A Popular History of Astronomy During the Nineteenth Century Part 10

[Footnote 208: _Ibid._, p. 218. In a letter to Von Zach of June 24, 1802, he speaks of Pallas as "almost incredibly small," and makes it only seventy English miles in diameter. _Monat. Corr._, vol. vi., pp.

89, 90.]

[Footnote 209: Olbers, _Monat. Corr._, vol. vi., p. 88.]

[Footnote 210: _Conn. d. Tems_ for 1814, p. 218.]

[Footnote 211: _Popular Astronomy_, p. 327.]

[Footnote 212: _Month. Not._, vol. vii., p. 299; vol. viii., p. 1.]

[Footnote 213: _Ibid._, p. 146.]

[Footnote 214: Airy, _Mem. R. A. S._, vol. xvi., p. 386.]

[Footnote 215: See Newcomb's _Pop. Astr._, p. 359. The error of Ura.n.u.s amounted, in 1844, to 2'; but even the tailor of Breslau, whose extraordinary powers of vision Humboldt commemorates (_Kosmos_, Bd. ii., p. 112), could only see Jupiter's first satellite at its greatest elongation, 2' 15". He might, however, possibly have distinguished two objects of _equal_ l.u.s.tre at a lesser interval.]

[Footnote 216: J. W. L. Glaisher, _Observatory_, vol. xv., p. 177.]

[Footnote 217: _Mem. R. A. S._, vol. xvi., p. 399.]

[Footnote 218: For an account of D'Arrest's share in the detection see _Copernicus_, vol. ii., pp. 63, 96.]

[Footnote 219: _Mem. R. A. S._, vol. xvi., p. 412.]

[Footnote 220: He had recorded the places of 3,150 stars (three of which were different positions of the planet), and was preparing to map them, when, October 1, news of the discovery arrived from Berlin. Prof.

Challis's _Report_, quoted in Obituary Notice, _Month. Not._, Feb., 1883, p. 170.]

[Footnote 221: See Airy in _Mem. R. A. S._, vol. xvi., p. 411.]

[Footnote 222: He died January 21, 1892, in his 71st year.]

[Footnote 223: Ledger, _The Sun, its Planets and their Satellites_, p.

414.]

[Footnote 224: Presented by the Misses La.s.sell, after their father's death, to the Greenwich Observatory.]

[Footnote 225: _Astr. Jour._, No. 508.]

[Footnote 226: _Report of U.S. Naval Observatory for 1900_, p. 15.]

[Footnote 227: Grant, _Hist. of Astr._, p. 271.]

[Footnote 228: _Month. Not._, vol. ix., p. 91.]

[Footnote 229: _Month. Not._, vol. xi., p. 21.]

[Footnote 230: _Astr. Nach._, No. 756 (May 2, 1851).]

[Footnote 231: _Phil. Trans._, vol. i., p. 246. See H. T. Vivian, _Engl.

Mech._, April 20, 1894.]

[Footnote 232: Secchi, _Month. Not._, vol. xiii., p. 248.]

[Footnote 233: Hind, _ibid._, vol. xv., p. 32.]

[Footnote 234: Lynn, _Observatory_, Oct. 1, 1883; Hadley, _Phil.

Trans._, vol. x.x.xii., p. 385.]

[Footnote 235: Proctor, _Saturn and its System_, p. 64.]

[Footnote 236: _Phil. Trans._, vol. lxxvii., p. 125.]

[Footnote 237: _Month. Not._, vol. xi., p. 248.]

[Footnote 238: _Ibid._, vol. x.x.xv., pp. 16-22.]

[Footnote 239: _Ibid._, p. 26.]

[Footnote 240: _Ibid._, vol. xli., p. 190.]

CHAPTER V

_COMETS_

Newton showed that the bodies known as "comets," or _hirsute_ stars, obey the law of gravitation; but it was by no means certain that the individual of the species observed by him in 1680 formed a permanent member of the solar system. The velocity, in fact, of its rush round the sun was quite possibly sufficient to carry it off for ever into the depths of s.p.a.ce, there to wander, a celestial casual, from star to star.

With another comet, however, which appeared two years later, the case was different. Edmund Halley, who afterwards succeeded Flamsteed as Astronomer Royal, calculated the elements of its...o...b..t on Newton's principles, and found them to resemble so closely those similarly arrived at for comets observed by Peter Apian in 1531, and by Kepler in 1607, as almost to compel the inference that all three were apparitions of a single body. This implied its revolution in a period of about seventy-six years, and Halley accordingly fixed its return for 1758-9.

So fully alive was he to the importance of the announcement that he appealed to a "candid posterity," in the event of its verification, to acknowledge that the discovery was due to an Englishman. The prediction was one of the test-questions put by Science to Nature, on the replies to which largely depend both the development of knowledge and the conviction of its reality. In the present instance, the answer afforded may be said to have laid the foundation of this branch of astronomy.

Halley's comet punctually reappeared on Christmas Day, 1758, and effected its perihelion pa.s.sage on the 12th of March following, thus proving beyond dispute that some at least of these erratic bodies are domesticated within our system, and strictly conform, if not to its unwritten customs (so to speak), at any rate to its fundamental laws.

Their movements, in short, were demonstrated by the most unanswerable of all arguments--that of verified calculation--to be _calculable_, and their investigation was erected into a legitimate department of astronomical science.

This notable advance was the chief _result_ obtained in the field of inquiry just now under consideration during the eighteenth century. But before it closed, its cultivation had received a powerful stimulus through the invention of an improved _method_. The name of Olbers has already been brought prominently before our readers in connection with asteroidal discoveries; these, however, were but chance excursions from the path of cometary research which he steadily pursued through life. An early predilection for the heavens was fixed in this particular direction by one of the happy inspirations of genius. As he was watching, one night in the year 1779, by the sick-bed of a fellow-student in medicine at Gottingen, an important simplification in the mode of computing the paths of comets occurred to him. Although not made public until 1797, "Olbers's method" was then universally adopted, and is still regarded as the most expeditious and convenient in cases where absolute rigour is not required. By its introduction, not only many a toilsome and thankless hour was spared, but workers were multiplied, and encouraged in the prosecution of labours more useful than attractive.

The career of Heinrich Olbers is a brilliant example of what may be done by an amateur in astronomy. He at no time did regular work in an observatory; he was never the possessor of a transit or any other fixed instrument; moreover, all the best years of his life were absorbed in the a.s.siduous exercise of a toilsome profession. Born in 1758 at the village of Arbergen, where his father was pastor, he settled in 1781 as a physician in the neighbouring town of Bremen, and continued in active practice there for over forty years. It was thus only the hours which his robust const.i.tution enabled him to spare from sleep that were available for his intellectual pleasures. Yet his recreation was, as Von Zach remarked,[241] no less prolific of useful results than the severest work of other men. The upper part of his house in the Sandga.s.se was fitted up with such instruments and appliances as restrictions of s.p.a.ce permitted, and there, night after night during half a century and upwards, he discovered, calculated, or observed the cometary visitants of northern skies. Almost as effective in promoting the interests of science as the valuable work actually done by him, was the influence of his genial personality. He engaged confidence by his ready and discerning sympathy; he inspired affection by his benevolent disinterestedness; he quickened thought and awakened zeal by the suggestions of a lively and inventive spirit, animated with the warmest enthusiasm for the advancement of knowledge. Nearly every astronomer in Germany enjoyed the benefits of a frequently active correspondence with him, and his communications to the scientific periodicals of the time were numerous and striking. The motive power of his mind was thus widely felt and continually in action. Nor did it wholly cease to be exerted even when the advance of age and the progress of infirmity rendered him incapable of active occupation. He was, in fact, _alive_ even to the last day of his long life of eighty-one years; and his death, which occurred March 2, 1840, left vacant a position which a rare combination of moral and intellectual qualities had conspired to render unique.

Amongst the many younger men who were attracted and stimulated by intercourse with him was Johann Franz Encke. But while Olbers became a mathematician because he was an astronomer, Encke became an astronomer because he was a mathematician. A born geometer, he was naturally sent to Gottingen and placed under the tuition of Gauss. But geometers are men; and the contagion of patriotic fervour which swept over Germany after the battle of Leipsic did not spare Gauss's promising pupil. He took up arms in the Hanseatic Legion, and marched and fought until the oppressor of his country was safely ensconced behind the ocean-walls of St. Helena. In the course of his campaigning he met Lindenau, the militant director of the Seeberg Observatory, and by his influence was appointed his a.s.sistant, and eventually, in 1822, became his successor.

Thence he was promoted in 1825 to Berlin, where he superintended the building of the new observatory, so actively promoted by Humboldt, and remained at its head until within some eighteen months of his death in August, 1865.

On the 26th of November, 1818, Pons of Ma.r.s.eilles discovered a comet, whose inconspicuous appearance gave little promise of its becoming one of the most interesting objects in our system. Encke at once took the calculation of its elements in hand, and brought out the unexpected result that it revolved round the sun in a period of about 3-1/3 years.[242] He, moreover, detected its ident.i.ty with comets seen by Mechain in 1786, by Caroline Herschel in 1795, by Pons, Huth, and Bouvard in 1805, and after six laborious weeks of research into the disturbances experienced by it from the planets during the entire interval since its first ascertained appearance, he fixed May 24, 1822, as the date of its next return to perihelion. Although on that occasion, owing to the position of the earth, invisible in the northern hemisphere, Sir Thomas Brisbane's observatory at Paramatta was fortunately ready equipped for its recapture, which Rumker effected quite close to the spot indicated by Encke's ephemeris.

The importance of this event can be better understood when it is remembered that it was only the second instance of the recognised return of a comet (that of Halley's, sixty-three years previously, having, as already stated, been the first); and that it, moreover, established the existence of a new cla.s.s of celestial objects, somewhat loosely distinguished as "comets of short period." These bodies (of which about thirty have been found to circulate within the orbit of Saturn) are remarkable as showing certain planetary affinities in the manners of their motions not at all perceptible in the wider travelling members of their order. They revolve, without exception, in the same direction as the planets--from west to east; they exhibit a marked tendency to conform to the zodiacal track which limits planetary excursions north and south; and their paths round the sun, although much more eccentric than the approximately circular planetary orbits, are far less so than the extravagantly long ellipses in which comets comparatively untrained (as it were) in the habits of the solar system ordinarily perform their revolutions.

No _great_ comet is of the "planetary" kind. These are, indeed, only by exception visible to the naked eye; they possess extremely feeble tail-producing powers, and give small signs of central condensation.

Thin wisps of cosmical cloud, they flit across the telescopic field of view without sensibly obscuring the smallest star. Their appearance, in short, suggests--what some notable facts in their history will presently be shown to confirm--that they are bodies already effete, and verging towards dissolution. If it be asked what possible connection can be shown to exist between the shortness of period by which they are essentially characterised, and what we may call their _superannuated_ condition, we are not altogether at a loss for an answer. Kepler's remark,[243] that comets are consumed by their own emissions, has undoubtedly a measure of truth in it. The substance ejected into the tail must, in overwhelmingly large proportion, be for ever lost to the central ma.s.s from which it issues. True, it is of a nature inconceivably tenuous; but unrepaired waste, however small in amount, cannot be persisted in with impunity. The incitement to such self-spoliation proceeds from the sun; it accordingly progresses more rapidly the more numerous are the returns to the solar vicinity. Comets of short period may thus reasonably be expected to _wear out_ quickly.

They are, moreover, subject to many adventures and vicissitudes. Their aphelia--or the farthest points of their orbits from the sun--are usually, if not invariably, situated so near to the path either of Jupiter or of Saturn, as to permit these giant planets to act as secondary rulers of their destinies. By their influence they were, in all likelihood, originally fixed in their present tracks; and by their influence, exerted in an opposite sense, they may, in some cases, be eventually ejected from them. Careers so varied, as can easily be imagined, are apt to prove instructive, and astronomers have not been backward in extracting from them the lessons they are fitted to convey.