Transit of Venus.

Southland Times, Issue 4494, 5 December 1882, Page 2

Transit of Venus.

— — ♦ This important astronomical event, which occurs on Thursday forenoon, and for which nearly every Government in the world has made preparation in order to ensure its correct observation, is the last transit of Venus that will take place for one hundred and twenty years. Before another will occur, the twenty -first century will have dawned, and the Victorian era glided down the obscuring* vista of the historical past. The advance which science has made since Gassendi, with his rude and imperfect instruments, set himself to the task of taking observations of the transit of Venus, two hundred and fifty years ago, has been immense ; but who can predict the limits it will have attained when the astronomers of 2004 undertake the ex. periment? Astronomical and scientifio knowledge will have reached a stage in that day of which even our imagination fails to concelvo, The great discoveries which have been ma ie within recent years, and which are as yet only in their infancy, will have reached their highest perfection, and been eclipsed by others far more wonderful than any that have gone before. Could we but obtain a glimpse into the future we would behold things undreamt of ii^ our philosophy. But as that is impossible, we can only turn to the past or busy ourselves witb the present. FIRST TRANSIT OBSERVATIONS. Transits of Venus usually occur in pairs, the two transits of a pair being separated by only eight years, but between the nearest transits of consecutive pairs more thany a century elapses. These transits, more/ over, can occur only in June and December. Upou the establishment of tho Coperaictu*

theory, it was immediately perceived that transits of tiie inferior planets across tho face of the sun must occur, and the recognition of the value of transits of Venus for determining the solar parallax was not long in following. The idea of utilising such transits for this purpose seems to have been vaguely conceived by James Gregory, or perhaps even by Horrocks, J-JiijhHalloy was the first to work it out completely, and to him is usually assigned the honor of the invention. His paper, published in 1716, was mainly insfrumental in inducing the Governments of Europe to undertake the observations of the transits of Venue in 1761 and 1769, from whicb our first accurate knowledge of the sun's distance was obtained. When Kepler had finished his Rudolpnine tables, they furnished, the means of predicting the places j of tlie planets with some, approach to accuracy, and in 1627 he announced that Mercury would cross the face of the sun on. November 7, 163i, and Venus on December 6of the same year; The intense interest with which Gassendi prepared to observe these transits can be imagined •when it is remembered that hitherto no such phenomena had ever greeted mortal eyes. He was destitute of , what would now* be regarded as the commonest instruments. The invention of telescopes was Only twenty years old, and a reasonably good clock had never been constructed. UNDER DIFFICULTIES. , Gassendi's observatory was situated in Paris, and its appliances were of the most primitive kind. By admitting the solar rays into a darkened room through a small round hole, an image of the sun nine or ten inches in diameter^was obtained upon a white screen. For the measurement of position angles a carefully divided circle was traced upon this screen, and the whole was so ai'ranged that the circle could be "made to coincide accurately -with the image of the sun. To determine the times of ingress and egress an assistant was stationed outside with a large quadrant, and he was instructed to observe the altitude of the sun whenever Gassendi stamped upon the floor. The transit of Venus was fixed to take place on December 6,1631. Gassendi knew that Kepler had assigned a time near sunset for first contact, but the tables were not sufficiently exact to forbid the possibility of the whole transit being visible at Paris. Alas ! alas ! these hopes were doomed to disappointment. A severe storm of wind and rain prevailed on 4th and sth December, and although the sun was visible at intervals on the 6th and 7th, not a trace of the planet could be seen.' We now know that the transit ; happened in the night, between the 6th and 7th, and was wholly invisible at Paris. THE FIRST RECORDED TRANSIT. "When Kepler predicted the transit of 1631, he found from his tables that at her inferior conjunction, on December 4, 1639. Venus would pass just south of the sun, and therefore he believed the second transit of the pair would fail. On the other hand the tables of the Belgian astronomer, ' Lansberg; indicated that the northern part of the sun's disc would be traversed by the planet. In the autumn of 1639, this discrepancy was investigated by Jeremiah Horrocks, a young curate only * twenty years old, living in the obscure village of Hoole, fifteen miles Dorth of Liverpool, and he found, apparently from his own observations, that although Kepler's tables were far more accurate than Lansberg's, the path of the planet would really be a little north of that assigned by Kelper, and a transit over the southern portion of the sun would occur. He communicated this discovery to his ' friend, William Crabtree, and these two ardent astronomers were tbe only ones who had the good fortune to witness this, the first recorded transit of Venus. Horrocks had great confidence in his corrected ephemeris of Venus, and it forbade him to expect the ingress of the planet upon the sun before three o'clock in the afternoon of Sunday, Nov. 24, old style (Dec. 4, new style) ; but as other astronomers assigned a date some hours earlier, he took the precaution to begin his observations on the 23rd. The 24th seems to have been partially cloudy, but he watched carefully from sunrise to nine o'clock, from a little before ten o'clock until noon, and at one o'clock in the afternoon ; having been called away in the interval by business of the highest importance — presumably the , celebration of divine service. About fifteen minutes past three^he was again at liberty, and, as the clouds had dispersed, he returned to his telescope, and was rejoiced to find Venus upon the sun's disc, second contact having just happened. Only thirty-five minutes remained before sunset, but during these precious moments he made determinations of the positions of Venus which are even yet of the highest value. Crabtree was less fortunate. At his station, near Manchester, there was but a momentary break in the clouds, a quarter of an hour before sunset. This sufficed to give him a glimpse of the transit, and he afterwards made a sketch from memory. ITS VISIBLE EFFECTS, The sun causes Venus to cast a shadow which has the form of a gigantic cone, its apex resting upon the planet, and its

diimetar coi)tiDUßlly;iincreasiug as' it recedes into apace. All the phenomena of transits ,are produced by thsparoage of ttjis ■hadow-cone over tbe eartb, and as eachr. Kint <rttKe'.lcdn*ck)rreßpondß!to a'particu- ■'■■ phase of a transit, any given : phase •will encounter the earth and *wilV first become visible' at soWe'tpbiht -Where 'the stan Is just rising. Between these two points It wfir traverse - nearly half the earth's circumference j 'ahd ih sddoitf^wilrcorisnme »bou«2olmintttek.>H*a v > « '•£ ■■.. . THB TRANSITS OF T^.KiaHTKENI'H CENTURY. The transit of iWi was visibll throughput Europe,; Md: was jwell; obseryed by astronomers iii i' Vail parts '-. ofl that continent. .Besides this, England V sent expeditions to St. Helena and itothe Gape of Good Hope, and English astronomers obaerved at Madras and' Calcutta ; >French Astronomers were sent toJTpbojsk, Rodrig«^Hmdi3Pdndicherry, < Russians ? to the confinsasof Taitary and China, and.Swedes to Lapland. No less than AXLiy stations ;*were - occupied . by 176 observers, and of these 137 published their ibsery|tj)ns. When this mass of data was- submitted to computation the result was far from satisifactqry./;Values of the solar parallax were obtained ranging from .-; Bv49a seconds to 10.10 Seconds, and in their disappointment the astronomere^f the eighteenth century c6nclncfea"th«t-^o much-reliahbe -had been placed u&iiSeihle'a i method of observation./ Thejprepafations for this trans.it of 1709* were most elaborate, and the results better than in 1761, but still not satisfac'fory.^VTlie'blackdroj) and other distortions disturbed the contacts in that transit as they had done in the previous one, and tho .■values of the parallax deduced by the best ,cotnptiters janged from 8.43 seconds' to j8»8p seconds. • Thus the matter rested till 1825 and' 1827, when Ehcke piiblisHed abstracts of his discussion of the transits' of (1761 -and 1769, from which he deduced^ t , parallax of 8.58 seconds. .This discussion Vwas not. printed in full until 1835, when it immediately. commanded the attention of astronomers; audits result, which Enckohad modifieS y td / 8.57 seconds, was universally accepted for more than a quarter of a cen- " time : wore ' oh certain gravitational investigations led to a Strong suspicion that the sun's distance had been 'over-estimated by at least three million " miles, and tho observations of Mars at its opposition in 1862 converted this suspicion into a conviction. The eighteenth century transits were: again rediscut-sed, and a parallax of 8.83 seconds was found from .them, by Powalky in 1865, and 8.91 r seconds by Mr.E. J. Stone in 1868. KTew- . combe's paper in 1867, also produced a marked impressioD. TRANSITS OF THE NINETEENTH CENTURY. , The transit of 1874 was then approaching, and in the discussion as to how it should be utilised Hailey's and Delia! e's methods, once more played a prominent part. It was recognised that the uncertainty in the observed times of contact of the eighteenth century transits was largely "due to, the black drop, and the causes of that phenomenon were caref ully considered. Among them most astronomers believed thatirradiation played an important if not the principal part ; but at the same time there -was a general feeling that the telescopes of a century ago were bad, and that the magnificent instruments of the present day would givebetter results. In view of - all the circumstances it was determined .■ that the contracts should be observed with equatorially mounted achromatic telescopes of from four to six inches - aperture, and that magnifying powers of from 150 to 2W diameters should be employed. The darmaoa and Russians adopted heliometcra of about three inches aperture for making exact determinations of the positions of Venn j during transit, but other nations ' did not follow their example. Photography, an agency undreamed of in the eighteenth century, was also available, and all saw the desirability of employing it ; but there was much difference of opinion as to how this should be done. The question of instrumental outfit having been disposed of, stations were selected and parties despatched to almost every available point. Nearly all tho nations of the civilised world took part in the operations. The weather was not altogether propitious on the day »f the transit, but nevertheless a mass of da'a was accumulated which will require years for its thorough discussion. Wticn the parties returned home the contiact observations were first attacked, but it was soon found that they were little better than those of the eighteenth century. The black drop, und the atmospheres of Venus and the earth, had again produced a series of complicated phenomena, extending over many seconds of time, from among which it was extremely difficult to pick out the true contact, lt was uncertain whether cr not different observers had really recorded the same phase, and in every ca-e that question had to be decided before the observations could be used. Thus it came about that, within certain rather wide limits, the resulting parallax was unavoidably dependent upon the judgment of tho computer, and to that extent was mere guesswork. Attention wan next directed to the photograph?, and soon it began to be whispered about th-it thc-as taken by European astronomers were a failure ; and so it proved. Those, however, taken by the American astronomers were more suceesasfnl, but at an international convention of astronomers held in Purls to consider how the transit of 1832 i-hould be observed, it was declared that photognphy was a failure and should not he died again. The American observers, however, are to give it another trial. Tho real argument for expending so much money upon transi's of Venus is that bcinar an important factor in determining the so'ar parallax, their extreme rarity renders it Unpardonable to neglect any oppoituuity of observing them. — Exchange.