,[810] derived by Zollner from photometric observations of Mercurian phases. A similar study of the waxing and waning moon had afforded him the curious discovery that light-changes dependent upon phase vary with the nature of the reflecting surface, following a totally different law on a smooth h.o.m.ogeneous globe and on a rugged and mountainous one. Now the phases of Mercury--so far as could be determined from only two sets of observations--correspond with the latter kind of structure. Strictly a.n.a.logous to those of the moon, they seem to indicate an a.n.a.logous mode of surface-formation. This conclusion was fully borne out by Muller's more extended observations at Potsdam during the years 1885-1893.[811]

Practical a.s.surance was gained from them that the innermost planet has a rough rind of dusky rock, absorbing all but 17 per cent. of the light poured upon it by the fierce adjacent sun. Its "albedo," in other words, is 017,[812] which is precisely that ascribed to the moon. The absence of any appreciable Mercurian atmosphere followed almost necessarily from these results.

On March 26, 1800, Schroter, observing with his 13-foot reflector in a peculiarly clear sky, perceived the southern horn of Mercury's crescent to be quite distinctly blunted.[813] Interception of sunlight by a Mercurian mountain rather more than eleven English miles high explained the effect to his satisfaction. By carefully timing its recurrence, he concluded rotation on an axis in a period of 24 hours 4 minutes. The first determination of the kind rewarded twenty years of unceasing vigilance. It received ostensible confirmation from the successive appearances of a dusky streak and blotch in May and June, 1801.[814]

These, however, were inferred to be no permanent markings on the body of the planet, but atmospheric formations, the streak at times drifting forwards (it was thought) under the fluctuating influence of Mercurian breezes. From a rediscussion of these somewhat doubtful observations Bessel inferred that Mercury rotates on an axis inclined 70 to the plane of its...o...b..t in 24 hours 53 seconds.

The rounded appearance of the southern horn seen by Schroter was more or less doubtfully caught by n.o.ble (1864), Burton, and Franks (1877);[815]

but was obvious to Mr. W. F. Denning at Bristol on the morning of November 5, 1882.[816] That the southern polar regions are usually less bright than the northern is well ascertained; but the cause of the deficiency remains dubious. If inequalities of surface are in question, they must be on a considerable scale; and a similar explanation might be given of the deformations of the "terminator"--or dividing-line between darkness and light in the planet's phases--first remarked by Schroter, and again clearly seen by Trouvelot in 1878 and 1881.[817] The displacement, during four days, of certain brilliant and dusky s.p.a.ces on the disc indicated to Mr. Denning in 1882 rotation in about twenty-five hours; while the general aspect of the planet reminded him of that of Mars.[818] But the difficulties in the way of its observation are enormously enhanced by its constant close attendance on the sun.

In his sustained study of the features of Mercury, Schroter had no imitator until Schiaparelli took up the task at Milan in 1882. His observations were made in daylight. It was found that much more could be seen, and higher magnifying powers used, high up in the sky near the sun, than at low alt.i.tudes, through the agitated air of morning or evening twilight. A notable discovery ensued.[819] Following the planet hour by hour, instead of making necessarily brief inspections at intervals of about a day, as previous observers had done, it was found that the markings faintly visible remained sensibly fixed, hence, that there was no rotation in a period at all comparable with that of the earth. And after long and patient watching, the conclusion was at last reached that Mercury turns on his axis in the same time needed to complete a revolution in his...o...b..t. One of his hemispheres, then, is always averted from the sun, as one of the moon's hemispheres from the earth, while the other never shifts from beneath his torrid rays. The "librations," however, of Mercury are on a larger scale than those of the moon, because he travels in a more eccentric path. The temporary inequalities arising between his "even pacing" on an axis and his alternately accelerated and r.e.t.a.r.ded elliptical movement occasion, in fact, an oscillation to and fro of the boundaries of light and darkness on his globe over an arc of 47 22', in the course of his year of 88 days. Thus the regions of perpetual day and perpetual night are separated by two segments, amounting to one-fourth of the entire surface, where the sun rises and sets once in 88 days. Else there is no variation from the intense glare on one side of the globe, and the nocturnal blackness on the other.

To Schiaparelli's scrutiny, Mercury appeared as a "spotty globe,"

enveloped in a tolerably dense atmosphere. The brownish stripes and streaks, discerned on his rose-tinged disc, and judged to be permanent, were made the basis of a chart. They were not indeed always equally well seen. They disappeared regularly near the limb, and were at times veiled even when centrally situated. Some of them had been clearly perceived by De Ball at Bothkamp in 1882.[820]

Mr. Lowell followed Schiaparelli's example by observing Mercury in the full glare of noon. "The best time to study him," he remarked, "is when planetary almanacs state 'Mercury invisible.'" A remarkable series of drawings executed, some at Flagstaff in 1896, the remainder at Mexico in 1897, supplied grounds for the following, among other, conclusions.[821]

Mercury rotates synchronously with its revolution--that is, once in 88 days--on an axis sensibly perpendicular to its...o...b..tal plane. No certain signs of a Mercurian atmosphere are visible. The globe is seamed and furrowed with long narrow markings, explicable as cracks in cooling. It is, and always was, a dead world. From micrometrical measures, moreover, the inferences were drawn that the planet's ma.s.s has a probable value about 1/20 that of the earth, while its mean density falls considerably short of the terrestrial standard.

The theory of Mercury's movements has always given trouble. In Lalande's,[822] as in Mastlin's time, the planet seemed to exist for no other purpose than to throw discredit on astronomers; and even to Leverrier's powerful a.n.a.lysis it long proved recalcitrant. On the 12th of September, 1869, however, he was able to announce before the Academy of Sciences[823] the terms of a compromise between observation and calculation. They involved the addition of a new member to the solar system. The hitherto unrecognised presence of a body about the size of Mercury itself revolving at somewhat less than half its mean distance from the sun (or, if farther, then of less ma.s.s, and _vice versa_), would, it was pointed out, produce exactly the effect required, of displacing the perihelion of the former planet 38" a century more than could otherwise be accounted for. The planes of the two orbits, however, should not lie far apart, as otherwise a nodal disturbance would arise not perceived to exist. It was added that a ring of asteroids similarly placed would answer the purpose equally well, and was more likely to have escaped notice.

Upon the heels of this forecast followed promptly a seeming verification. Dr. Lescarbault, a physician residing at Orgeres, whose slender opportunities had not blunted his hopes of achievement, had, ever since 1845, when he witnessed a transit of Mercury, cherished the idea that an unknown planet might be caught thus projected on the solar background. Unable to observe continuously until 1858, he, on March 26, 1859, saw what he had expected--a small perfectly round object slowly traversing the sun's disc. The fruitless expectation of reobserving the phenomenon, however, kept him silent, and it was not until December 22, after the news of Leverrier's prediction had reached him, that he wrote to acquaint him with his supposed discovery.[824] The Imperial Astronomer thereupon hurried down to Orgeres, and by personal inspection of the simple apparatus used, by searching cross-examination and local inquiry, convinced himself of the genuine character and substantial accuracy of the reported observation. He named the new planet "Vulcan,"

and computed elements giving it a period of revolution slightly under twenty days.[825] But it has never since been seen. M. Liais, director of the Brazilian Coast Survey, thought himself justified in a.s.serting that it never had been seen. Observing the sun for twelve minutes after the supposed ingress recorded at Orgeres, he noted those particular regions of its surface as "tres uniformes d'intensite."[826] He subsequently, however, admitted Lescarbault's good faith, at first rashly questioned. The planet-seeking doctor was, in truth, only one among many victims of similar illusions.

Waning interest in the subject was revived by a fresh announcement of a transit witnessed, it was a.s.serted, by Weber at Peckeloh, April 4, 1876.[827] The pseudo-planet, indeed, was detected shortly afterwards on the Greenwich photographs, and was found to have been seen by M. Ventosa at Madrid in its true character of a sun-spot without penumbra; but Leverrier had meantime undertaken the investigation of a list of twenty similar dubious appearances, collected by Haase, and republished by Wolf in 1872.[828] From these, five were picked out as referring in all likelihood to the same body, the reality of whose existence was now confidently a.s.serted, and of which more or less probable transits were fixed for March 22, 1877, and October 15, 1882.[829] But, widespread watchfulness notwithstanding, no suspicious object came into view at either epoch.