ASTRONOMICAL NOTES.

Press, Volume LVII, Issue 17157, 28 May 1921, Page 6

ASTRONOMICAL NOTES.

FOR JUNE. (SPECIALLY I'OR "THE press.") (Br E. G. Hogg, M.A., F.R.A.S.) The sun outers the zodiac 1 sign Cancer at 11.6 a.m. on June 22nd. This dav will be the shortest of the year, the duration of daylight being only Bhrs olmins. It is at the time of the solstices that the length of daylight alters most slow.lv; during the ten days before or after the shortest day. the period of daylight lengthens only by about two minutes, while in the ten days lie fore or after an equinox, the change is as much its 33mins. At the winter solstice the meridian altitude'of the sun is only 23 degrees as against 70 degrees at the summer solstice. The distance of tho earth from the sun on June 22nd will bo about 94,370,000 miles. The planet Mercury sets on June Ist at 5.42 p.m., and on June 15th at 6.9 p.m. Venus rises on those dates at 3;51 a.m. and 3.44 aim. respectively; it is now a brilliant object in the eastern sky before sunrise, its apparent magnitude being about —4.2. Mars is too close to the sun during the month to be seen. Jupiter sets on June Ist at 11.35 p.m. and on June 15th at 10.48 p.m., and Saturn sets on those dates at 0.16 a.m...and 11.24 p.m. respectively. Observers should have a good opportunity for sighting ~.he planet Mercury just now; it sets-on Juno. Ist about 36deg. north ol wost and, will be a . brilliant object in the sky, its apparent magnitude being 0.0, i.e., it will be brighter than any of the fixed stars except Sirius and Canopus By June loth its apparent magnitude: will nave declined to 1.0. There can be no doubt that Mercury is a difficult object to see without a telescope, as the elongation or angular distance trom the sun seldom exceeds 1& degrees. There is a legend to the effect that Copernicus, despite all he wrote on the planetu.y system, never during his long life of seventy jears certainly saw Mercury. Brahe thought fit to p-ai-ft on record the-fact that while in Denmark he mado several observations of the planet. One Goad, who published in 1686 a folio volume of astro-meteorological aphorisms, unveiling the choicest , secrets of nature, contemptuously calls Mercury a 'squmtin.T lacquey of . the sun, who seldom shows his head in these parts; as .it ■ ho was in debt." More recently Mr W.J. Dinning, tho well-known authority^ on meteors, in a communication to Mature," stated that according to the register he had kept of his observations of the planet, lie had seen it. 150 times during the years 1868 to*l9oo. It is to be hoped that some of our younger observers who have not yet made acquaintance with this elusive heavenly body will score their -first sight of it during the present montbConsiderable interest a/ttaches to tho return of the Pons-Winnecke comet to perihelion during this month. It traverses its orbrfc round the wun twice while Jupiter makes one . revolution, and during the past-fifty years it has be;*n so greatly disturbed in its pdth by tTie attractive influence of; that planet that its periodic time: has been increased by about four months, and its perihelion distance has been altered trom about 0.77 to unity, i.e., when nearest to. the sun its distance from that body is equal to the earth's ihean, !• distance-: hence, if \tlic/;paths of the comet and the earth do not actually intersect, they will como very close to. each other. The knowledge of this hear approach of the two' bodies- has evidently caused some excitement in journalistic circles, • and - some • sensational head-lines have in consequence been excogitated: "Look out for.the Bump," "Great Comet's Mad Orbit," "Alas! Poor Pons," "Great Firetfcirk Show on June'26th," "Fatal Bump for tho'Cyclonic Traveller": these are a few' specimens, and it is evident that the .authors of'these scare notices regard the impending catastrophe with some levity of spirit. It is unfortunate that this "sliow"—possibly the first of it,s kind if it com:-s off—is confined to. the northern hemisphere, and that we who live south oif tne line are out of all the fun: Bat,' after, all, it may not come off. The exact date when the comet will pass through the. earth's orbit is not known to a few days, and there may «e as much as an. interval of'a fortnight elapsing before the earth reaches the point in its orbit through which. the comet plunged. Many years ago ;it was calculated that encounters between the earth; and a comet. will not; occur more -than once in fifteen or twenty million years, on the average; at prer sent we have no certain knowledge that the earth has ever been struck by a comet ; there is no record—■reolasicul or' historical —of sticli. a collision, so there is a faiijt sporting (.'linnce .that it may como' off this time, and tiv.it the' almost trftal ignorance under which we labour concerning the physical constitution of a: comet'b head may be dissipated. We do riot anticipate nitv very untoward results' from the "blimp," it is unlikely to affect- the cost of living or cause <i postponement of the Di-mpsey-Carperitier fialit; ; "Ihe eardi luis on more man one occasion passed, through, tile tail, of a comet; Such an event happened in all pri/bability on June ibth; jLoii), but nothing was known ot it until n&rly a. month later, when the tact of. its occurrence eni.Tged from the calculations of Olbers. On June 3Jl'h, 1861, the earth passed through the tail of a great comet; no perceptible effects were, however, proclucvd by the meeting. The magnetic instruments of the Greenwich Observatory were, indeed, disturbed on the following night, but it would be rash to infer that the comet was the cause : of tlie-ir, agitation. It is probable that tho tail of Halley's comet during its recent visit to ihe vicinity of the sun enveloped. the earth, but'lio peculiar atmospheric or other phenomena were noted. It would seem that the material expelled frotn the comet to form its tfail (and 'presumably lost to the comet torever) is so fide-grained that in spite of I the fact that it may be moving with a very high' velocity when it readies our atmosphere it is unable to manifest its presence by any luminous or other effects, and our .past experience therefore justifies us" in anticipating that no ill effects will follow a passage of the earth through the tail of the PonsWinnecke comet. Turning our attention to the head of the comet, we -may note that, besides its action in producing the tail, the sun exerts a powerful disintegrating influence upon a comet: the head may be regarded as a loosely agglomerated mass of solid bodies, occupying a large volume of space, and as the parts of this mass lying nearest to the sun are more powerfully attracted bv that body than those now remote from it, they will tend to move ahead relative to the other parts, so . that after many revolutions round the sun the comet pursues its course, followed in the same orbit approximately by aa ever lengthening cliain of cometaiy matter, which reveals itself to us in the periodic showers of meteors or shooting stars, which appear in the heavens. With the progression of time the chain may, as in tiie case of the "Perseids," form a closed .ring, which intersects the earth's orhit ( and so gives rise to an annual display of meteois. Now this disturbing, action of the sun on the head of the comet, which produces ultimately the meteor stream, is not a selective one; it affects

alike the large as well as the small masses, of which 'the head is cm posed, and therefore the resultiag chain of cometary matter or nieteo'" stream will contain not onlv the small, bodies originally forming the head, but also the Ijirge masses—if anv were present there "Wo can understand that , tne minute members of a meteor stream entering the earth's atmosphere with a ! nigti velocity will encounter a fri' tional capable of developing nn amount of heat sufficient to speedily reduce them to a gaseous condition,* but ve should expect, the larger masses to survive their collision with the at-tuo-spher". and rench the earth's surface as meteorites. There is, however, no evidence which connects the fall of meteorites with periodic displavs of meteors, and it therefore seems reasonable to infer that the meteor stream, I and consequently the head of the comet from which it is derived, is composed almost exclusively of small fragments or matter. There are other rensons for neJievmg that meteors and meteorites ,- 1 P f!r m their origin, but that must be v. j a • ure occas ion. -If, then, I the head of a comet is merelv a meteor I swarm containing in addition to small i so'id particles, much gaseous material, : aR tne spectroscope appears to indicate, the impact of the Pons-Winnecke comet, the earth's atmosphere, is liKcly to do no more than give rise to a display of shooting stars, which may rival m intensity and beauty the wonderful phenomena associated with t' ! e recurrent Le nid showers of 1833, 1856, and other years. There is, j however, another contingency which is worth a moment's attention: the head of the comet may miss the earth and yet pass close enough to our companion to alter its motion Ihe moon makes one rotation on its axiß while revolves once round the earth, and consequently the moon always presents the same face to tho earth: if, then, the comet produces a change in the.moon's orbit, and so disturbs the balance between the times of rotation and revolution, it would como to pa9s that -the well-known face which the moon has shown us in the past- | would be slowly turned away from us j and a lunar surface; which no eye has yet seen would be carried into'view, new features would come into sight, and . selenography would be the popular stiidv for a time. Man has often speculated what the face of the moon turned from us is like: let us then hope, that the comet will sntisfv.our craving for knowledge in this respect. On June 16th, 1916, Mr W. F. Denning went out into his garden at 10.31) p.m.- to watch for meteors, and in a few minutes he realised that a very special shower was in progress: shooting stars were radiating from a small '.area in the sky at the rate of about .30 or 40 per hour, and subsequent examination of the circumstances of -the fall led him to associate it with the PonsWinnecke comet. Other observations made in America confirmed Mr Den- : ning's view, and showed that-outlying meteors;of the system had reached the earth's atmosphere as early as May 13th and as late as July 3rd. The parent comet had at that return to the neigh- : bourhood of the sun passed through perihelion in September, '1915, and from i this it may be calculated that the length - of the meteor chain was not less than | 560,000,000 miles. There does not-ap-i pear to have been a recurrent display of the meteors in June. 1917, from which a finite length for the stream may be inferred. Astronomers are anticipating that there will be a return of this shooting-star system ..this year,, and the date of it has been somewhat confidently fixed its June 28th: it is quite likely, however, .that the perturbing action of. Jupiter, which is supposed- to have displaced tlie comet'-s orbit outwards^from the sun during the period 1909-1915 by some three million miles, may have .brought about changes in the orbit of both, comet, and _ meteor stream which will materially . alter the time 6f the " expected shower, and it is highly probable that the continuation of this disturbing potion may in.a : , few"'years carry the Whole system well, beyond the earth's orbit" wh'6h t%he meteor -display will- no ' lon'ger occur.. It is perhaps significant that wb'le the comet, was photoffrirhed in 1915 five months before perihelion, it had not been detected on this return by the beginning of March, or less than fonr months before nerihelion, in Bpite of the faet that it is very favourably nWced for observation in the morning sky. '