[Footnote 162: _De Facie in Orbe Lunae_, xix., 10. Cf. Grant, _Astr.

Nach._, No. 1838. As to the phenomenon mentioned by Philostratus in his _Life of Apollonius_ (viii. 23), see W. T. Lynn, _Observatory_, vol.

ix., p. 128.]

[Footnote 163: Schmidt, _Astr. Nach._, No. 1832.]

[Footnote 164: _Astronomiae Pars Optica, Op. omnia_, t. ii., p. 317.]

[Footnote 165: _De Stella Nova, Op._, t. ii., pp. 696, 697.]

[Footnote 166: _Astr. Pars Op._, p. 320.]

[Footnote 167: _Mem. de l'Ac. des Sciences_, 1706, p. 119.]

[Footnote 168: A digit = 1/12 of the solar diameter.]

[Footnote 169: _Phil. Trans._, vol. xxix., pp. 247-249.]

[Footnote 170: _Mem. de l'Ac. des Sciences_, 1715; _Histoire_, p. 49; _Memoires_, pp. 93-98.]

[Footnote 171: _Ibid._, 1724, p. 178.]

[Footnote 172: _Mem. de l'Ac. des Sciences_, 1715, pp. 161, 166-169.]

[Footnote 173: _Ed. Ency._, art. _Astronomy_, p. 635.]

[Footnote 174: _Trans. Am. Phil. Soc._, vol. vi., p. 274.]

[Footnote 175: _Memoir of Caroline Herschel_, p. 327.]

[Footnote 176: _Phil. Trans._, vol. xxv., p. 2240.]

[Footnote 177: _Ibid._, vol. xl., p. 182.]

[Footnote 178: _Ibid._, vol. xlv., p. 586.]

[Footnote 179: _Mem. R. A. S._, vol. i., pp. 145, 148.]

[Footnote 180: _American Journal of Science_, vol. xlii., p. 396.]

[Footnote 181: _Phil. Trans._, vol. x.x.xviii., p. 134. Father Secchi, however, adverted to a distinct mention of a prominence observed in 1239 A.D. A description of a total eclipse of that date includes the remark, "Et quoddam foramen erat ignitum in circulo solis ex parte inferiore"

(Muratori, _Rer. It. Scriptores_, t. xiv., col. 1097). The "circulus solis" of course signifies the corona.]

[Footnote 182: _Phil. Trans._, vol. lxix., p. 114.]

[Footnote 183: _Trans. Am. Phil. Soc._, vol. vi., 1809, p. 267.]

[Footnote 184: _Annuaire_, 1846, p. 460.]

[Footnote 185: _Ibid._, p. 439, _note_.]

[Footnote 186: _Ibid._, p. 416.]

[Footnote 187: _Mem. R. A. S._, vol. xxi., p. 82.]

[Footnote 188: _Ibid._, p. 90.]

[Footnote 189: _Ibid._, pp. 7, 8.]

[Footnote 190: _Le Soleil_, t. i., p. 386.]

[Footnote 191: By Williams and Stanistreet, _Mem. R. A. S._, vol. xxi., pp. 54, 56. Santini had made a similar observation at Padua in 1842.

Grant, _Hist. Astr._, p. 401.]

[Footnote 192: La.s.sell in _Month. Not._, vol. xii., p. 53.]

[Footnote 193: _Comptes Rendus_, t. x.x.xiv., p. 155.]

[Footnote 194: _Optische Untersuchungen_, and _Zeitschrift fur populare Mittheilungen_, Bd. i., 1860, p. 201.]

CHAPTER IV

_PLANETARY DISCOVERIES_

In the course of his early gropings towards a law of the planetary distances, Kepler tried the experiment of setting a planet, invisible by reason of its smallness, to revolve in the vast region of seemingly desert s.p.a.ce separating Mars from Jupiter.[195] The disproportionate magnitude of the same interval was explained by Kant as due to the overweening size of Jupiter. The zone in which each planet moved was, according to the philosopher of Konigsberg, to be regarded as the empty storehouse from which its materials had been derived. A definite relation should thus exist between the planetary ma.s.ses and the planetary intervals.[196] Lambert, on the other hand, sportively suggested that the body or bodies (for it is noticeable that he speaks of them in the plural) which once bridged this portentous gap in the solar system, might, in some remote age, have been swept away by a great comet, and forced to attend its wanderings through s.p.a.ce.[197]

These speculations were destined before long to a.s.sume a more definite form. Johann Daniel t.i.tius, a professor at Wittenberg (where he died in 1796), pointed out in 1772, in a note to a translation of Bonnet's _Contemplation de la Nature_,[198] the existence of a remarkable symmetry in the disposition of the bodies const.i.tuting the solar system.

By a certain series of numbers, increasing in regular progression,[199]

he showed that the distances of the six known planets from the sun might be represented with a close approach to accuracy. But with one striking interruption. The term of the series succeeding that which corresponded to the orbit of Mars was without a celestial representative. The orderly flow of the sequence was thus singularly broken. The s.p.a.ce where a planet should--in fulfilment of the "Law"--have revolved, was, it appeared, untenanted. Johann Elert Bode, then just about to begin his long career as leader of astronomical thought and work at Berlin, marked at once the anomaly, and filled the vacant interval with a hypothetical planet. The discovery of Ura.n.u.s, at a distance falling but slightly short of perfect conformity with the law of t.i.tius, lent weight to a seemingly hazardous prediction, and Von Zach was actually at the pains, in 1785, to calculate what he termed "a.n.a.logical" elements[200] for this unseen and (by any effect or influence) _unfelt_ body. The search for it, through confessedly scarcely less chimerical than that of alchemists for the philosopher's stone, he kept steadily in view for fifteen years, and at length (September 21, 1800) succeeded in organising, in combination with five other German astronomers a.s.sembled at Lilienthal, a force of what he jocularly termed celestial police, for the express purpose of tracking and intercepting the fugitive subject of the sun.

The zodiac was accordingly divided for purposes of scrutiny into twenty-four zones; their apportionment to separate observers was in part effected, and the a.s.sociation was rapidly getting into working order, when news arrived that the missing planet had been found, through no systematic plan of search, but by the diligent, though otherwise directed labours of a distant watcher of the skies.

Giuseppe Piazzi was born at Ponte in the Valtelline, July 16, 1746. He studied at various places and times under Tiraboschi, Beccaria, Jacquier, and Le Sueur; and having entered the Theatine order of monks at the age of eighteen, he taught philosophy, science, and theology in several of the Italian cities, as well as in Malta, until 1780, when the chair of mathematics in the University of Palermo was offered to and accepted by him. Prince Caramanico, then viceroy of Sicily, had scientific leanings, and was easily won over to the project of building an observatory, a commodious foundation for which was afforded by one of the towers of the viceregal palace. This architecturally incongruous addition to an ancient Saracenic edifice--once the abode of Kelbite and Zirite Emirs--was completed in February, 1791. Piazzi, meanwhile, had devoted nearly three years to the a.s.siduous study of his new profession, acquiring a practical knowledge of Lalande's methods at the ecole Militaire, and of Maskelyne's at the Royal Observatory; and returned to Palermo in 1789, bringing with him, in the great five-foot circle which he had prevailed upon Ramsden to construct, the most perfect measuring instrument hitherto employed by an astronomer.

He had been above nine years at work on his star-catalogue, and was still profoundly unconscious that a place amongst the Lilienthal band[201] of astronomical detectives was being held in reserve for him, when, on the first evening of the nineteenth century, January 1, 1801, he noticed the position of an eighth-magnitude star in a part of the constellation Taurus to which an error of Wollaston's had directed his special attention. Reobserving, according to his custom, the same set of fifty stars on four consecutive nights, it seemed to him, on the 2nd, that the one in question had slightly shifted its position to the west; on the 3rd he a.s.sured himself of the fact, and believed that he had chanced upon a new kind of comet without tail or coma. The wandering body, whatever its nature, exchanged retrograde for direct motion on January 14,[202] and was carefully watched by Piazzi until February 11, when a dangerous illness interrupted his observations. He had, however, not omitted to give notice of his discovery; but so precarious were communications in those unpeaceful times, that his letter to Oriani of January 23 did not reach Milan until April 5, while a missive of one day later addressed to Bode came to hand at Berlin, March 20. The delay just afforded time for the publication, by a young philosopher of Jena named Hegel, of a "Dissertation" showing, by the clearest light of reason, that the number of the planets could not exceed seven, and exposing the folly of certain devotees of induction who sought a new celestial body merely to fill a gap in a numerical series.[203]

Unabashed by speculative scorn, Bode had scarcely read Piazzi's letter when he concluded that it referred to the precise body in question. The news spread rapidly, and created a profound sensation, not unmixed with alarm lest this latest addition to the solar family should have been found only to be again lost. For by that time Piazzi's moving star was too near the sun to be any longer visible, and in order to rediscover it after conjunction a tolerably accurate knowledge of its path was indispensable. But a planetary orbit had never before been calculated from such scanty data as Piazzi's observation afforded;[204] and the attempts made by nearly every astronomer of note in Germany to compa.s.s the problem were manifestly inadequate, failing even to account for the positions in which the body had been actually seen, and _a fortiori_ serving only to mislead as to the places where, from September, 1801, it ought once more to have become discernible. It was in this extremity that the celebrated mathematician Gauss came to the rescue. He was then in his twenty-fifth year, and was earning his bread by tuition at Brunswick, with many possibilities, but no settled career before him.

The news from Palermo may be said to have converted him from an arithmetician into an astronomer. He was already in possession of a new and more general method of computing elliptical orbits; and the system of "least squares," which he had devised though not published, enabled him to extract the most probable result from a given set of observations. Armed with these novel powers, he set to work; and the communication in November of his elements and ephemeris for the lost object revived the drooping hopes of the little band of eager searchers.

Their patience, however, was to be still further tried. Clouds, mist, and sleet seemed to have conspired to cover the retreat of the fugitive; but on the last night of the year the sky cleared unexpectedly with the setting in of a hard frost, and there, in the north-western part of Virgo, nearly in the position a.s.signed by Gauss to the runaway planet, a strange star was discerned by Von Zach[205] at Gotha, and on a subsequent evening--the anniversary of the original discovery--by Olbers at Bremen. The name of Ceres (as the tutelary G.o.ddess of Sicily) was, by Piazzi's request, bestowed upon this first known of the numerous, and probably all but innumerable family of the minor planets.

The recognition of the second followed as the immediate consequence of the detection of the first. Olbers had made himself so familiar with the positions of the small stars along the track of the long-missing body, that he was at once struck (March 28, 1802) with the presence of an intruder near the spot where he had recently identified Ceres. He at first believed the new-comer to be a variable star usually inconspicuous, but just then at its maximum of brightness; but within two hours he had convinced himself that it was no _fixed_ star, but a rapidly moving object. The aid of Gauss was again invoked, and his prompt calculations showed that this fresh celestial acquaintance (named "Pallas" by Olbers), revolved round the sun at nearly the same mean distance as Ceres, and was beyond question of a strictly a.n.a.logous character.

This result was perplexing in the extreme. The symmetry and simplicity of the planetary scheme appeared fatally compromised by the admission of many, where room could, according to old-fashioned rules, only be found for one. A daring hypothesis of Olbers's invention provided an exit from the difficulty. He supposed that both Ceres and Pallas were fragments of a primitive trans-Martian planet, blown to pieces in the remote past, either by the action of internal forces or by the impact of a comet; and predicted that many more such fragments would be found to circulate in the same region. He, moreover, pointed out that these numerous...o...b..ts, however much they might differ in other respects, must all have a common line of intersection,[206] and that the bodies moving in them must consequently pa.s.s, at each revolution, through two opposite points of the heavens, one situated in the Whale, the other in the constellation of the Virgin, where already Pallas had been found and Ceres recaptured.

The intimation that fresh discoveries might be expected in those particular regions was singularly justified by the detection of two bodies now known respectively as Juno and Vesta. The first was found near the predicted spot in Cetus by Harding, Schroter's a.s.sistant at Lilienthal, September 2, 1804; the second by Olbers himself in Virgo, after three years of persistent scrutiny, March 29, 1807.

The theory of an exploded planet now seemed to have everything in its favour. It required that the mean or average distances of the newly-discovered bodies should be nearly the same, but admitted a wide range of variety in the shapes and positions of their orbits, provided always that they preserved common points of intersection. These conditions were fulfilled with a striking approach to exactness. Three of the four "asteroids" (a designation introduced by Sir. W.