Mabch 2, 1896.] 



KNOWLEDGE. 



59 



[The "Crab" nebula, discovered in 1731 by Bevis (who, 

 after the death of Bliss, the fourth Astronomer Royal, 

 became a competitor for the vacant place), was rediscovered 

 in 1758 by Messier, the " comet-ferret " of Louis XV., and 

 was the occasion of Messier's drawing up his catalogue of 

 nebulfE. This, practically the first catalogue of the kind, 

 was published in 1771, and contained lorty-five objects, at 

 the head of which was the nebula with which we are now 

 concerned. His second catalogue, published ten years 

 later, included one hundred and three. 



The " Crab " owes its name to the filaments springing 

 from it, as the arms spring from a cuttle-fish, which are 

 represented in the well-known drawing by the elder Lord 

 Rosse. Lassell also represents various projecting appen- 

 dages, but these do not confirm those recorded by Lord 

 Rosse. There is, however, nothing crab-like in the beau- 

 tiful photograph of it which Dr. Roberts has produced, 

 nor do the later observations made with Lord Rosse's 

 reflector bear out the earlier descriptions of the claws. 

 The name, therefore, as indicating the shape of the 

 nebula, appears a misnomer. It might, however, be stiU 

 applicable in the oarsman's sense, for not a few tyros in 

 the art of comet-seeking have been deluded by Messier 1, 

 and have had sorrowfully to confess that they have " caught 

 a crab." Perhaps no object in the heavens has been so 

 often mistaken by beginners for a comet. — E. W. Mauncer.] 



ANOTHER DARK STAR. 



By Miss A. M. Clerke, Authoress of " The System of the 



Stars," and " A Popular History of Astronomy during tlie 



Nineteenth Century," dc, dc, 



ONE hundred and seventeen years ago the star 

 catalogued by Flamsteed as 70 Ophiuchi was dis- 

 covered by William Herschel to be an easily- 

 divided, unequal pair. Their mutual revolution 

 quickly became apparent, and by 1872 the com- 

 panion had resumed its original position precisely east of its 

 primary. A period of ninety-three years was thus, it might 

 be thought, emphatically asserted ; yet the assertion has not 

 been borne out by facts. The star is now certainly known 

 to have been behind time in returning to the starting point. 

 Left to itself, it would have accomplished the circuit in 

 eighty-eight years. But it was not left to itself; and its 

 unmanageability on the supposition of undisturbed elliptic 

 motion is forcibly illustrated by the circumstance that 

 periods ranging from seventy-three to ninety-eight years 

 have been assigned to it by skilled computers, most of them 

 with ample materials at command. 



Few double stars have received so much attention as 70 

 Ophiuchi. It is a beautiful object to view, and a tempting 

 one to measure ; hence, all sorts and conditions of star- 

 gazers have tried their hand at it. Much " grey matter," 

 too, has been wasted in efl'orts to rationalize its move- 

 ments. Encke exemplified with it, in 1830, his brand- 

 new method for determining stellar orbits ; Sir John 

 Herschel followed with a less rigorous, though more satis- 

 factory wodim oprrandi : and they have had not a few 

 imitators. The stars, meanwhile, preserved their indepen- 

 dence, taking slight heed of the various and sundry 

 orbits assigned to them, from which they diverged un- 

 accountably and at once. Perturbations were tolerably 

 evident ; and Captain Jacob, the East India Company's 

 astronomer at Madras from 1818 to 1859, hazarded the 

 conjecture that the visible companion, while circuiting its 

 primary in eighty-seven and a half years, described a 

 secondary ellipse round an invisible or at least undisccrncd 



body once in twenty-six years.* Sir John Herschel held 

 a similar opinion, and the persuasion of the star's triplicity 

 was so general that Mr. Burnham thought its r.elescopic 

 verification worth some pains ; but, neither with the 

 Dearborn eighteen-iuch, nor with the Lick thu-ty-six inch, 

 could either of the components be subdivided ; each alike 

 was pronounced " round, with all powers." 



At last, in 1893, Dr. Schur, of Gouingen, collected all 

 his Teutonic patience for an attack in form upon the star.t 

 His work was ably done. If the anomalies which had 

 occasioned so much perplexity could have been got rid of, 

 his skilful manipulation would have accomplished the feat. 

 But his ellipse had not been obtained, so to speak, in the 

 natural course. The observations of distance proving 

 incompatible with the observations of position-angle, he 

 had uncompromisingly rejected the former, and relied exclu- 

 sively upon the latter ; and even these had been subjected 

 to suspicious corrections, implying a consensus of error 

 between various excellent observers, now in one direction, 

 again, after some lapse of time, in the opposite. Moreover, 

 his elements stood very ill the test of predxtion. 



" While engaged recently," Dr. See writes,* " in the 

 observation of double stars at the Leander McCormick 

 Observatory of the University of Virginia, I took occasion 

 to measure 70 Ophiuchi on three good nights. On 

 comparing the results with Schur's ephemeris, four months 

 later, I noticed with surprise that the observed angle was 

 over four degrees in advance of the theoretical place. As 

 the Virginia measures had been made under favourable 

 conditions and with extreme care, it became evident that 

 the orbit to which Professor Schur had devoted so much 

 attention would need revision." 



He accordingly undertook the unruly star, and laid down 

 for it an ellipse difi'ering little from Mr. Burnham's in 

 1893, § or from Mr. Gore's in 1888. |i But this did not 

 satisfy him. Comparing one by one the recorded and 

 theoretical places of the revolvicg object, he detected 

 systematic discrepancies, acceleration repeatedly alteruaiing 

 with retardation. Their genuineness was coutirmeJ by an 

 examination of Dr. Schur's apparent orbit — obtained by 

 projection from the elemencs of his real one — showing 

 departures from tbe observed distances corresponding to 

 the departures from the observed rate of progress by which 

 Dr. See's own calculations were, to a certain extent, 

 vitiated. " We were thus," he remarks, " confronted with 

 a case in which it was apparently impossible to satisfy 

 both angles and distances." 



In the accompanying figure, the dotted line represents 

 Schur's ellipse; the smooth line, that constructed to accord 

 with the measures of last year. Both are projections of 

 the actual orbit upon a plane at right angles to the line of 

 sight. So that tbey profess to portray just the visible 

 facts of revolution, the larger star, as usual, being 

 assumed for convenience to be stationary. The two 

 curves — Dr. Schur's, it will be remembered, based upon 

 angles alone, Dr. See's upon angles and distances alike — 

 dill'er with curious regularity, one cutting the other at 

 fixed intervals, as it runs in and out. Both are evidently 

 drawn with dexterity and judgment; it would be 

 time thrown away to try and improve npon them ; yet 

 neither accommodates itself within plausible limits to the 

 given conditions. The stars were, iu point of fact, by 

 turns closer together and farther apart than they ought to 

 have been according to the Gottingen calculations ; they 



• H. Sadler, Enolisk Mechanic, Vol. XLI., p. 410. 



+ Aitr. .Vac 4., Xo3. 322 J-1. 



I Axtronomical Jottrn U, Xo 363. 



§ Astrottomii and Astroph'/sict, June, 1S93. 



1' Honlhhi yoticrs. W.l. XLVIII., Xo. ■'>. 



