A Popular History of Astronomy During the Nineteenth Century Part 4

The distances even of the few stars found to have measurable parallaxes are on a scale entirely beyond the powers of the human mind to conceive.

In the attempt both to realize them distinctly, and to express them conveniently, a new unit of length, itself of bewildering magnitude, has originated. This is what we may call the _light-journey_ of one year.

The subtle vibrations of the ether, propagated on all sides from the surface of luminous bodies, travel at the rate of 186,300 miles a second, or (in round numbers) six billions of miles a year. Four and a third such measures are needed to span the abyss that separates us from the nearest fixed star. In other words, light takes four years and four months to reach the earth from Alpha Centauri; yet Alpha Centauri lies some ten billions of miles nearer to us (so far as is yet known) than any other member of the sidereal system!

The determination of parallax leads, in the case of stars revolving in known orbits, to the determination of ma.s.s; for the distance from the earth of the two bodies forming a binary system being ascertained, the seconds of arc apparently separating them from each other can be translated into millions of miles; and we only need to add a knowledge of their period to enable us, by an easy sum in proportion, to find their combined ma.s.s in terms of that of the sun. Thus, since--according to Dr. Doberck's elements--the components of Alpha Centauri revolve round their common centre of gravity at a mean distance nearly 25 times the radius of the earth's...o...b..t, in a period of 88 years, the attractive force of the two together must be just twice the solar. We may gather some idea of their relations by placing in imagination a second luminary like our sun in circulation between the orbits of Neptune and Ura.n.u.s.

But systems of still more majestic proportions are reduced by extreme remoteness to apparent insignificance. A double star of the fourth magnitude in Ca.s.siopeia (Eta), to which a small parallax is ascribed on the authority of O. Struve, appears to be above eight times as ma.s.sive as the central orb of our world; while a much less conspicuous pair--85 Pegasi--exerts, if the available data can be depended upon, no less than thirteen times the solar gravitating power.

Further, the actual rate of proper motions, so far as regards that part of them which is projected upon the sphere, can be ascertained for stars at known distance. The annual journey, for instance, of 61 Cygni _across the line of sight_ amounts to 1,000, and that of Alpha Centauri to 446 millions of miles. A small star, numbered 1,830 in Groombridge's Circ.u.mpolar Catalogue, "devours the way" at the rate of at least 150 miles a second--a speed, in Newcomb's opinion, beyond the gravitating power of the entire sidereal system to control; and Mu Ca.s.siopeiae possesses above two-thirds of that surprising velocity; while for both objects, radial movements of just sixty miles a second were disclosed by Professor Campbell's spectroscopic measurements.

Herschel's conclusion as to the advance of the sun among the stars was not admitted as valid by the most eminent of his successors. Bessel maintained that there was absolutely no preponderating evidence in favour of its supposed direction towards a point in the constellation Hercules.[78] Biot, Burckhardt, even Herschel's own son, shared his incredulity. But the appearance of Argelander's prize-essay in 1837[79]

changed the aspect of the question. Herschel's first memorable solution in 1783 was based upon the motions of thirteen stars, imperfectly known; his second, in 1805, upon those of no more than six. Argelander now obtained an entirely concordant result from the large number of 390, determined with the scrupulous accuracy characteristic of Bessel's work and his own. The reality of the fact thus persistently disclosed could no longer be doubted; it was confirmed five years later by the younger Struve, and still more strikingly in 1847[80] by Galloway's investigations, founded exclusively on the apparent displacements of southern stars. In 1859 and 1863, Sir George Airy and Mr. Dunkin (1821-1898),[81] employing all the resources of modern science, and commanding the wealth of material furnished by 1,167 proper motions carefully determined by Mr. Main, reached conclusions closely similar to that indicated nearly eighty years previously by the first great sidereal astronomer; which Mr. Plummer's reinvestigation of the subject in 1883[82] served but slightly to modify. Yet astronomers were not satisfied. Dr. Auwers of Berlin completed in 1866 a splendid piece of work, for which he received in 1888 the Gold Medal of the Royal Astronomical Society. It consisted in reducing afresh, with the aid of the most refined modern data, Bradley's original stars, and comparing their places thus obtained for the year 1755 with those a.s.signed to them from observations made at Greenwich after the lapse of ninety years. In the interval, as was to be antic.i.p.ated, most of them were found to have travelled over some small span of the heavens, and there resulted a stock of nearly three thousand highly authentic proper motions. These ample materials were turned to account by M. Ludwig Struve[83] for a discussion of the sun's motion, of which the upshot was to shift its point of aim to the bordering region of the constellations Hercules and Lyra. And the more easterly position of the solar apex was fully confirmed by the experiments, with variously a.s.sorted lists of stars, of Lewis Boss of Albany,[84] and Oscar Stumpe of Bonn.[85] Fresh precautions of refinement were introduced into the treatment of the subject by Ristenpart of Karlsruhe,[86] by Kapteyn of Groningen,[87] by Newcomb[88] and Porter[89] in America, who ably availed themselves of the copious materials acc.u.mulated before the close of the century. Their results, although not more closely accordant than those of their predecessors, combined to show that the journey of our system is directed towards a point within a circle about ten degrees in radius, having the brilliant Vega for its centre. To determine its rate was a still more arduous problem. It involved the a.s.sumption, very much at discretion, of an average parallax for the stars investigated; and Otto Struve's estimate of 154 million miles as the span yearly traversed was hence wholly unreliable. Fortunately, however, as will be seen further on, a method of determining the sun's velocity independently of any knowledge of star-distances, has now become available.

As might have been expected, speculation has not been idle regarding the purpose and goal of the strange voyage of discovery through s.p.a.ce upon which our system is embarked; but altogether fruitlessly. The variety of the conjectures hazarded in the matter is in itself a measure of their futility. Long ago, before the construction of the heavens had as yet been made the subject of methodical inquiry, Kant was disposed to regard Sirius as the "central sun" of the Milky Way; while Lambert surmised that the vast Orion nebula might serve as the regulating power of a subordinate group including our sun. Herschel threw out the hint that the great cl.u.s.ter in Hercules might prove to be the supreme seat of attractive force;[90] Argelander placed his central body in the constellation Perseus;[91] Fomalhaut, the brilliant of the Southern Fish, was set in the post of honour by Boguslawski of Breslau. Madler (who succeeded Struve at Dorpat in 1839) concluded from a more formal inquiry that the ruling power in the sidereal system resided, not in any single prepondering ma.s.s, but in the centre of gravity of the self-controlled revolving mult.i.tude.[92] In the former case (as we know from the example of the planetary scheme), the stellar motions would be most rapid near the centre; in the latter, they would become accelerated with remoteness from it.[93] Madler showed that no part of the heavens could be indicated as a region of exceptionally swift movements, such as would result from the presence of a gigantic (though possibly obscure) ruling body; but that a community of extremely sluggish movements undoubtedly existed in and near the group of the Pleiades, where, accordingly, he placed the centre of gravity of the Milky Way.[94] The bright star Alcyone thus became the "central sun," but in a purely pa.s.sive sense, its headship being determined by its situation at the point of neutralisation of opposing tendencies, and of consequent rest.

By an avowedly conjectural method, the solar period of revolution round this point was fixed at 18,200,000 years.

The scheme of sidereal government framed by the Dorpat astronomer was, it may be observed, of the most approved const.i.tutional type; deprivation, rather than increase of influence accompanying the office of chief dignitary. But while we are still ignorant, and shall perhaps ever remain so, of the fundamental plan upon which the Galaxy is organised, recent investigations tend more and more to exhibit it, not as monarchical (so to speak), but as federative. The community of proper motions detected by Madler in the vicinity of the Pleiades may accordingly possess a significance altogether different from what he imagined.

Bessel's so-called "foundation of an Astronomy of the Invisible" now claims attention.[95] His prediction regarding the planet Neptune does not belong to the present division of our subject; a strictly a.n.a.logous discovery in the sidereal system was, however, also very clearly foreshadowed by him. His earliest suspicions of non-uniformity in the proper motion of Sirius dated from 1834; they extended to Procyon in 1840; and after a series of refined measurements with the new Repsold circle, he announced in 1844 his conclusion that these irregularities were due to the presence of obscure bodies round which the two bright Dog-stars revolved as they pursued their way across the sphere.[96] He even a.s.signed to each an approximate period of half a century. "I adhere to the conviction," he wrote later to Humboldt, "that Procyon and Sirius form real binary systems, consisting of a visible and an invisible star.

There is no reason to suppose luminosity an essential quality of cosmical bodies. The visibility of countless stars is no argument against the invisibility of countless others."[97]

An inference so contradictory to received ideas obtained little credit, until Peters found, in 1851,[98] that the apparent anomalies in the movements of Sirius could be completely explained by an orbital revolution in a period of fifty years. Bessel's prevision was destined to be still more triumphantly vindicated. On the 31st of January, 1862, while in the act of trying a new 18-inch refractor, Mr. Alvan G. Clark (one of the celebrated firm of American opticians) actually discovered the hypothetical Sirian companion in the precise position required by theory. It has now been watched through nearly an entire revolution (period 494 years), and proves to be very slightly luminous in proportion to its ma.s.s. Its attractive power, in fact, is nearly half that of its primary, while it emits only 1/10000th of its light. Sirius itself, on the other hand, possesses a far higher radiative intensity than our sun. It gravitates--admitting Sir David Gill's parallax of 038" to be exact--like two suns, but shines like twenty. Possibly it is much distended by heat, and undoubtedly its atmosphere intercepts a very much smaller proportion of its light than in stars of the solar cla.s.s.

As regards Procyon, visual verification was awaited until November 13, 1896, when Professor Schaeberle, with the great Lick refractor, detected the long-sought object in the guise of a thirteenth-magnitude star. Dr.

See's calculations[99] showed it to possess one-fifth the ma.s.s of its primary, or rather more than half that of our sun.[100] Yet it gives barely 1/20000th of the sun's light, so that it is still nearer to total obscurity than the dusky satellite of Sirius. The period of forty years a.s.signed to the system by Auwers in 1862[101] appears to be singularly exact.

But Bessel was not destined to witness the recognition of "the invisible" as a legitimate and profitable field for astronomical research. He died March 17, 1846, just six months before the discovery of Neptune, of an obscure disease, eventually found to be occasioned by an extensive fungus-growth in the stomach. The place which he left vacant was not one easy to fill. His life's work might be truly described as "epoch-making." Rarely indeed shall we find one who reconciled with the same success the claims of theoretical and practical astronomy, or surveyed the science which he had made his own with a glance equally comprehensive, practical, and profound.

The career of Friedrich Georg Wilhelm Struve ill.u.s.trates the maxim that science _differentiates_ as it develops. He was, while much besides, a specialist in double stars. His earliest recorded use of the telescope was to verify Herschel's conclusion as to the revolving movement of Castor, and he never varied from the predilection which this first observation at once indicated and determined. He was born at Altona, of a respectable yeoman family, April 15, 1793, and in 1811 took a degree in philology at the new Russian University of Dorpat. He then turned to science, was appointed in 1813 to a professorship of astronomy and mathematics, and began regular work in the Dorpat Observatory just erected by Parrot for Alexander I. It was not, however, until 1819 that the acquisition of a 5-foot refractor by Troughton enabled him to take the position-angles of double stars with regularity and tolerable precision. The resulting catalogue of 795 stellar systems gave the signal for a general resumption of the Herschelian labours in this branch. His success, so far, and the extraordinary facilities for observation afforded by the Fraunhofer achromatic encouraged him to undertake, February 11, 1825, a review of the entire heavens down to 15 south of the celestial equator, which occupied more than two years, and yielded, from an examination of above 120,000 stars, a harvest of about 2,200 previously unnoticed composite objects. The ensuing ten years were devoted to delicate and patient measurements, the results of which were embodied in _Mensurae Micrometricae_, published at St. Petersburg in 1837.

This monumental work gives the places, angles of position, distances, colours, and relative brightness of 3,112 double and multiple stars, all determined with the utmost skill and care. The record is one which gains in value with the process of time, and will for ages serve as a standard of reference by which to detect change or confirm discovery.

It appears from Struve's researches that about one in forty of all stars down to the ninth magnitude is composite, but that the proportion is doubled in the brighter orders.[102] This he attributed to the difficulty of detecting the faint companions of very remote orbs. It was also noticed, both by him and Bessel, that double stars are in general remarkable for large proper motions. Struve's catalogue included no star of which the components were more than 32" apart, because beyond that distance the chances of merely optical juxtaposition become considerable; but the immense preponderance of extremely close over (as it were) loosely yoked bodies is such as to demonstrate their physical connection, even if no other proof were forthcoming. Many stars previously believed to be single divided under the scrutiny of the Dorpat refractor; while in some cases, one member of a supposed binary system revealed itself as double, thus placing the surprised observer in the unexpected presence of a triple group of suns. Five instances were noted of two pairs lying so close together as to induce a conviction of their mutual dependence;[103] besides which, 124 examples occurred of triple, quadruple, and multiple combinations, the reality of which was open to no reasonable doubt.[104]

It was first pointed out by Bessel that the fact of stars exhibiting a common proper motion might serve as an unfailing test of their real a.s.sociation into systems. This was, accordingly, one of the chief criteria employed by Struve to distinguish true binaries from merely optical couples. On this ground alone, 61 Cygni was admitted to be a genuine double star; and it was shown that, although its components appeared to follow almost strictly rectilinear paths, yet the probability of their forming a connected pair is actually greater than that of the sun rising to-morrow morning.[105] Moreover, this tie of an identical movement was discovered to unite bodies[106] far beyond the range of distance ordinarily separating the members of binary systems, and to prevail so extensively as to lead to the conclusion that single do not outnumber conjoined stars more than twice or thrice.[107]

In 1835 Struve was summoned by the Emperor Nicholas to superintend the erection of a new observatory at Pulkowa, near St. Petersburg, destined for the special cultivation of sidereal astronomy. Boundless resources were placed at his disposal, and the inst.i.tution created by him was acknowledged to surpa.s.s all others of its kind in splendour, efficiency, and completeness. Its chief instrumental glory was a refractor of fifteen inches aperture by Merz and Mahler (Fraunhofer's successors), which left the famous Dorpat telescope far behind, and remained long without a rival. On the completion of this model establishment, August 19, 1839, Struve was installed as its director, and continued to fulfil the important duties of the post with his accustomed vigour until 1858, when illness compelled his virtual resignation in favour of his son Otto Struve, born at Dorpat in 1819. He died November 23, 1864.

An inquiry into the laws of stellar distribution, undertaken during the early years of his residence at Pulkowa, led Struve to confirm in the main the inferences arrived at by Herschel as to the construction of the heavens. According to his view, the appearance known as the Milky Way is produced by a collection of irregularly condensed star-cl.u.s.ters, within which the sun is somewhat eccentrically placed. The nebulous ring which thus integrates the light of countless worlds was supposed by him to be made up of stars scattered over a bent or "broken plane," or to lie in two planes slightly inclined to each other, our system occupying a position near their intersection.[108] He further attempted to show that the limits of this vast a.s.semblage must remain for ever shrouded from human discernment, owing to the gradual extinction of light in its pa.s.sage through s.p.a.ce,[109] and sought to confer upon this celebrated hypothesis a definiteness and certainty far beyond the aspirations of its earlier advocates, Cheseaux and Olbers; but arbitrary a.s.sumptions vitiated his reasonings on this, as well as on some other points.[110]

In his special line as a celestial explorer of the most comprehensive type, Sir William Herschel had but one legitimate successor, and that successor was his son. John Frederick William Herschel was born at Slough, March 17, 1792, graduated with the highest honours from St.

John's College, Cambridge, in 1813, and entered upon legal studies with a view to being called to the Bar. But his share in an early compact with Peac.o.c.k and Babbage, "to do their best to leave the world wiser than they found it," was not thus to be fulfilled. The acquaintance of Dr. Wollaston decided his scientific vocation. Already, in 1816, we find him reviewing some of his father's double stars; and he completed in 1820 the 18-inch speculum which was to be the chief instrument of his investigations. Soon afterwards, he undertook, in conjunction with Mr.

(later Sir James) South, a series of observations, issuing in the presentation to the Royal Society of a paper[111] containing micrometrical measurements of 380 binary stars, by which the elder Herschel's inferences of orbital motion were, in many cases, strikingly confirmed. A star in the Northern Crown, for instance (Eta Coronae), had completed more than one entire circuit since its first discovery; another, Tau Ophiuchi, had _closed up_ into apparent singleness; while the motion of a third, Xi Ursae Majoris, in an obviously eccentric orbit, was so rapid as to admit of being traced and measured from month to month.

It was from the first confidently believed that the force retaining double stars in curvilinear paths was identical with that governing the planetary revolutions. But that ident.i.ty was not ascertained until Savary of Paris showed, in 1827,[112] that the movements of the above-named binary in the Great Bear could be represented with all attainable accuracy by an ellipse calculated on orthodox gravitational principles with a period of 58-1/4 years. Encke followed at Berlin with a still more elegant method; and Sir John Herschel, pointing out the uselessness of a.n.a.lytical refinements where the data were necessarily so imperfect, described in 1832 a graphical process by which "the aid of the eye and hand" was brought in "to guide the judgment in a case where judgment only, and not calculation, could be of any avail."[113]

Improved methods of the same kind were published by Dr. See in 1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance with stellar orbits is steadily gaining precision, certainty, and extent.

In 1825 Herschel undertook, and executed with great a.s.siduity during the ensuing eight years, a general survey of the northern heavens, directed chiefly towards the verification of his father's nebular discoveries.

The outcome was a catalogue of 2,306 nebulae and cl.u.s.ters, of which 525 were observed for the first time, besides 3,347 double stars discovered almost incidentally.[116] "Strongly invited," as he tells us himself, "by the peculiar interest of the subject, and the wonderful nature of the objects which presented themselves," he resolved to attempt the completion of the survey in the southern hemisphere. With this n.o.ble object in view, he embarked his family and instruments on board the _Mount Stewart Elphinstone_, and, after a prosperous voyage, landed at Cape Town on the 16th of January, 1834. Choosing as the scene of his observations a rural spot under the shelter of Table Mountain, he began regular "sweeping" on the 5th of March. The site of his great reflector is now marked with an obelisk, and the name of Feldhausen has become memorable in the history of science; for the four years' work done there may truly be said to open the chapter of our knowledge as regards the southern skies.

The full results of Herschel's journey to the Cape were not made public until 1847, when a splendid volume[117] embodying them was brought out at the expense of the Duke of Northumberland. They form a sequel to his father's labours such as the investigations of one man have rarely received from those of another. What the elder observer did for the northern heavens, the younger did for the southern, and with generally concordant results. Reviving the paternal method of "star-gauging," he showed, from a count of 2,299 fields, that the Milky Way surrounds the solar system as a complete annulus of minute stars; not, however, quite symmetrically, since the sun was thought to lie somewhat nearer to those portions visible in the southern hemisphere, which display a brighter l.u.s.tre and a more complicated structure than the northern branches. The singular cosmical agglomerations known as the "Magellanic Clouds" were now, for the first time, submitted to a detailed, though admittedly incomplete, examination, the almost inconceivable richness and variety of their contents being such that a lifetime might with great profit be devoted to their study. In the Greater Nubecula, within a compa.s.s of forty-two square degrees, Herschel reckoned 278 distinct nebulae and cl.u.s.ters, besides fifty or sixty outliers, and a large number of stars intermixed with diffused nebulosity--in all, 919 catalogued objects, and, for the Lesser Cloud, 244. Yet this was only the most conspicuous part of what his twenty-foot revealed. Such an extraordinary concentration of bodies so various led him to the inevitable conclusion that "the Nubeculae are to be regarded as systems _sui generis_, and which have no a.n.a.logues in our hemisphere."[118] He noted also the blankness of surrounding s.p.a.ce, especially in the case of Nubecula Minor, "the access to which on all sides," he remarked, "is through a desert;" as if the cosmical material in the neighbourhood had been swept up and garnered in these mighty groups.[119]

Of southern double stars, he discovered and gave careful measurements of 2,102, and described 1,708 nebulae, of which at least 300 were new. The list was ill.u.s.trated with a number of drawings, some of them extremely beautiful and elaborate.

Sir John Herschel's views as to the nature of nebulae were considerably modified by Lord Rosse's success in "resolving" with his great reflectors a crowd of these objects into stars. His former somewhat hesitating belief in the existence of phosph.o.r.escent matter, "disseminated through extensive regions of s.p.a.ce in the manner of a cloud or fog,"[120] was changed into a conviction that no valid distinction could be established between the faintest wisp of cosmical vapour just discernible in a powerful telescope, and the most brilliant and obvious cl.u.s.ter. He admitted, however, an immense range of possible variety in the size and mode of aggregation of the stellar const.i.tuents of various nebulae. Some might appear nebulous from the closeness of their parts; some from their smallness. Others, he suggested, might be formed of "discrete luminous bodies floating in a non-luminous medium;"[121] while the annular kind probably consisted of "hollow sh.e.l.ls of stars."[122] That a physical, and not merely an optical, connection unites nebulae with the _embroidery_ (so to speak) of small stars with which they are in many instances profusely decorated, was evident to him, as it must be to all who look as closely and see as clearly as he did. His description of No. 2,093 in his northern catalogue as "a network or tracery of nebula following the lines of a similar network of stars,"[123] would alone suffice to dispel the idea of accidental scattering; and many other examples of a like import might be quoted. The remarkably frequent occurrence of one or more minute stars in the close vicinity of "planetary" nebulae led him to infer their dependent condition; and he advised the maintenance of a strict watch for evidences of circulatory movements, not only over these supposed stellar satellites, but also over the numerous "double nebulae," in which, as he pointed out, "all the varieties of double stars as to distance, position, and relative brightness, have their counterparts."

He, moreover, investigated the subject of nebular distribution by the simple and effectual method of graphic delineation or "charting," and succeeded in showing that while a much greater uniformity of scattering prevails in the southern than in the northern heavens, a condensation is nevertheless perceptible about the constellations Pisces and Cetus, roughly corresponding to the "nebular region" in Virgo by its vicinity (within 20 or 30) to the opposite pole of the Milky Way. He concluded "that the nebulous system is distinct from the sidereal, though involving, and perhaps to a certain extent intermixed with, the latter."[124]

Towards the close of his residence at Feldhausen, Herschel was fortunate enough to witness one of those singular changes in the aspect of the firmament which occasionally challenge the attention even of the incurious, and excite the deepest wonder of the philosophical observer.

Immersed apparently in the Argo nebula is a star denominated Eta Carinae. When Halley visited St. Helena in 1677, it seemed of the fourth magnitude; but Lacaille in the middle of the following century, and others after him, cla.s.sed it as of the second. In 1827 the traveller Burch.e.l.l, being then at St. Paul, near Rio Janeiro, remarked that it had unexpectedly a.s.sumed the first rank--a circ.u.mstance the more surprising to him because he had frequently, when in Africa during the years 1811 to 1815, noted it as of only fourth magnitude. This observation, however, did not become generally known until later. Herschel, on his arrival at Feldhausen, registered the star as a bright second, and had no suspicion of its unusual character until December 16, 1837, when he suddenly perceived its light to be almost tripled. It then far outshone Rigel in Orion, and on the 2nd of January following it very nearly matched Alpha Centauri. From that date it declined; but a second and even brighter maximum occurred in April, 1843, when Maclear, then director of the Cape Observatory, saw it blaze out with a splendour approaching that of Sirius. Its waxings and wanings were marked by curious "trepidations" of brightness extremely perplexing to theory. In 1863 it had sunk below the fifth magnitude, and in 1869 was barely visible to the naked eye; yet it was not until eighteen years later that it touched a minimum of 76 magnitude. Soon afterwards a recovery of brightness set in, but was not carried very far; and the star now shines steadily as of the seventh magnitude, its reddish light contrasting effectively with the silvery rays of the surrounding nebula. An attempt to include its fluctuations within a cycle of seventy years[125] has signally failed; the extent and character of the vicissitudes to which it is subject stamping it rather as a species of connecting link between periodic and temporary stars.[126]

Among the numerous topics which engaged Herschel's attention at the Cape was that of relative stellar brightness. Having contrived an "astrometer" in which an "artificial star," formed by the total reflection of moonlight from the base of a prism, served as a standard of comparison, he was able to estimate the l.u.s.tre of the _natural_ stars examined by the distances at which the artificial object appeared equal respectively to each. He thus constructed a table of 191 of the princ.i.p.al stars,[127] both in the northern and southern hemispheres, setting forth the numerical values of their apparent brightness relatively to that of Alpha Centauri, which he selected as a unit of measurement. Further, the light of the full moon being found by him to exceed that of his standard star 27,408 times, and Dr. Wollaston having shown that the light of the full moon is to that of the sun as 1:801,072[128] (Zollner made the ratio 1:618,000), it became possible to compare stellar with solar radiance. Hence was derived, in the case of the few stars at ascertained distances, a knowledge of real l.u.s.tre.

Alpha Centauri, for example, emits less than twice, Capella one hundred times as much light as our sun; while Arcturus, at its enormous distance, must display the splendour of 1,300 such luminaries.

Herschel returned to England in the spring of 1838, bringing with him a wealth of observation and discovery such as had perhaps never before been ama.s.sed in so short a time. Deserved honours awaited him. He was created a baronet on the occasion of the Queen's coronation (he had been knighted in 1831); universities and learned societies vied with each other in showering distinctions upon him; and the success of an enterprise in which scientific zeal was tinctured with an attractive flavour of adventurous romance, was justly regarded as a matter of national pride. His career as an observing astronomer was now virtually closed, and he devoted his leisure to the collection and arrangement of the abundant trophies of his father's and his own activity. The resulting great catalogue of 5,079 nebulae (including all then certainly known), published in the _Philosophical Transactions_ for 1864, is, and will probably long remain, the fundamental source of information on the subject;[129] but he unfortunately did not live to finish the companion work on double stars, for which he had acc.u.mulated a vast store of materials.[130] He died at Collingwood in Kent, May 11, 1871, in the eightieth year of his age, and was buried in Westminster Abbey, close beside the grave of Sir Isaac Newton.

The consideration of Sir John Herschel's Cape observations brings us to the close of the period we are just now engaged in studying. They were given to the world, as already stated, three years before the middle of the century, and accurately represent the condition of sidereal science at that date. Looking back over the fifty years traversed, we can see at a glance how great was the stride made in the interval. Not alone was acquaintance with individual members of the cosmos vastly extended, but their mutual relations, the laws governing their movements, their distances from the earth, ma.s.ses, and intrinsic l.u.s.tre, had begun to be successfully investigated. _Begun to be_; for only regarding a scarcely perceptible minority had even approximate conclusions been arrived at.

Nevertheless the whole progress of the future lay in that beginning; it was the thin end of the wedge of exact knowledge. The principle of measurement had been subst.i.tuted for that of probability; a basis had been found large and strong enough to enable calculation to ascend from it to the sidereal heavens; and refinements had been introduced, fruitful in performance, but still more in promise. Thus, rather the kind than the amount of information collected was significant for the time to come--rather the methods employed than the results actually secured rendered the first half of the nineteenth century of epochal importance in the history of our knowledge of the stars.

FOOTNOTES:

[Footnote 58: Bessel, _Populare Vorlesungen_, pp. 6, 408.]

[Footnote 59: Fitted to the old transit instrument, July 11, 1772.]

[Footnote 60: _Briefwechsel mit Olbers_, p. xvi.]

[Footnote 61: R. Wolf, _Gesch. der Astron._, p. 518.]

[Footnote 62: Bessel, _Pop. Vorl._, p. 22.]

[Footnote 63: A new reduction of the observations upon which they were founded was undertaken in 1896 by Herman S. Davis, of the U.S. Coast Survey.]

[Footnote 64: Bessel, _Pop. Vorl._, p. 440.]

[Footnote 65: Durege, _Bessel's Leben und Wirken_, p. 28.]

[Footnote 66: _Bonner Beobachtungen_, Bd. iii.-v., 1859-62.]

[Footnote 67: Bessel, _Pop. Vorl._, p. 238.]

[Footnote 68: The heads of the screws applied to move the halves of the object-gla.s.s in the Konigsberg heliometer are of so considerable a size that a thousandth part of a revolution, equivalent to 1/20 of a second of arc, can be measured with the utmost accuracy. Main, _R. A. S. Mem._, vol. xii., p. 53.]

[Footnote 69: _Specola Astronomica di Palermo_, lib. vi., p. 10, _note_.]

[Footnote 70: _Monatliche Correspondenz_, vol. xxvi., p. 162.]

[Footnote 71: _Astronomische Nachrichten_, Nos. 365-366. It should be explained that what is called the "annual parallax" of a star is only half its apparent displacement. In other words, it is the angle subtended at the distance of that particular star by the _radius_ of the earth's...o...b..t.]

[Footnote 72: _Astr. Nach._, Nos. 401-402.]

[Footnote 73: Sir R. Ball's measurements at Dunsink gave to 61 Cygni a parallax of 047"; Professor Pritchard obtained, by photographic determinations, one of 043".]