LECTURE ON THE PLANET VENUS.

Globe, Volume XXIV, Issue 2686, 16 November 1882, Page 3

LECTURE ON THE PLANET VENUS.

“Venus and her Transits” formed the subject of a lecture by the Rev. H J. C. Gilbert at Phillipstown on Wednesday evening. After a hymn had been sung and prayer offered, Mr Gilbert said he would offer no apology for departing from the usual course of Divine service on the occasion. Ho had hoped that someone in this learned city of Christchurch would have come forward to instruct the public mind in a popular way on a subject which was occupying the minds of thoughtful men. As no one had done so he felt if his duty to do what he could for those of his parishioners who had neither the time nor the opportunity to study the subject for themselves. They would bo kind enough to bear in mind that ho spoke with no special knowledge of astronomy, but as one who had road something and was anxious to impart the result of that reading. After a brief explanation, with the aid of a diagram of the solar system, the lecturer proceeded to show in what way those planets whose orbits were interior to that of the earth enabled us by their transits to determine the sun’s distance. They all know what was meant by a transit, namely, a planet passing between the earth and the sun in such a manner that the intermediate body is scon projected on the solar disc. Venus and Mercury, being interior planets, had made several such transits, but the first that proved of any practical value was the transit of Venus in 1769. The English observers on that occasion were stationed at Tahiti, under the command of Captain Cook, and with him were Mr Green, Dr. Solander, and Sir Joseph Banks —names familiar to Now Zealanders —when they thought of Cook’s Straits, the Solanders, Green Island, and Banks’ Peninsula. Prom calculations based upon that transit it had been reckoned that the sun’s mean distance is somewhere about ninety-two millions of miles. Still there was an error of measurement, and one which the transit of eight years ago did not seem to have satisfactorily corrected. The compilers of the “ Nautical Almanac ” still found it as troublesome as ever to find the exact position of the moon on a given day some time hence. It ought to be remembered that the exact distance of the sun was that which enabled astronomers to measure the distance of the other heavenly bodies, for although the relative distances of all the planets are accurately known, their real distances are not accurately known, and could not be known until astronomers got hold of the standard measure —the sun’s distance —by which all other measures could be adjusted. Mr Gilbert next proceeded to point out by the aid of several diagrams the principle followed in ascertaining the sun’s distance. It was impossible to measure it in the ordinary way in which we gained the distance of other inaccessible objects—namely, by the direction of the object from both ends of a base-line. They knew that when two angles of a triangle were supplied it was easy to discover the remaining angle and two sides. Thus the surveyor gained the distance of a mountain-top the sailor the distance of a lighthouse. On the same principle the moon’s distance was measured; but in the case of the sun the difficulty lay in our not having a sufficient base line on the earth. Venus helped us out of the difficulty by giving us an opportunity of laying a base line on the sun. To make the matter simple let them suppose the earth to be at rest and only Venus in motion, and just entering on the sun’s disc ; thou let them suppose observers stationed one at the North and another at the South Poles. These observers would both see Venus crossing the sun at the same time, but not in the same place. The southern observer would note the apparent path of Venus much higher up on the sun’s face than he of the North. And since the lines of sight of these observers are quite straight, and therefore make equal angles on either side of Venus, it follows that the space between the two transit lines on the sun’s face would be to the distance between the two observers (the earth’s diameter) as the distance of Venus from the sun is to its distance from the earth. To make this as simple as possible, let them suppose Venus to be exactly midway between the sun and the earth, in that case the distance between the transit lines would be equal to that between the two observers, or the earth’s diameter. But Venus is much nearer to the earth than to the sun. If the sun’s distance were divided into 100 equal parts, Venus would stand about 72 from the sun and only 28 from the earth. The space, therefore, between the transit lines would be just two and a half times as great as the earth’s diameter. They would now see that when it was once known that such and such a portion of the sun’s diameter equalled the distance separating the observers, the whole diameter became known. And knowing how large the sun is, and knowing also how large it looks, it would soon be discovered how far off an object so largo must be to look so small. The lecture, which occupied an hour and and a half in delivery, was both interesting and instructive. The meeting closed with prayer.