THE COMING TRANSIT OF MERCURY,

Otago Witness, Volume 01, Issue 2123, 1 November 1894, Page 54

THE COMING TRANSIT OF MERCURY,

By George D. Cobmack.

On Sunday, 11th inst., we shall have the opportunity of witnessing one of those astronomical phenomena so full of interest to astronomers— namely, a transit of the planet Mercurj acroßß the sun's disc. To observers in Otago fcbe transit may "be seen from sunrise ,till about half-past 8. For further particulars regarding Its occurrence in other places the reader could consult an almanac, a« I intend the following lines to give a short description of Mercury and the part he plays in the phenomena of the eolar system. Mercury is the nearest planet we know of to the sun, situated at a distance from him of about 35,000,000 of miles, and comi-ljt.ing a revolution li. 88 dayn. He is a very small planet, about 3140 miles in diameter; and this, together with bi« olose position to the sun, which always keeps him more or leas immersed in the solar rays, renders him a very uaattraotlre object for observation. The discovery of this small planet certainly reflects great credit on the ancient astronomers when we taka into consideration the fact that under the most favourable circumstances he Is never seen above the horizon more than two hours in the absence of the sun ; so that an observer wishing to get a glimpse of Mercury must make the observation either a couple of hours before sunrise or the same period after sunset. Mercury's comparatively small dimensions and closeness to the sun therefore puts him almost out of the reacn of convenient observation, for even with the bsst of telescopic power the constant glara of the sun, together with bo small an object, makes it impossible for any definite markings to be visible on his surface which might; lead to successful computation of his diurnal motion. Is has been computed by eomo that the planet rotates on his axis in about 24ur smln ; but others again maintain that his rotation has never beep ascertained, no markings ever having been observed upon his diec with any degree of accuracy. Of lato years a good deal of attention has been paid to this raemberofour solar system by astronomers, and It is to be hoped that with the appliances of modern invention now at hand, the nature and motions of Mercury will be more assiduously investigated. Perhaps the most interesting part of the vißible aspect of Mercury Is his phanes. These were apparently, according to Ddtmivg, firht observed in the year 1630, and anyone possessed of a small instrument may observe these changes which we see the moon undergo monthly. Any planet whose orbit or path is inferior to that of. our earth, as in the case of Venus and Mercury, must go through all the phases of the moon. The cause of the phenomenon is this, and will ba readily understood. We all know that the planets shine by reflected light, and it is Obvious in cmsequence that only one side of a planetary body can be illumined at the one tine. Now, in the case of Mercury, for instance, while this body is In that position between us and the sun oilled inferior conjunction, he will be invisible to us, because his illumined half will ba .facing the sun and not us. But as the planet moveß round on his path, we will begin to sea a faint crescent, or the flr^t appearance of his lighted surface. As he proceeds still further we see more of his illumined surface, until at last, when he is directly opposite the earth— i,c , on ths other side of the sun from us— he will be observed as a bright star, because that part of him illumined will be facing us as well as the sun.

Occasionally, and at periods of varying lengths, Mercury happens to be situated exactly between the earth and the sun, and he is then visible as a small dark spot upon the sun's disc. This phenomenon is termed "a transit of Mercury," and was first noted by Gassendl in Paris on November 7, 1631. If the orbits of Mercury and the earth were in the same plane exactly, these transits would occur more frequently, but the orbit of Mercury is inolined about 7deg. to the plane of the ecliptic, so that the planet Is seldom in that plane. The time during whjob an occurrence of this kind lasts is about four hours, though on some occasions it has lasted as long as seven hours, as on May 7, 1799, and as short as lhr 14min, as on November 12, 1782. Transits of Meroury as a rule occur either in the months of May or November, and for this reason. The sun, in his apparenty early revolution round the earth, passes, of course, through every point of the eoliptio, and he consequently outs the path of Meroury at bis ascending and descending nodes. The places where these nodes are situated are In precisely opposite parts of the eoliptio, bo the sun therefore crosses them at intervals of clx months — i «., in May and November, and it is, of course, at these crossings that transits ooour. With Venus, the months are Jane and December. Transits of Venus, however, are of greater rarity and importance than those of Meronry, and they are observed with considerable Interest, ajj the phenomenon aids 09 Id 4fiter^nlni th:6j?ari'*dirtanojjtroath»

In making observations In connection with transits of Mercury, some strange phenonema have been noticed. Bright spots are supposed to have been distinguished on the surface of the planet as it passes over the eun ; and at times a bright ring has been seen to encircle it-. However, fhe accounts given by the different observers of these transits is of such a contradictory nature that it is generally accepted the phenonema in question are the results of mere optical illusions, for during the same transit, and with equally powerful telescopes, the results are seldom compatible. Some observers maintain strongly to having seen a blaok spot in the centre of the opaqu9 sphere, while others possessed of as good, if not better, means express themselves confident a« to its non-Bxißtence. These diversities of opinion also occur in connection with the luminous ring enciroliog the planet. For instance, during the transit of 1878, which was observed with great carefulness bj many distinguished telescopists. we have truly a diversity of evidence. Dr Dunkin, with a 6in equatorial, says he saw "a very minute point of light near the centre of the planet " ; also 11 the ring or oorona of light around Mercury was clearly visible." Captain Tupman, with a similar instrument, could detect no «gn whatever of either of these features. In fact, he was struck with the definite aharpneßS of the planet s outline. Again, Mr Christio with the great, equatorial of 12 Sin aperture saw "a minute s^ot near the centre of. the planet's disc," also " a bright halo of somewhat irregular outline was seen round the planet." Mr Downing with a 67in equatorial observed tfa.a ting round Mercury, but no ppofc on his disc ; and Mr W. Nash with a Gin equatorial could get no glimpse of either spot or luminous ring.

I could quote more evidence of a like cmqtiadictory nature, but the above will sullioe to incline the reader to regard the spot and hal:> as mere optical effects scan under different aspects, and probably arising from a variety of causes. No such appearances have nver been observed excopt during transits, and it is consequently more tlun likely tnat the sun* tflare, together with the solar corona that intervenes between Mercury and the sun, produces these telescopic imperfections. The next transit that will occur after the one of this present year will be on November 12, 1907. The little planet will complete nearly 54 revolutions round his great centre before we will observe another of these events. Thirteen long years— the third of a generation — will elapse, and aßtronomer.fi will agaia witness the little world as a dark spot projected upon the silvery disc of the sun, unchanged in appearance, »lowly and silently wending the same course as he did when observed by the ancient a<?tronomorG hundreds of years before the Christian era. A thousand years is but a speck upon the tira«plece of the universe. When the aßtronomer reflects upon the brilliant and harmonious order of the heavens, he suraly feelu the ufcter innlgnjfiennce of all terrestrial porup. Ha seos world beyond world, star beyond star, system beyond system, till even bis imagination is confused and bewildored in a galaxy of indescribable grandness, wonder, harmony, and perfection — wonders that, though they roveal their existence to him, baffl.B his intelligence from forming an adequate conception. The universe to him holds no lirait, no boundaries; life, force, and motion, midst uulimitable fields of ethe", pervade its boundless realms.