HALLEY'S COMET.

Dominion, Volume 3, Issue 646, 25 October 1909, Page 3

HALLEY'S COMET.

I -...."'■ > TAKING ITS PHOTOGRAPH. : A LETTER FROM. MEEANEE OBSERVA- ' '~ .'TORY. Writing from "The Observatory,' St. Mary's Seminary, Mceaneo," under date October 22, : Mr. S. J.- Cullen states: V ,"I enclose an enlargement of a-'photograph' ' of Halley's Comet, magnified from the original ' negative eleven' diameters. Of course,, the-first ' photograph of an expected comet is necessarily 1 very taint. Were it large,- and very clearly \ defined, the comet could have been photo- ? -graphed before. It was not till within the ;• last week-or so that so long an exposure as two and a half -hours was possibje. During the whole of >this time, tho sky in that direction must be perfectly cloudless. Tho plate used was extremely sensitive, and the lens was a three and a hall inch Dalmeyer portrait lens at open aperture of 3.5. Later on,-when" the comet becomes' tclescopically visible, photo- •■ graphs for reproduction can be taken. Then ) tho camera will be nblo to follow the comet ■ itself, and not the adjacent stars, as it must i at present. So accurately was its position ) gauged at the' timo-of exposure, that it ap- ■ pears almost in the centre of tho plato which I photographs clearly a portion of the heavens I the size of the Southern Cross only; . The i comet rises at a. quarter past twelve at prc- , sent, just where the sun rises in winter.. It is, howovcr, invisible as yet, even through tho largest telescopes.. Its feeble-light was-.allowed to add minute by, minute to an extremely faint ■ impression, till at dawn a distinct, though very small, imago of .the comet revealed itself 1 on the developed, plate.", '--' 1 THE ROMANCE OF ASTRONOMY. ; The reappearance of Halley's Comet calls to : mind.ono of the most interesting chapters in the history of astronomy, Ncarly\ two: centuries I and a quarter, havo elapsed since, Newton ani nouueed the theory of the motion of comets, I in accordance with which it was henceforward f understood that many of these bodies were . probably moving .in elliptic orbits round tho i sun, and might, be. expected to appear regularly when in those parts of their orbits which were nearest him. A hundred years ago, only one comet was known to hove actually reappeared. ,At the present time, the number , recognised.as-periodical is-very considerable; ;■ but most of those which move in ellipses of i SlTr' ol! i and return at short- intervnls, are bodies of feeble luminosity, frequently I requiring powerful telescopes to 'see,them at ' nL'Tn " ercr P r f e » ti ns-'a brilliant appearance hko many of. those, which make longer excursions- into space, and attract general attention when they return, to the neighbourhood of the sun and of, our planet. Comets which acquire thousands, or oven hundreds, of 3 cars to complete, a circuit round tho sun,' are - co«ri 0 tl,^?f m?!1 ;- a P ro .i;i rti ' )H "f their, whole oZtswtl to determine their tni, fll Mi ver accuracy, to ascertivelv iritl, P nT° dS ' °' r to mentify then, posilarlv *lL 1 Prc \ l ° us particu,' El . obs , or , vatl ?Ds.of f centuries ago, on' H w knowledge o^their, motions at sue SrSS ?„*?«" (I " iV:ed ' ,VCr, ' ! ""t «>'fficiently preciso Tor this purpose. JLi S in , (< ;" stin e here to- remark that all the comets .which revolve round the sun in very HolV P " 10<,S n (ft - nd wh, > h ' as »«»* «een ment oned, are all inconspicuous bodies) move™." tho same direction as do all the plauets, and h„°J^M 0f m « n . s 3dina«oii to theirs? whereas those of long periods move quite, as frequently m the opposite direction, and in planes inV °7- 'fl-e ; *rißl«S'' (sometimes very nearly perpendicular) to those of the planets. Halley's- Prediction, n the ' COf "- fet ? short and those of very long periods, there are few which / oqu ' rin r 1 somewhat mo r<> than se\pnty years to complete a revolution, and about the same distanco from him as Neptune, the most distant:khown planet,• One• of these !?p£m?S' CU °i US '■?' >i « ct wl,0 » >t returns to the neighbourhood of the sun; and it was in reference, to it that,, the first successful prediction of the return of. a conict was made. Ihe. astronomer ..who .did so. it need scarcely bo montioned, was tho illustrious Hallcy, who 'became the second Astronomer Royal at Greenwich in 1.20. Ho made the prediction in ques-tion-in 1/05. 'Applying Newton's principlesto the .motions ot comets, now considered 'to i "i 01 ; 1 "!! "ko tho planets in conic sections , about tho sun, ,he .discussed t| lo motions-of "." these-bodies-of this class of whicli- 1 Stt'ffiS ' cient observations ;wcr'e and findina; thnt two discovered respectively in : August, 1531, and in September 1007, were revolving in elliptic-orbits very "similar to that'vpurs'uedi by the. comet observed by himself and others ■m- the summer of .1082,.■iho'conjectured that - all these were appearances of the same body movnig.roHiulthe sun (in. the opposite direction to that, of- all the. planets),: in aii elongated ellipse, a complcto revolution round which occupied about, seventy-six years. Ho was further confirmed in his views by the accounts . of the remarkable comet seen in 1450, which, al- , though no strictly astronomical observations of its course-were made, appeared'to havo ■ been m all probability,, moving in nearly the same manner, and was thus led confidently to predict another rettrn of tho same body in 1/50. (:;omewhat too fast, as is often the case) ho .considered that all comets would, by degrees, be found to be moving in elliptic orbits, and to. return regularly to view, when they should como into those parts of their courses which were nearest to theearth- and sun. As regards the comet in' particular whose orbit he: calculated, it is well known that his prediction was - fulfilled ■ and that body returned in tho winter of. 1753-59 again in the autumn of 1835, 'that it has always been called sineo : tho first,of these return's by tho name of the astronomer who predicted its return, and that it has since-been shown, with a degree of probability amounting'al- ■ most to a certainty, that it can bo' identified with many previous appearances, tho'first of which took .placo about eleven years before the Christian- era. ,"'. ''.Identification'of .Comets.' 'Several subsequent attempts were made to identify comets .as seeiv at more . than one appearance,. and to predict their future returns. Maskelyne (Astronomer Royal from liGS'.to 1811)-thought that the comet seen in 1532 was identical with that of 1661, and that it would return in 1700. The Russian astronomer, Lexell;. finding that a comet, which was discovered by Messier at Paris in tho month of' June, 1770, appeared to bo moving -in an ellipse of short period, predicted that it would return after an interval of about five and a half, years. But those expectations were-not fulfilled; nor was it until about ninety years ago Uiat other predictions of-tho returns of comets were made,.one of which very soon proved to be successful. This was in regard to one discovered by Pons, at Marseilles, on November 26, 1818. Encke, applying the resources of improved mathematics to tho theory ;of -.the. motions'' ■ of■ ■ this ■ body,. found that it' was identical with one discovered in' 1786 by Mechain, .in 1795 by Miss Herschel, and ■in ISOS by- Thnlis and others. He found that its orbit was very short, and that.it,only required about threo and one-third years to "com-: plete a revolution-round the sun. Hence ho predicted its return accordingly took place, tho comet being first seen on June 2. ': But before this tho Teturns of two other cometswith periods nearly as'long as that of. Halley's, had been' predicted. One of these was discovered ■ in-1812—also' by Pons; but it was not until 181G that its orbit was determined by Encke himself, who proved that the period was a little more than seventy years. ' It has boon reserved for a later time to see this prediction fulfilled—tho returned comet being detected first on September 1, 1883, by Mr. Brooks, at Phelps, in the Stato of New York. In 1786 it passed its perihelion on September 15; at tne subsequent return on January 25. ISBi. • ~)'■■ '■ Less than three years after the first discovery of- the last-mentioned comet, or on March.fi, 1815,-nnother was discovered by Olbers at Bremen, which passed its perihelion on April_2o, and was calculated to be moving in an elliptic orbit with- a period of about seventytwo years, or a little longer than the other. It returned accordingly and was detected by Mr. Brooks at Phelps, N.Y., on the night of August 24, 1887, and passed its perihelion early in October. CAMBRIDGE OBSERVATIONS. ! (By Telegraph—Prnss Association—Copyright.) (Rec. October 24, 5.5 p.m.) London, October 23. '. Mr. H. 'F. Newall, astronomer at the Cambridge Observatory, saw Halley's . Comet on Thursday with a large reflector. ~