A Popular History of Astronomy During the Nineteenth Century Part 3

[Footnote 40: Bullialdus, _De Nebulosa Stella in Cingulo Andromedae_ (1667); see also G. P. Bond, _Mem. Am. Ac._, vol. iii., p. 75, Holden's Monograph on the Orion Nebula, _Washington Observations_, vol. xxv., 1878 (pub. 1882), and Lady Huggins's drawing, _Atlas of Spectra_, p.

119.]

[Footnote 41: _Mathemata Astronomica_, p. 75.]

[Footnote 42: _Systema Saturnium_, p. 9.]

[Footnote 43: _Phil. Trans._, vol. xxix., p. 390.]

[Footnote 44: _Mem. Ac. des Sciences_, 1755.]

[Footnote 45: _Conn. des Temps_, 1784 (pub. 1781), p. 227. A previous list of forty-five had appeared in _Mem. Ac. des Sciences_, 1771.]

[Footnote 46: _Phil. Trans._, vol. lxxiv., p. 442.]

[Footnote 47: _Ibid._, vol. lxxix., p. 213.]

[Footnote 48: _Ibid._, vol. lxxv., p. 254.]

[Footnote 49: _Ibid._, vol. lxxix., p. 225.]

[Footnote 50: _Phil. Trans._, vol. lxxix., p. 226.]

[Footnote 51: _Ibid._, vol. lx.x.xi., p. 72.]

[Footnote 52: _Ibid._, p. 85.]

[Footnote 53: _Phil. Trans._, vol. ci., p. 271.]

[Footnote 54: _Ibid._, p. 277.]

[Footnote 55: J. Herschel, _Phil. Trans._, vol. cxvi., part iii., p. 1.]

[Footnote 56: His own words to the poet Campbell cited by Holden, _Life and Works_, p. 109.]

[Footnote 57: _Phil. Trans._, vol. civ., p. 283.]

CHAPTER II

_PROGRESS OF SIDEREAL ASTRONOMY_

We have now to consider labours of a totally different character from those of Sir William Herschel. Exploration and discovery do not const.i.tute the whole business of astronomy; the less adventurous, though not less arduous, task of gaining a more and more complete mastery over the problems immemorially presented to her, may, on the contrary, be said to form her primary duty. A knowledge of the movements of the heavenly bodies has, from the earliest times, been demanded by the urgent needs of mankind; and science finds its advantage, as in many cases it has taken its origin, in condescension to practical claims.

Indeed, to bring such knowledge as near as possible to absolute precision has been defined by no mean authority[58] as the true end of astronomy.

Several causes concurred about the beginning of the last century to give a fresh and powerful impulse to investigations having this end in view.

The rapid progress of theory almost compelled a corresponding advance in observation; instrumental improvements rendered such an advance possible; Herschel's discoveries quickened public interest in celestial inquiries; royal, imperial, and grand-ducal patronage widened the scope of individual effort. The heart of the new movement was in Germany.

Hitherto the observatory of Flamsteed and Bradley had been the acknowledged centre of practical astronomy; Greenwich observations were the standard of reference all over Europe; and the art of observing prospered in direct proportion to the fidelity with which Greenwich methods were imitated. Dr. Maskelyne, who held the post of Astronomer Royal during forty-six years (from 1765 to 1811), was no unworthy successor to the eminent men who had gone before him. His foundation of the _Nautical Almanac_ (in 1767) alone const.i.tutes a valid t.i.tle to fame; he introduced at the Observatory the important innovation of the systematic publication of results; and the careful and prolonged series of observations executed by him formed the basis of the improved theories, and corrected tables of the celestial movements, which were rapidly being brought to completion abroad. His catalogue of thirty-six "fundamental" stars was besides excellent in its way, and most serviceable. Yet he was devoid of Bradley's instinct for divining the needs of the future. He was fitted rather to continue a tradition than to found a school. The old ways were dear to him; and, indefatigable as he was, a definite purpose was wanting to compel him, by its exigencies, along the path of progress. Thus, for almost fifty years after Bradley's death, the acquisition of a small achromatic[59] was the only notable change made in the instrumental equipment of the Observatory. The transit, the zenith sector, and the mural quadrant, with which Bradley had done his incomparable work, retained their places long after they had become deteriorated by time and obsolete by the progress of invention; and it was not until the very close of his career that Maskelyne, compelled by Pond's detection of serious errors, ordered a Troughton's circle, which he did not live to employ.

Meanwhile, the heavy national disasters with which Germany was overwhelmed in the early part of the nineteenth century seemed to stimulate rather than impede the intellectual revival already for some years in progress there. Astronomy was amongst the first of the sciences to feel the new impulse. By the efforts of Bode, Olbers, Schroter, and Von Zach, just and elevated ideas on the subject were propagated, intelligence was diffused, and a firm ground prepared for common action in mutual sympathy and disinterested zeal. They received powerful aid through the foundation, in 1804, by a young artillery officer named Von Reichenbach, of an Optical and Mechanical Inst.i.tute at Munich. Here the work of English instrumental artists was for the first time rivalled, and that of English opticians--when Fraunhofer entered the new establishment--far surpa.s.sed. The development given to the refracting telescope by this extraordinary man was indispensable to the progress of that fundamental part of astronomy which consists in the exact determination of the places of the heavenly bodies. Reflectors are brilliant engines of discovery, but they lend themselves with difficulty to the prosaic work of measuring right ascensions and polar distances. A signal improvement in the art of making and working flint-gla.s.s thus most opportunely coincided with the rise of a German school of scientific mechanicians, to furnish the instrumental means needed for the reform which was at hand. Of the leader of that reform it is now time to speak.

Friedrich Wilhelm Bessel was born at Minden, in Westphalia, July 22, 1784. A certain taste for figures, coupled with a still stronger distaste for the Latin accidence, directed his inclination and his father's choice towards a mercantile career. In his fifteenth year, accordingly, he entered the house of Kuhlenkamp and Sons, in Bremen, as an apprenticed clerk. He was now thrown completely upon his own resources. From his father, a struggling Government official, heavily weighted with a large family, he was well aware that he had nothing to expect; his dormant faculties were roused by the necessity for self-dependence, and he set himself to push manfully forward along the path that lay before him. The post of supercargo on one of the trading expeditions sent out from the Hanseatic towns to China and the East Indies was the aim of his boyish ambition, for the attainment of which he sought to qualify himself by the industrious acquisition of suitable and useful knowledge. He learned English in two or three months; picked up Spanish with the casual aid of a gunsmith's apprentice; studied the geography of the distant lands which he hoped to visit; collected information as to their climates, inhabitants, products, and the courses of trade. He desired to add some acquaintance with the art (then much neglected) of taking observations at sea; and thus, led on from navigation to astronomy, and from astronomy to mathematics, he groped his way into a new world.

It was characteristic of him that the practical problems of science should have attracted him before his mind was as yet sufficiently matured to feel the charm of its abstract beauties. His first attempt at observation was made with a s.e.xtant, rudely constructed under his own directions, and a common clock. Its object was the determination of the longitude of Bremen, and its success, he tells us himself,[60] filled him with a rapture of delight, which, by confirming his tastes, decided his destiny. He now eagerly studied Bode's _Jahrbuch_ and Von Zach's _Monatliche Correspondenz_, overcoming each difficulty as it arose with the aid of Lalande's _Traite d'Astronomie_, and supplying, with amazing rapidity, his early deficiency in mathematical training. In two years he was able to attack a problem which would have tasked the patience, if not the skill, of the most experienced astronomer. Amongst the Earl of Egremont's papers Von Zach had discovered Harriot's observations on Halley's comet at its appearance in 1607, and published them as a supplement to Bode's Annual. With an elaborate care inspired by his youthful ardour, though hardly merited by their loose nature, Bessel deduced from them an orbit for that celebrated body, and presented the work to Olbers, whose reputation in cometary researches gave a special fitness to the proffered homage. The benevolent physician-astronomer of Bremen welcomed with surprised delight such a performance emanating from such a source. Fifteen years previously, the French Academy had crowned a similar work; now its equal was produced by a youth of twenty, busily engaged in commercial pursuits, self-taught, and obliged to s.n.a.t.c.h from sleep the hours devoted to study. The paper was immediately sent to Von Zach for publication, with a note from Olbers explaining the circ.u.mstances of its author; and the name of Bessel became the common property of learned Europe.

He had, however, as yet no intention of adopting astronomy as his profession. For two years he continued to work in the counting-house by day, and to pore over the _Mecanique Celeste_ and the Differential Calculus by night. But the post of a.s.sistant in Schroter's observatory at Lilienthal having become vacant by the removal of Harding to Gottingen in 1805, Olbers procured for him the offer of it. It was not without a struggle that he resolved to exchange the desk for the telescope. His reputation with his employers was of the highest; he had thoroughly mastered the details of the business, which his keen practical intelligence followed with lively interest; his years of apprenticeship were on the point of expiring, and an immediate, and not unwelcome prospect of comparative affluence lay before him. The love of science, however, prevailed; he chose poverty and the stars, and went to Lilienthal with a salary of a hundred thalers yearly. Looking back over his life's work, Olbers long afterwards declared that the greatest service which he had rendered to astronomy was that of having discerned, directed, and promoted the genius of Bessel.[61]

For four years he continued in Schroter's employment. At the end of that time the Prussian Government chose him to superintend the erection of a new observatory at Konigsberg, which after many vexatious delays, caused by the prostrate condition of the country, was finished towards the end of 1813. Konigsberg was the first really efficient German observatory.

It became, moreover, a centre of improvement, not for Germany alone, but for the whole astronomical world. During two-and-thirty years it was the scene of Bessel's labours, and Bessel's labours had for their aim the reconstruction, on an amended and uniform plan, of the entire science of observation.

A knowledge of the places of the stars is the foundation of astronomy.[62] Their configuration lends to the skies their distinctive features, and marks out the shifting tracks of more mobile objects with relatively fixed, and generally unvarying points of light. A more detailed and accurate acquaintance with the stellar mult.i.tude, regarded from a purely uranographical point of view, has accordingly formed at all times a primary object of celestial science, and was, during the last century, cultivated with a zeal and success by which all previous efforts were dwarfed into insignificance. In Lalande's _Histoire Celeste_, published in 1801, the places of no less than 47,390 stars were given, but in the rough, as it were, and consequently needing laborious processes of calculation to render them available for exact purposes. Piazzi set an example of improved methods of observation, resulting in the publication, in 1803 and 1814, of two catalogues of about 7,600 stars--the second being a revision and enlargement of the first--which for their time were models of what such works should be.[63] Stephen Groombridge at Blackheath was similarly and most beneficially active. But something more was needed than the diligence of individual observers. A systematic reform was called for; and it was this which Bessel undertook and carried through.

Direct observation furnishes only what has been called the "raw material" of the positions of the heavenly bodies.[64] A number of highly complex corrections have to be applied before their _mean_ can be disengaged from their _apparent_ places on the sphere. Of these, the most considerable and familiar is atmospheric refraction, by which objects seem to stand higher in the sky than they in reality do, the effect being evanescent at the zenith, and attaining, by gradations varying with conditions of pressure and temperature, a maximum at the horizon. Moreover, the points to which measurements are referred are themselves in motion, either continually in one direction, or periodically to and fro. The _precession_ of the equinoxes is slowly progressive, or rather retrogressive; the _nutation_ of the pole oscillatory in a period of about eighteen years. Added to which, the non-instantaneous transmission of light, combined with the movement of the earth in its...o...b..t, causes a small annual displacement known as _aberration_.

Now it is easy to see that any uncertainty in the application of these corrections saps the very foundations of exact astronomy. Extremely minute quant.i.ties, it is true, are concerned; but the life and progress of modern celestial science depends upon the sure recognition of extremely minute quant.i.ties. In the early years of the nineteenth century, however, no uniform system of "reduction" (so the complete correction of observational results is termed) had been established.

Much was left to the individual caprice of observers, who selected for the several "elements" of reduction such values as seemed best to themselves. Hence arose much hurtful confusion, tending to hinder united action and mar the usefulness of laborious researches. For this state of things, Bessel, by the exercise of consummate diligence, sagacity, and patience, provided an entirely satisfactory remedy.

His first step was an elaborate investigation of the precious series of observations made by Bradley at Greenwich from 1750 until his death in 1762. The catalogue of 3,222 stars which he extracted from them gave the earliest example of the systematic reduction on a uniform plan of such a body of work. It is difficult, without entering into details out of place in a volume like the present, to convey an idea of the arduous nature of this task. It involved the formation of a theory of the errors of each of Bradley's instruments, and a difficult and delicate inquiry into the true value of each correction to be applied, before the entries in the Greenwich journals could be developed into a finished and authentic catalogue. Although completed in 1813, it was not until five years later that the results appeared with the proud, but not inappropriate t.i.tle of _Fundamenta Astronomiae_. The eminent value of the work consisted in this, that by providing a ma.s.s of entirely reliable information as to the state of the heavens at the epoch 1755, it threw back the beginning of _exact_ astronomy almost half a century. By comparison with Piazzi's catalogues the amount of precession was more accurately determined, the proper motions of a considerable number of stars became known with certainty, and definite prediction--the certificate of initiation into the secrets of Nature--at last became possible as regards the places of the stars. Bessel's final improvements in the methods of reduction were published in 1830 in his _Tabulae Regiomontanae_. They not only const.i.tuted an advance in accuracy, but afforded a vast increase of facility in application, and were at once and everywhere adopted. Thus astronomy became a truly universal science; uncertainties and disparities were banished, and observations made at all times and places rendered mutually comparable.[65]

More, however, yet remained to be done. In order to verify with greater strictness the results drawn from the Bradley and Piazzi catalogues, a third term of comparison was wanted, and this Bessel undertook to supply. By a course of 75,011 observations, executed during the years 1821-33, with the utmost nicety of care, the number of accurately known stars was brought up to above 50,000, and an ample store of trustworthy facts laid up for the use of future astronomers. In this department Argelander, whom he attracted from finance to astronomy, and trained in his own methods, was his a.s.sistant and successor. The great "Bonn Durchmusterung,"[66] in which 324,198 stars visible in the northern hemisphere are enumerated, and the corresponding "Atlas" published in 1857-63, const.i.tuting a picture of our sidereal surroundings of heretofore unapproached completeness, may be justly said to owe their origin to Bessel's initiative, and to form a sequel to what he commenced.

But his activity was not solely occupied with the promotion of a comprehensive reform in astronomy; it embraced special problems as well.

The long-baffled search for a parallax of the fixed stars was resumed with fresh zeal as each mechanical or optical improvement held out fresh hopes of a successful issue. Illusory results abounded. Piazza in 1805 perceived, as he supposed, considerable annual displacements in Vega, Aldebaran, Sirius, and Procyon; the truth being that his instruments were worn out with constant use, and could no longer be depended upon.[67] His countryman, Calandrelli, was similarly deluded. The celebrated controversy between the Astronomer Royal and Dr. Brinkley, Director of the Dublin College Observatory, turned on the same subject.

Brinkley, who was in possession of a first-rate meridian-circle, believed himself to have discovered relatively large parallaxes for four of the brightest stars; Pond, relying on the testimony of the Greenwich instruments, a.s.serted their nullity. The dispute, protracted for fourteen years, from 1810 until 1824, was brought to no definite conclusion; but the strong presumption on the negative side was abundantly justified in the event.

There was good reason for incredulity in the matter of parallaxes.

Announcements of their detection had become so frequent as to be discredited before they were disproved; and Struve, who investigated the subject at Dorpat in 1818-21, had clearly shown that the quant.i.ties concerned were too small to come within the reliable measuring powers of any instrument then in use. Already, however, the means were being prepared of giving to those powers a large increase.

On the 21st July, 1801, two old houses in an alley of Munich tumbled down, burying in their ruins the occupants, of whom one alone was extricated alive, though seriously injured. This was an orphan lad of fourteen named Joseph Fraunhofer. The Elector Maximilian Joseph was witness of the scene, became interested in the survivor, and consoled his misfortune with a present of eighteen ducats. Seldom was money better bestowed. Part of it went to buy books and a gla.s.s-polishing machine, with the help of which young Fraunhofer studied mathematics and optics, and secretly exercised himself in the shaping and finishing of lenses; the remainder purchased his release from the tyranny of one Weichselberger, a looking-gla.s.s maker by trade, to whom he had been bound apprentice on the death of his parents. A period of struggle and privation followed, during which, however, he rapidly extended his acquirements; and was thus eminently fitted for the task awaiting him, when, in 1806, he entered the optical department of the establishment founded two years previously by Von Reichenbach and Utzschneider. He now zealously devoted himself to the improvement of the achromatic telescope; and, after a prolonged study of the theory of lenses, and many toilsome experiments in the manufacture of flint-gla.s.s, he succeeded in perfecting, December 12, 1817, an object-gla.s.s of exquisite quality and finish, 9-1/2 inches in diameter, and of 14 feet focal length.

This (as it was then considered) gigantic lens was secured by Struve for the Russian Government, and the "great Dorpat refractor"--the first of the large achromatics which have played such an important part in modern astronomy--was, late in 1824, set up in the place which it still occupies. By ingenious improvements in mounting and fitting, it was adapted to the finest micrometrical work, and thus offered unprecedented facilities both for the examination of double stars (in which Struve chiefly employed it), and for such subtle measurements as might serve to reveal or disprove the existence of a sensible stellar parallax.

Fraunhofer, moreover, constructed for the observatory at Konigsberg the first really available heliometer. The principle of this instrument (termed with more propriety a "divided object-gla.s.s micrometer") is the separation, by a strictly measurable amount, of two distinct images of the same object. If a double star, for instance, be under examination, the two half-lenses into which the object-gla.s.s is divided are shifted until the upper star (say) in one image is brought into coincidence with the lower star in the other, when their distance apart becomes known by the amount of motion employed.[68]

This virtually new engine of research was delivered and mounted in 1829, three years after the termination of the life of its deviser. The Dorpat lens had brought to Fraunhofer a t.i.tle of n.o.bility and the sole management of the Munich Optical Inst.i.tute (completely separated since 1814 from the mechanical department). What he had achieved, however, was but a small part of what he meant to achieve. He saw before him the possibility of nearly quadrupling the light-gathering capacity of the great achromatic acquired by Struve; he meditated improvements in reflectors as important as those he had already effected in refractors; and was besides eagerly occupied with investigations into the nature of light, the momentous character of which we shall by-and-by have an opportunity of estimating. But his health was impaired, it is said, from the weakening effects of his early accident, combined with excessive and unwholesome toil, and, still hoping for its restoration from a projected journey to Italy, he died of consumption, June 7, 1826, aged thirty-nine years. His tomb in Munich bears the concise eulogy, _Approximavit sidera_.

Bessel had no sooner made himself acquainted with the exquisite defining powers of the Konigsberg heliometer, than he resolved to employ them in an attack upon the now secular problem of star-distances. But it was not until 1837 that he found leisure to pursue the inquiry. In choosing his test-star he adopted a new principle. It had hitherto been a.s.sumed that our nearest neighbours in s.p.a.ce must be found among the brightest ornaments of our skies. The knowledge of stellar proper motions afforded by the critical comparison of recent with earlier star-places, suggested a different criterion of distance. It is impossible to escape from the conclusion that the apparently swiftest-moving stars are, _on the whole_, also the nearest to us, however numerous the individual exceptions to the rule. Now, as early as 1792,[69] Piazzi had noted as an indication of relative vicinity to the earth, the unusually large proper motion (52" annually) of a double star of the fifth magnitude in the constellation of the Swan. Still more emphatically in 1812[70]

Bessel drew the attention of astronomers to the fact, and 61 Cygni became known as the "flying star." The _seeming_ rate of its flight, indeed, is of so leisurely a kind, that in a thousand years it will have shifted its place by less than 3-1/2 lunar diameters, and that a quarter of a million would be required to carry it round the entire circuit of the visible heavens. Nevertheless, it has few rivals in rapidity of movement, the apparent displacement of the vast majority of stars being, by comparison, almost insensible.

This interesting, though inconspicuous object, then, was chosen by Bessel to be put to the question with his heliometer, while Struve made a similar and somewhat earlier trial with the bright gem of the Lyre, whose Arabic t.i.tle of the "Falling Eagle" survives as a time-worn remnant in "Vega." Both astronomers agreed to use the "differential"

method, for which their instruments and the vicinity to their selected stars of minute, physically detached companions offered special facilities. In the last month of 1838 Bessel made known the result of one year's observations, showing for 61 Cygni a parallax of about a third of a second (03136").[71] He then had his heliometer taken down and repaired, after which he resumed the inquiry, and finally terminated a series of 402 measures in March 1840.[72] The resulting parallax of 03483" (corresponding to a distance about 600,000 times that of the earth from the sun), seemed to be ascertained beyond the possibility of cavil, and is memorable as the first _published_ instance of the fathom-line, so industriously thrown into celestial s.p.a.ce, having really and indubitably _touched bottom_. It was confirmed in 1842-43 with curious exactness by C. A. F. Peters at Pulkowa; but later researches showed that it required increase to nearly half a second.[73]

Struve's measurements inspired less confidence. They extended over three years (1835-38), but were comparatively few, and were frequently interrupted. The parallax, accordingly, of about a quarter of a second (02613") which he derived from them for Alpha Lyrae, and announced in 1840,[74] has proved considerably too large.[75]

Meanwhile a result of the same kind, but of a more striking character than either Bessel's or Struve's, had been obtained, one might almost say casually, by a different method and in a distant region. Thomas Henderson, originally an attorney's clerk in his native town of Dundee, had become known for his astronomical attainments, and was appointed in 1831 to direct the recently completed observatory at the Cape of Good Hope. He began observing in April, 1832, and, the serious shortcomings of his instrument notwithstanding, executed during the thirteen months of his tenure of office a surprising amount of first-rate work. With a view to correcting the declination of the l.u.s.trous double star Alpha Centauri (which ranks after Sirius and Canopus as the third brightest orb in the heavens), he effected a number of successive determinations of its position, and on being informed of its very considerable proper motion (36" annually), he resolved to examine the observations already made for possible traces of parallactic displacement. This was done on his return to Scotland, where he filled the office of Astronomer Royal from 1834 until his premature death in 1844. The result justified his expectations. From the declination measurements made at the Cape and duly reduced, a parallax of about one second of arc clearly emerged (diminished by Gill's and Elkin's observations, 1882-1883, to O75"); but, by perhaps an excess of caution, was withheld from publication until fuller certainty was afforded by the concurrent testimony of Lieutenant Meadows's determinations of the same star's right ascension.[76] When at last, January 9, 1839, Henderson communicated his discovery to the Astronomical Society, he could no longer claim the priority which was his due. Bessel had antic.i.p.ated him with the parallax of 61 Cygni by just two months.

Thus from three different quarters, three successful and almost simultaneous a.s.saults were delivered upon a long-beleaguered citadel of celestial secrets. The same work has since been steadily pursued, with the general result of showing that, as regards their overwhelming majority, the stars are far too remote to show even the slightest trace of optical shifting from the revolution of the earth in its...o...b..t. In nearly a hundred cases, however, small parallaxes have been determined, some certainly (that is, within moderate limits of error), others more or less precariously. The list is an instructive one, in its omissions no less than in its contents. It includes stars of many degrees of brightness, from Sirius down to a nameless telescopic star in the Great Bear;[77] yet the vicinity to the earth of this minute object is so much greater than that of the brilliant Vega, that the latter transported to its place would increase in l.u.s.tre thirty-eight times. Moreover, many of the brightest stars are found to have no sensible parallax, while the majority of those ascertained to be nearest to the earth are of fifth, sixth, even ninth magnitudes. The obvious conclusions follow that the range of variety in the sidereal system is enormously greater than had been supposed, and that estimates of distance based upon apparent magnitude must be wholly futile. Thus, the splendid Canopus, Betelgeux, and Rigel can be inferred, from their indefinite remoteness, to exceed our sun thousands of times in size and l.u.s.tre; while many inconspicuous objects, which prove to be in our relative vicinity, must be notably his inferiors. The limits of real stellar magnitude are then set very widely apart. At the same time, the so-called "optical" and "geometrical"

methods of relatively estimating star-distances are both seen to have a foundation of fact, although so disguised by complicated relations as to be of very doubtful individual application. On the whole, the chances are in favour of the superior vicinity of a bright star over a faint one; and, on the whole, the stars in swiftest _apparent_ motion are amongst those whose _actual_ remoteness is least. Indeed, there is no escape from either conclusion, unless on the supposition of special arrangements in themselves highly improbable, and, we may confidently say, non-existent.