Part 33 (1/2)
The next announcement of the discovery of ”Vulcan” was on the occasion of the total solar eclipse of July 29, 1878.[830] This time it was stated to have been seen at some distance south-west of the obscured sun, as a ruddy star with a minute planetary disc; and its simultaneous detection by two observers--the late Professor James C. Watson, stationed at Rawlins (Wyoming Territory), and Professor Lewis Swift at Denver (Colorado)--was at first readily admitted. But their separate observations could, on a closer examination, by no possibility be brought into harmony, and, if valid, certainly referred to two distinct objects, if not to four; each astronomer eventually claiming a pair of planets. Nor could any one of the four be identified with Lescarbault's and Leverrier's Vulcan, which, if a substantial body revolving round the sun, must then have been found on the _east_ side of that luminary.[831]
The most feasible explanation of the puzzle seems to be that Watson and Swift merely saw each the same two stars in Cancer: haste and excitement doing the rest.[832] Nevertheless, they strenuously maintained their opposite conviction.[833]
Intra-Mercurian planets have since been diligently searched for when the opportunity of a total eclipse offered, especially during the long obscuration at Caroline Island. Not only did Professor Holden ”sweep” in the solar vicinity, but Palisa and Trouvelot agreed to divide the field of exploration, and thus make sure of whatever planetary prey there might be within reach; yet with only negative results. Photographic explorations during recent eclipses have been equally fruitless. Belief in the presence of any considerable body or bodies within the orbit of Mercury is, accordingly, at a low ebb. Yet the existence of the anomaly in the Mercurian movements indicated by Leverrier has been made only surer by further research.[834] Its elucidation const.i.tutes one of the ”pending problems” of astronomy.
From the observation at Bologna in 1666-67 of some very faint spots, Domenico Ca.s.sini concluded a rotation or libration of Venus--he was not sure which--in about twenty-three hours.[835] By Bianchini in 1726 the period was augmented to twenty-four _days_ eight hours. J. J. Ca.s.sini, however, in 1740, showed that the data collected by both observers were consistent with rotation in twenty-three hours twenty minutes.[836] So the matter rested until Schroter's time. After watching nine years in vain, he at last, February 28, 1788, perceived the ordinarily uniform brightness of the planet's disc to be marbled with a filmy streak, which returned periodically to the same position in about twenty-three hours twenty-eight minutes. This approximate estimate was corrected by the application of a more definite criterion. On December 28, 1789, the southern horn of the crescent Venus was seen truncated, an outlying lucid point interrupting the darkness beyond. Precisely the same appearance recurred two years later, giving for the planet's rotation a period of 23h. 21m.[837] To this only twenty-two seconds were added by De Vico, as the result of over 10,000 observations made with the Cauchoix refractor of the Collegio Romano, 1839-41.[838] The axis of rotation was found to be much more bowed towards the orbital plane than that of the earth, the equator making with it an angle of 53 11'.
These conclusions inspired, it is true, much distrust, consequently there were no received ideas on the subject to be subverted.
Nevertheless, a shock of surprise was felt at Schiaparelli's announcement, early in 1890,[839] that Venus most probably rotates after the fas.h.i.+on just previously ascribed to Mercury. A continuous series of observations, from November, 1877, to February, 1878, with their records in above a hundred drawings, supplied the chief part of the data upon which he rested his conclusions. They certainly appeared exceptionally well-grounded; and the doubts at first qualifying them were removed by a fresh set of determinations in July, 1895.[840] Most observers had depended, in their attempts to ascertain the rotation-period of Venus, upon evanescent shadings, most likely of atmospheric origin, and scarcely recognisable from day to day. Schiaparelli fixed his attention upon round, defined, l.u.s.trously white spots, the presence of which near the cusps of the illuminated crescent has been attested for close upon two centuries. His steady watch over them showed the invariability of their position with regard to the terminator; and this is as much as to say that the regions of day and night do not s.h.i.+ft on the surface of the planet. In other words, she keeps the same face always turned towards the sun. Moreover, since her orbit is nearly circular, libratory effects are very small. They amount in fact to only just one-thirtieth of those serving to modify the severe contrasts of climate in Mercury.
Confirmatory evidence of Schiaparelli's result for Venus is not wanting.
Thus, observations irreconcilable with a swift rate of rotation were made at Bothkamp in 1871 by Vogel and Lohse;[841] and a drawing executed by Professor Holden with the great Was.h.i.+ngton reflector, December 15, 1877, showed the same markings in the positions recorded at Milan to have been occupied by them eight hours previously. Further, a series of observations, carried out by M. Perrotin at Nice, May 15 to October 4, 1890, and from Mount Mounier in 1895-6, with the special aim of testing the inference of synchronous rotation and revolution, proved strongly corroborative of it.[842] A remarkable collection of drawings made by Mr. Lowell in 1896 appeared decisive in its favour;[843] Tacchini at Rome,[844] Mascari at Catania and Etna,[845] Cerulli at Terano,[846]
obtained in 1892-6 evidence similar in purport. On the other hand, Niesten of Brussels found reason to revert to Vico's discarded elements for the planet's rotation;[847] and Trouvelot,[848] Stanley Williams,[849] Villiger,[850] and Leo Brenner,[851] so far agreed with him as to adopt a period of approximately twenty-four hours. Finally, E.
Von Oppolzer suggested an appeal to the spectroscope;[852] and Belopolsky secured in 1900[853] spectrograms apparently marked by the minute displacements corresponding to a rapid rate of axial movement.
But they were avowedly taken only as an experiment, with unsuitable apparatus; and the desirable verification of their supposed import is not yet forthcoming. Until it is, Schiaparelli's period of 225 days must be allowed to hold the field.
Effects attributed to great differences of level in the surface of Venus have struck many observers. Francesco Fontana at Naples in 1643 noticed irregularities along the inner edge of the crescent.[854] Lahire in 1700 considered them--regard being had to difference of distance--to be much more strongly marked than those visible in the moon.[855] Schroter's a.s.sertions to the same effect, though scouted with some unnecessary vehemence by Herschel,[856] have since been repeatedly confirmed; amongst others by Madler, De Vico, Langdon, who in 1873 saw the broken line of the terminator with peculiar distinctness through a veil of auroral cloud;[857] by Denning,[858] March 30, 1881, despite preliminary impressions to the contrary, as well as by C. V. Zenger at Prague, January 8, 1883. The great mountain ma.s.s, presumed to occasion the periodical blunting of the southern horn, was precariously estimated by the Lilienthal observer to rise to the prodigious height of nearly twenty-seven miles, or just five times the elevation of Mount Everest!
Yet the phenomenon persists, whatever may be thought of the explanation.
Moreover, the speck of light beyond, interpreted as the visible sign of a detached peak rising high enough above the encircling shadow to catch the first and last rays of the sun, was frequently discerned by Baron Van Ertborn in 1876;[859] while an object near the northern horn of the crescent, strongly resembling a lunar ring-mountain, was delineated both by De Vico in 1841 and by Denning forty years later.
We are almost equally sure that Venus, as that the earth is encompa.s.sed with an atmosphere. Yet, notwithstanding luminous appearances plainly due to refraction during the transits both of 1761 and 1769, Schroter, in 1792, took the initiative in coming to a definite conclusion on the subject.[860] It was founded, first, on the rapid diminution of brilliancy towards the terminator, attributed to atmospheric absorption; next, on the extension beyond a semicircle of the horns of the crescent; lastly, on the presence of a bluish gleam illuminating the early hours of the Cytherean night with what was taken to be genuine twilight. Even Herschel admitted that sunlight, by the same effect through which the heavenly bodies show _visibly above_ our horizons while still _geometrically below_ them, appeared to be bent round the shoulder of the globe of Venus. Ample confirmation of the fact has since been afforded. At Dorpat in May, 1849, the planet being within 3 26' of inferior conjunction, Madler found the arms of waning light upon the disc to embrace no less than 240 of its extent;[861] and in December, 1842, Mr. Guthrie, of Bervie, N.B., actually observed, under similar conditions, the whole circ.u.mference to be lit up with a faint nebulous glow.[862] The same curious phenomenon was intermittently seen by Mr.
Leeson Prince at Uckfield in September, 1861;[863] but with more satisfactory distinctness by Mr. C. S. Lyman of Yale College,[864]
before and after the conjunction of December 11, 1866, and during nearly five hours previous to the transit of 1874, when the yellowish ring of refracted light showed at one point an approach to interruption, possibly through the intervention of a bank of clouds. Again, on December 2, 1898, Venus being 1 45' from the sun's centre, Mr. H. N.
Russell, of the Halsted Observatory, descried the coalescence of the cusps, and founded on the observation a valuable discussion of such effects.[865] Taking account of certain features in the case left unnoticed by Neison[866] and Proctor,[867] he inferred from them the presence of a Cytherean atmosphere considerably less refractive than our own, although possibly, in its lower strata, enc.u.mbered with dust or haze.
Similar appearances are conspicuous during transits. But while the Mercurian halo is characteristically seen on the sun, the ”silver thread” round the limb of Venus commonly shows on the part _off_ the sun. There are, however, instances of each description in both cases.
Mr. Grant, in collecting the records of physical phenomena accompanying the transits of 1761 and 1769, remarks that no one person saw both kinds of annulus, and argues a dissimilarity in their respective modes of production.[868] Such a dissimilarity probably exists, in the sense that the inner section of the ring is illusory, the outer, a genuine result of the bending of light in a gaseous envelope; but the distinction of separate visibility has not been borne out by recent experience. Several of the Australian observers during the transit of 1874 witnessed the complete phenomenon. Mr. J. Macdonnell, at Eden, saw a ”shadowy nebulous ring” surround the whole disc when ingress was two-thirds accomplished; Mr. Tornaghi, at Goulburn, perceived a halo, entire and unmistakable, at half egress.[869] Similar observations were made at Sydney,[870] and were renewed in 1882 by Lescarbault at Orgeres, by Metzger in Java, and by Barnard at Vanderbilt University.[871]
Spectroscopic indications of aqueous vapour as present in the atmosphere of Venus, were obtained in 1874 and 1882, by Tacchini and Ricc in Italy, and by Young in New Jersey.[872] Janssen, however, who made a special study of the point subsequently to the transit of 1882, found them much less certain than he had antic.i.p.ated;[873] and Vogel, by repeated examinations, 1871-73, could detect only the very slightest variations from the pattern of the solar spectrum. Some additions there indeed seem to be in the thickening of a few water and oxygen-lines; but so nearly evanescent as to induce the persuasion that most of the light we receive from Venus has traversed only the tenuous upper portion of its atmosphere.[874] It is reflected, at any rate, with comparatively slight diminution. On the 26th and 27th of September, 1878, a close conjunction gave Mr. James Nasmyth the rare opportunity of watching Venus and Mercury for several hours side by side in the field of his reflector; when the former appeared to him like clean silver, the latter as dull as lead or zinc.[875] Yet the light _incident_ upon Mercury is, on an average, three and a half times as strong as the light reaching Venus. Thus, the reflective power of Venus must be singularly strong.
And we find, accordingly, from a combination of Zollner's with Muller's results, that its albedo is but little inferior to that of new-fallen snow; in other words, it gives back 77 per cent. of the luminous rays impinging upon it.
This extraordinary brilliancy would be intelligible were it permissible to suppose that we see nothing of the planet but a dense canopy of clouds. But the hypothesis is discountenanced by the Flagstaff observations, and is irreconcilable with the visibility of mountainous elevations, and permanent surface-markings. To Mr. Lowell these were so distinct and unchanging as to furnish data for a chart of the Cytherean globe, and the peculiar arrangement of divergent shading exhibited in it cannot off-hand be set down as unreal, in view of Perrotin's earlier discernment of a.n.a.logous linear traces. Gruithuisen's ”snow-caps,”[876]
however--it is safe to say--do not exist as such; although s.h.i.+ning regions near the poles form a well-attested trait of the strange Cytherean landscape.
The ”secondary,” or ”ashen light,” of Venus was first noticed by Riccioli in 1643; it was seen by Derham about 1715, by Kirch in 1721, by Schroter and Harding in 1806;[877] and the reality of the appearance has since been authenticated by numerous and trustworthy observations. It is precisely similar to that of the ”old moon in the new moon's arms”; and Zenger, who witnessed it with unusual distinctness, January 8, 1883,[878] supposes it due to the same cause--namely, to the faint gleam of reflected earth-light from the night-side of the planet. When we remember, however, that ”full earth-light” on Venus, at its nearest, has little more than 1/12000 its intensity on the moon, we see at once that the explanation is inadequate. Nor can Professor Safarik's,[879] by phosph.o.r.escence of the warm and teeming oceans with which Zollner[880]
regarded the globe of Venus as mainly covered, be seriously entertained.
Vogel's suggestion is more plausible. He and O. Lohse, at Bothkamp, November 3 to 11, 1871, saw the dark hemisphere _partially_ illuminated by secondary light, extending 30 from the terminator, and thought the effect might be produced by a very extensive twilight.[881] Others have had recourse to the a.n.a.logy of our aurorae, and J. Lamp suggested that the grayish gleam, visible to him at Bothkamp, October 21 and 26, 1887,[882] might be an accompaniment of electrical processes connected with the planet's meteorology. Whatever the origin of the phenomenon, it may serve, on a night-enwrapt hemisphere, to dissipate some of the thick darkness otherwise encroached upon only by ”the pale light of stars.”
Venus was once supposed to possess a satellite. But belief in its existence has died out. No one, indeed, has caught even a deceptive glimpse of such an object during the last 125 years. Yet it was repeatedly and, one might have thought, well observed in the seventeenth and eighteenth centuries. Fontana ”discovered” it in 1645; Ca.s.sini--an adept in the art of seeing--recognised it in 1672, and again in 1686; Short watched it for a full hour in 1740 with varied instrumental means; Tobias Mayer in 1759, Montaigne in 1761; several astronomers at Copenhagen in March, 1764, noted what they considered its unmistakable presence; as did Horrebow in 1768. But M. Paul Stroobant,[883] who in 1887 submitted all the available data on the subject to a searching examination, identified Horrebow's satellite with Theta Librae, a fifth-magnitude star; and a few other apparitions were, by his industry, similarly explained away. Nevertheless, several withstood all efforts to account for them, and together form a most curious case of illusion. For it is quite certain that Venus has no such conspicuous attendant.
The third planet encountered in travelling outward from the sun is the abode of man. He has in consequence opportunities for studying its physical habitudes altogether different from the baffling glimpse afforded to him of the other members of the solar family.
Regarding the earth, then, a ma.s.s of knowledge so varied and comprehensive has been acc.u.mulated as to form a science--or rather several sciences--apart. But underneath all lie astronomical relations, the recognition and investigation of which const.i.tute one of the most significant intellectual events of the present century.
It is indeed far from easy to draw a line of logical distinction between items of knowledge which have their proper place here, and those which should be left to the historian of geology. There are some, however, of which the cosmical connections are so close that it is impossible to overlook them. Among these is the ascertainment of the solidity of the globe. At first sight it seems difficult to conceive what the apparent positions of the stars can have to do with subterranean conditions; yet it was from star measurements alone that Hopkins, in 1839, concluded the earth to be solid to a depth of at least 800 or 1,000 miles.[884] His argument was, that if it were a mere sh.e.l.l filled with liquid, precession and nutation would be much larger than they are observed to be. For the sh.e.l.l alone would follow the pull of the sun and moon on its equatorial girdle, leaving the liquid behind; and being thus so much the lighter, would move the more readily. There is, it is true, grave reason to doubt whether this reasoning corresponds with the actual facts of the case;[885] but the conclusion to which it led has been otherwise affirmed and extended.
Indications of an identical purport have been derived from another kind of external disturbance, affecting our globe through the same agencies.
Lord Kelvin (then Sir William Thomson) pointed out in 1862[886] that tidal influences are brought to bear on land as well as on water, although obedience to them is perceptible only in the mobile element.