HALLEY’S COMET.

Lyttelton Times, Volume CXXI, Issue 15309, 19 May 1910, Page 7

HALLEY’S COMET.

TO-DAY’S TRANSIT OF THE SUN. THE EARTH AND THE TAIL. ENVELOPED, BUT SECURE. To-day the earth will havo its second experience within the Christian era of envelopment in the tail of a comet. Its return was predicted with marvellous accuracy, and Halley’s comet was first observed on its present apparition in September of last year, ana since then it has been rushing through space towards the sun and also the earth. On April 2U it attained its perihelion, and tins afternoon it will be in conjunction vvitn tne sun, an astronomical circumstance that is unique in scientinc records. During the penod occupied oy the head of tne comet m crossing the disc ol the sun it will be at its nearest position during the present visit to tiie ean-li, and tne magnitude ol its tail is so great tuat it will enveiop tne pianet and its protecting atmospneric covering.

'ilnco-quarters of a century elapse between tne periods at winch tins remarkable ceiesbiai wanderer is visiole from the earth, and tins is the third occasion on which it lias appeared since its period was nrst computed by the enstiuguisned scientist by wuose name it is now known. On previous occasions, notably in 1682, when it approached tlie earth’s path within a semi-diameter of the earth, its presence within the terrestrial ken has been attended with almost sensational circumstances. On tiie present occasion the comet will cross the sun directly in line with the earth, making tko hast transit of the sun’s disc by a comet that has been recorded. The progress of scientific knowledge within tiie past century has enabled astronomers to alleviate the popular fear that gained very general acceptance that the passage of tiie earth through the tail of a comet would bo attended with disastrous results to its inhabitants. The last apparition of Halley’s corner in earthly shies was in 18bo-6, its periholion, being attained on November 10 of the former year. It was computed to return to perihelion during the present year, and search for it was begun as early as 1908, hut it was not until last year that it was rediscovered. ■ It was located bv Professor Alas Wolf at the Komgstuhl Observatory in Heidelberg on beptember 11. Photographs showing it had been obtained at the Greenwich Observatory with the 30inch reflector on September 9, but owing to the proximity of the moon tiie two exposures were limited to thirty and twenty-five minutes respectively, and the very faint cometavy images wero not identified until after the receipt of the telegram announcing _ the discovery of the comet from Hciuc.-bc-rg. It was then of the sixteenth magnitude. There are irregularities ill the motion of the comet which make accurate prediction of its- return difficult, but Messrs Cowell and Cronimehn predicted its place to within four minutes of the actual spot determined by photographs taken by the observatories at Greenwich, Heidelberg and Lick, ft triumph in computational astronomy. From the tiny nobulous spot percoived in September of last year, the comet has grown in splendour as it has come plunging through space. It appeared to visual observation as an insignificant morning star some weens ago, but within the past few weeks its head has gained in brilliancy and its tail lias stretched out until it extends its twenty millions of miles over about 100 degrees of tlio heavens, a much greater lengtn than it ever attained before, and more than double the length that was anticipated. This afternoon it will pass between the earth, and the sun. The transit will commence at 1.52 and conclude at 2.52 o clock. It. cannot bo said with certainty then this coni unction of the two bodies will he visible even to powerful ’to.escopic observation, but on the other hand it »ay he possiblo to ueteev the presence of the intervening body by unaided visual investigation, with no apparatus other than a pieco of smoked glass for the protection of the observer’s eyes. The head of tlio comet may be detected as a small object somewhat darker than the disc upon which it will Owing to the irregularities in the motions of a cometary tail, it is impossible to determine the time at which the earth will enter the taij, but it will probably bo early this evening. As tlio earth and the comet are moving in opposite directions, the planet will pass through the tail at a rate equal to the sum of their motions, which is fortyfive miles a second, Tho diameter of the tail is estimated at two million miles, so that the earth will bo enveloped for at least twelve hours. OBSERVATIONS IN CHRISTCHURCH. ASTRONOMICAL AND MAGNETIC OBSERVATORIES. Tlio observations that will be made in Christchurch this afternoon of the comet’s transit of the sun and the attendant phenomena cannot be very elaborate, as the' instruments necessary for the delicate investigation required to ascertain any data are not available in the city. The astronomical observatory at Canterbury College is equipped with a six-incli telescope of high power and with it Dr C. Coleridge Farr and his assistant, Mr D. B. M’Leod, will endeavour to detect the nucleus of the comot in its progress across the solar disc. In addition to the interesting feature of the actual transit, there will be scope for careful observation in regard to tho phenomena consequent upon the envelopment of the earth. Mr H. F. Skey, who is in charge of the magnetic observatory, will endeavour to record any phenomenal conditions attributable to the proximity of tho comet.

This morning the rising of the comet and the sun were virtually simultaneous, there being a difference of only fivo minutes between them. It was not improbable that owing to the great length of tho comet’s tail its otter end would have been risible in the eastern sky in the early hours of this morning. At two o'clock, however, a heavy bank of clouds lay above the eastern horizon, and there was no appearance of a cometary glow to bo seen. ORIGIN AND CHARACTER OF COMETS. UNITS IN THE SOLAR SYSTEM. THEIR ORBITS AND PERIODS. Comets form a class of bodies belonging to the solar system, distinguished from the planets by their physical appearance and the-great eccentricity, of their orbits. There are several plain points of difference between comets and planets. The planets move all in the same direction, from west to east, which is astronomically called “ direct motion,” but the movements of comets are often from east to west, or retrograde. The principal motion of Halley’s comet is retrograde. The orbits of all the planets are confined to a zono of no great breadth on either side of tho ecliptic, but the paths of comets cut tho ecliptic in every direction, some being even perpendicular to it. The orbits of all the planets are nearly circular, or, more property speaking, are

ellipses of very small eccentricity. The orbits of comets present every variety of eccentricity, some of them almost rectilinear. Hovelius seems to have been the first who discovered by means of a geometrical construction that the orbits might be represented by parabolas, and Dorfel first calculated their elements on this hypothesis. After it was known that certain comets return to tho sun in tho same orbits, it became necessary to adopt the opinion that, in conformity with the laws of Kepler, the cometary orbits are ( ellipses having the sun in one of the foci. As tho ellipses are in general extremely elongated, and the comets are visible only while they describe a small portion of their orbits on either side of their perihelia, their paths during the time of their appearance diner very little from paiaociae. Thero are twenty known periodical comets, eleven of which have been observed at more than one perihelion passage. These latter have periods in years as follows:—Encke’s 3.3, Winnecke’s 5.6, Brorseu’s 5.5, Tempel’s L 6, D’Arrest's 6.4, Biela’s 6.6, Faye’s 7.4, Tuttle's 13.8, Halley’s 76, Tempel’s 11. 5/2, Swift’s 5.5. THE NUCLEUS. DIAPHANOUS AND UNSUBSTANTIAL. NO GRAVITATIONAL POWER. SUBJECTED TO ii\CONCEIVABLE ■ HEAT. The nucleus, whicn is the densest and most luminous part, may be said to torm tne true bouy of tile comet. It is tar, however, from Having tne dense and solid appearance oi planets, anu astronomers consider it to bo diaphanous, and believe _ that they have observed stars through it. It is extremely dmicuit to distinguish the nucleus from the surrounding nebulosity, if tne nucleus were an opaque gioouiar body it would exhibit pnasus lino Venus or Mars, according to its different positions with relation to the sun and tne earth, and such wore supposed to have been observed in the comet of 1682 by Hereiius, Picard and Laiiire. But tne neDUlosity renders tho phases ooseurc, and pievents the true body of the comet ti'om being seen. The real nucleus has probably never been soen by any astronomer. Comets are of ’ small density, for they produce no appreciable effect on tiie motions of the planets. Tiie comet of 1770 traversed the system of Jupiter’s satellites without causing any sensible perturbation ot tnese small bodies. It also passed very near to the earth, and Laplace calculated that if its mass had been equal in density to that of the earth, the effect of its attraction would have increased the sidereal year by 2hr 28min. Since its influence was altogether inssnsioie, it is certain that its mass was not equal to the five-tlicusandth part of that of tlio earth, and was 1 probably much less than even this quantity. If the nuclei of comets were solid," the matter of which they are composed must be extremely fine, in order to enable them to resist the intense heat they necessarily experience in their approach to tho sun. Newton computed that the great comet of 1680 was at its perihelion distant from the sun only 163rd part of the semi-diameter of the earth’s orbit, where it would be exposed to a heat more than two thousand times greater than that of red hot iron, a temperature of which the lay mind can form no conception, and a heat- which would instantly dissipate any substance with which man is acquainted. THE TAIL. SIR JOHN HERSCHEL’S CONCLUSIONS. A WONDERFUL ENERGY. During the last apparition of Halley’i comet, it was observed by Sir John Herschel and Sir Thomas Maclear from the Cape of Good Hope from January until the middle of Slay, 1836. Sir John directed his attention chiefly te a consideration of the cause which operated to produce the immense stream of matter called the tail, though on its outward journey this offshoot precedes: tho comet. He asked: “What is tint secret of its development within view of the observer; of its outward direction, always pointing from tho sun, in its approach, in its perihelion passage and outward flight.” After a careful examination of all the conditions, ho found himself driven to the conclusion that an energy of an entirely different kind from gravity and far more powerful, must exist in the sun'to produce such effects. In his “ Results of Astronomical Observations” (1847) he says: “ Nor let anyone be startlod in the assumption of such a repulsive force as is here supposed. Let it be borne in mind 'that we are dealing .... with, phenomena utterly incompatible with ordinary notions of gravitating matter. If they' be material in that ordinarily received sense which assigns to them only inertia and attractive gravitation, where, I would ask, is the force which can carry them round in the perihelion passage of the nucleus, in a direction continually pointing from tils sun in the manner of a rigid red swept round by some strong directive force, and in contravention to all the laws or planetary motion, which would require a slower angular movement of the mors remote particles, such as no attraction to the nucleus would give them, supposing it ever so intense?” Tho mystery grow upon him when he considered the immense stream of matter which issued from the body of the great comet of 16S0, and which in five days after passing the perihelion, stretched out far past the earth’s orbit : in tkqt brief interval it had shifted its angular direction nearly 150 degrees. Continuing liis argument, Sir John asks: “Whore are we to look if only gravity be admitted for any reasonable account cl its projection outward from the sun, putting its angular motion out of the question P” CONCLUSIVE EVIDENCE. ELECTRICITY THE CREATIVE FORCE. Professor Encke’s computation, shqwo ing tho comet of 1680 to have had a velocity at perihelion of a, million miiea an hour, is conclusive evidence against an angular motion of the tail. A continuous repulsion of new matter alone can explain the phenomenon. Astronomers of to-day identify the production of tho tail with electrical action exerted . by the sun on the lighter or volatile particles of cometary matter raised by the solar heat. The. German physicist, J. Karl Zollner, shows that, owing to evaporation ' and other changes produced by rapid approach to the sun, electrical processes of considerable intensity must take place in comets, and that their original lighters immediately connected with these, and depends upon solar radiation, rather through its electrifying effects than through its seemingly ther mal power. Comets are not bodies in candescent through heat, but glowing by electricity, and this is compatible with a comparatively low temperature. It is perfectly well ascertained that the energy of the push backwards produced by electricity depends upon the surface of the body acted upon, and that the energy of gravity depends upon its mass. Solar electrical repulsion increases as tho size of the body diminishes. Therefore very small cometary bodies, particles of matter indeed, will virtually cease to gravitate, and will ha wholly under tho repellent solar force. These discoveries in the physical domain of the comet-world afford a complete explanation of the origin and formatten of the long stream of almost ethereal matter projected from tho body of the comet, and of tho fact that it is always directed away from the sun. COMET SPECTRA. VALUABLE INVESTIGATIONS. COMETARY ELEMENTS DETERMINED. The comet of 1058-59 was subjected to a rough spectroscopic analysis by

'Donafci, after -whom the comet was ‘named, and the spectrum obtained, was •shown to be discontinuous, consisting (of a few broad bands. On August 5, '1864, Donati admitted the light of iTompol’s comet through a train of '.prisms, and found that it analysed into [three bright bands of yellow, green and •.blue, separated by wider dark intert'vals. This implied a great deal. ! Comets had previously been considered to shine mainly, if not wholly, bv rej fleeted sunlight. The spectroscope had I shown that they were self-luminous, and formed to a large extent of glowing gas. The next step was to determine what kind of gas it was that was thus glowing in them, and this was taken by Sir William Huggins, tho pioneer of spectroscopic astronomy, who died only a few days ago. In 1866 ie had examined the nucleus of Temjpel’s comet, and in 186 S Brorsen’s comet was analysed with similar results, but it was not until later in that (year that any successful attempt was I made to compare the band spectrum of [a comet’s nucleus with the spectrum of a known terrestrial element. A comet |of subordinate brilliancy, known ■as Comet 186 S 11., or some'times as Winnccke’s comet, was . the subject of the experiment. Huggins compared its spectrum with that of, the olefiant-gas “ vacuum ttibc” rendered luminous by electricity and 1 found tho agreement exact. All the [eighteen comets tested by light analysis [between IS6S and 1880 show the typical !hydro-carbon spectrum common to the (.whole of these compounds, but probI ably due immediately t-o the presence of [acetylene. .Some minor deviations from |the laboratory pattern in tho shift of [the maxima of light from the edge to [the middle of the yellow and blue bands [have been experimentally reproduced [by Vogel and Hasselberg in tubes containing a mixture of carbonic oxido with olefiant-gas. Their luminosity by disruptive electrical discharges was a necessary condition for the exhibition of the cometary type of spectrum. When a continuous current was employed the carbonic oxido bands asserted themselves to the exclusion of the hydrocarbons. The distinction is of great significance as regards the nature of (comets. Of particular interest in this 'connection is ‘the circumstance that 'carbonic oxide is one of the gases- evolved by meteoric stones and irons under stress of heat. The earliest comet of first-class lustre to present it.self for ‘spectroscopic examination was that disy»vered by Coggia at Marseilles on ’April 17, 1874. Bredikhine, Vogel and [Huggins were unanimous in pronouncing its spectrum to be that of marsh or olefiant-jjas. i Summarising the results of these investigations in her “Popular History ..of Astronomy in the Nineteenth Cenjtitry,” Agnes M. Clarke says:—“The (gaseous surroundings 'of comets are largely made up 'of a compound of (hydrogen and carbon. Other materials [are also present, but the hydro-carbon element is predominant. Its luminosity, (there is little doubt, is the effect of ,-electrical excitement.” PHENOMENA OP COMETS.

MAGNITUDE AND BRILLIANCE. 1 Tlie appearances exhibited by comets are diversified and sometimes remarkable. That which appeared in 134 B.C. 'at the birth of Mithridates is said to have had a disc equal in magnitude to 'that of the sun. Ten years before, this ■a comet was seen which, according to . Justin, extended over forty-five degrees. [Another, equally remarkable, appeared [in 117 A.D., and in 479 there was one of which the disc was of such magniitude that it might have occasioned the (extraordinary eclipse of the sun which ffcook place at that time. Tn 400 A.D. jone was observed which extended from .the zenith to the horizon. That of 531 'was of greater magnitude still and its 'appearance more terrific. Those which lappeared in 1066 and 1505 exhibited discs larger than that of the moon. It 'is, however, probable that these accounts were exaggerated. as since , comets have been, observed by astrono[iners no instances have occurred in which their magnitudes and appearances have been so extraordinary'. TAIL “ NEAREST THING TO NOTHING.” I DR KENNEDY INTERVIEWED. Some of the questions which people are asking about the comet, and esi pecially about the approaching contact of its tail with this planet, were put •to the Rev Dr Kennedy by a “Dominion ” reporter. I Can you tell me the exact times (when we shall enter the tail and emerge from it? ,

“ It is impossible to say exactly, because the motion of the tail is somewhat irregular. Wo shall pass through 'it during the night of Thursday next, May 19. The head, or nucleus, will transit across the sun from 1.50 to 2.50 p.m. on Thursday, and the earth should, '(therefore, enter the tail early the same .evening. We shall bo in the tail at '.least twelve hours. With the earth /travelling 15.3 miles a second and the '. comet 26.5 miles a second in *lie opposite direction, we shall pass through at /a velocity of, nearly 45 miles a second, and as the thickness of tho tail at that >part is estimated at two millions of ijniles, you will see that we should be in (it at least twelve.hours.” ! What part of tho earth will be the first to enter the tail ? i “ We cannot say, because we do not ; know, the exact time when we shall encounter it, but within three minutes of (the first contact, the whole of the earth mill be enveloped. Tho tail will appear i'to come to us from the east.” ! What would happen if the earth | came in contact with, the hoad instead of the tail P I “ There would probably be a very in)teresting shower of meteors. Some of .them might be largo enough to reach fthe surface of the earth, and might do 1 s- little damage. We do not- , know- the size of the particles in the nucleus, but / some think they are only as big as pins’ (heads and hundreds of feet apart from each other. If they arc as small as that, (they would bo vapourised' and lost bcIfore reaching the surface of the earth, lit is generally thought that no harm 'would result, even if the earth came (into contact with a comet’s head.” I Is it definitely known what the tail of Halley’s comet is.composed of? “ It is known to be composed of glowling gas at a very low pressure. Up to last January the result of photo- : graphs of the spectrum was that it contained cyanogen and nitrogen, hut I see now that a cablegram from Madrid .'states that the presence of cyanogen pas been disproved. Cyanogen, which iB one of the hydro-carbons,, is poison»Uß, but as comet’s tails are so rarefied jure should have no occasion to fear anything even if tho tail of the approaching comet consisted of nothing but cyanogen. I believe that nearly all the comets' tails that have been examined •with the spectroscope have been found ,rto contain one of the hydro-carbons—-tlie group of gases which includes cyanogen. / What shall we see as wo pass through the tail? “ Nothing, except perhaps n glow of iight in the heavens during the night, !but as the moon will be about full at (the time there is Very little prospect bf any unusual light of this kind being observed. There is a possibility 1 of a number of meteors being seeVi, because although the tail is mostly "as fihere may be a few very small particles Of solid'matter in it. They will ho 'suite harmless, as they will be turned (into.vapour in passing through our at‘Biosphere. There need• be no fear of 'any hurricane or storm, because, while I We shall go through the tail at erfuat ■Velocity, tho density will be so slight that such an effect will he utterly out *>f the question. The gas is at sure of less than one-thousandth part of the pressure of the atmospnere. It is as tenuous as the most perfect vacuum one can make .with an air pump. Now. vou wjl, agree A

1 could throw a vacuum—-not the vessel containing it, but only the vacuum — aa yon it, would not hurt you, however hard 1 threw it. A blow from a comet’s tail is like that. In fact, tho comet’s tail is tho nearest thing to nothing that wo have any knowledge ■of. The atmosphere around the earth i is like plate armour to Tv aid it off.” 1 Shall wo bo able to p.erceive any increase of temperature when we enter the tail? “ No. Tho glow of the tail is not due to ;t high temperature, but to electrical discharges in gas at a low .pressure. The tail is not very hot.” LECTURE BY AN AUCKLAND PROFESSOR. [From Ocii Correspondent.] AUCKLAND, May 18. Tho interest in Auckland with regard to the celestial visitor, Halley’s comet, was instanced last night when Professor Seagar delivered a lecture on the subject at St Andrew’s Hall. No fewer than two thousand people were turned away and the hall was packed. The lecturer, in discussing Halley’s comet and its previous visits and peculiarities, pointed out that it belonged to Neptune’s family of comets, as the indications were that it- was the attractive and perturbative influence of that planet which dragged Halley’s comet from its parabolic course into the elliptical one along which it at present travels. As the aphelion of that orbit was in the comparative vicinity of Neptune’s orb it, being only some 600,000,000 miles beyond it. tho. conclusion was reached that Neptune had caught the comet into our planetary system as it was speeding on toward tho sun. Tho lecturer also enlightened his hearers as to the methods of determining too course of a comet, of calculating its speed, and of even arriving at its composition. Halley’s comet, so called because tho Oxford professor was the first to verify its appearance and work nut its peribds, had an-orbit which extended something like 3.300,000,000 miles into space, yet took thirty-two years cf that time on its journey beyond Neptune’s orbit, which was 2,700,000,000 miles distant from the sun. Thus it was seen at what a tremendously increased pace it travelled as it approached the sun. Another remarkable feature which was common to comets, was the fact that the tail only became of any extent as it approached the sun, tho accepted theory being that some repellent action of the sun caused tho light matter and gases thrown off by the comet to whisk away to the rear, which accounted for the fact that the head .or coma was always nearest the sun, advancing or receding. Ho had little doubt that on Thursday, on which day the earth would only be about 14,000,000 miles distant from Halley’s comet, the earth would pass through its tail, which was estimated at about 18,009,0(10 miles in length, for in addition to this the tail was probably about 2,000,000 miles across, which, together would allow for more curve than would take tlie tail clear of our atmosphere. No one need liavo the slightest fear of the consequences, as so tenuous was the tail that although millions of miles thick, stars could easily be seen through it. It so happened on this rare occasion that the lino of nodes a creed so that on Thursday, in the afternoon some time, the comet would cross exactly between the sun. and tho earth, although only the most powerful telescopes might discover the actual transit. Tho professor went on to explain mat as an eclinse of the moon would occur on May 24, between about 4 p.m. and 0 p.m., the best opportunity of observing the comet as an evening star would probably then bo available if it. were a fine night, as until the full moon the moonlight would interfere with observations. Professor Seagar, interviewed this afternoon, said that to-morrow morning an exceptionally line, view of tho tail of Halley’s comet would be visible, provided that the weather conditions are favourable. “The head, or nucleus, will not be visible, however,” lie said, as it rises at about the same time as the sun. Wo shall be passing through the'tail during to-morrow afternoon or evening, and it is possible that part of it will be visible in the eastern sky tomorrow night after twilight.” Professor Seagar says there should be a fine view o.f tbe comet in the western sky on the following evening, just above the horizon. It will be visible to the naked eye for some two or three weeks to come, and will get fainter, and fainter. The moon will interfere to some extent with its luminosity, and it will be hardly as bright an object as it has been in. the morning sky. TO THE EDITOR. Sir, —As there seem to be seme very vague ideas afloat regarding comets, tbe following quotation from George F. Chambers, F.R.A.S., may be of interest :

“ Is there any reason to fear tho results of a collision between a comet and the earth? None whatever. Ho.wever vague it may bo, and in a certain sense must be, our answer to the question, * What is a comet?’ certain it is that every comet is ‘ a very imponderable body. At the same time, it always has been, and perhaps still is,- difficult to persuade the public that, wnatever might be the effect on a comet if it were to strike tho earth, the effect on the earth were it to be struck by a comet would be nil. This is not altogether a matter for speculation, for, according to a calculation by xiind, on June 30, 1881, the earth passed into and through the tail of the great comet of that year at about two-thirds of its distance from the nucleus. Assuredly tliero was no dynamical result; hut it seems, however, not unlikely that there was an optical result, at any rate traces of something of this sort were noted. Hind himself, iti Middlesex, observed a peculiar phosphorescence or illumination of tho sky which he attributed at the time to an auroral glaro. Lowe, in Nottinghamshire, confirmed Hind’s statement of tlie appearance of the heavens on tlie same day. Tlie sky had a yellow 7 auroral glare-like look, and the sun, though shining, gave but feeble light. Lowe adds that his vicar had the pulpit candles lighted in the parish church at seven o’clock (it was a Sunday), though only five days had elapsed since midsummer day, which itself proves that some sensation of darkness was felt even while tho sun was shining.” Prom this and from many oilier observations on comets I conclude that we shall probably sec (with suitable instruments) a small body cross the face of the sun, tho sunlight partly obscured as bv a cloud of smoko, tho blueness of the sky replaced by a yellowish light, and perhaps after sunset a display of “ shooting stars.”—l am, etc., NEW BRIGHTON. May IS, 1910.