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Transit of the Planet Venus.

By James Black, Esq.

The transit of Venus across the sun's disc which takes place in 1874 is already beginning to be a matter of engrossing interest to scientific men, in consequence of the great and important results that will accrue to astronomy from obtaining a favorable view of the phenomenon. The last that took place was in 1769, upon which occasion Captain Cook visited our shores here for the first time while commissioned by the British government to proceed to Tahiti for the purpose of observing it there. Recent news from England renders it probable that the neighbouring province of Canterbury will be selected as one station where that of 1874 will be officially observed. The construction of the proposed new observatory at Lyttelton will afford facilities for this much beyond what were at Cook's command more than a century ago. His apparatus were at that time somewhat hastily erected—had not that steady fixedness necessary in detecting those minute quantities that form the basis of astronomical measurements; and, besides, were not characterised by that refinement of mechanical structure that gives our modern instruments an efficiency far exceeding those of his day. The reason why Canterbury will be selected as a point of' observation is not because the phenomenon will be better observed there, but because our neighboring province will then possess the most southernly situated observatory in the world. The importance of this will appear presently. The truth is, the transit will be equally observable in every accessible latitude throughout the earth, and in this consists the primary element, or at all events one main element, of its usefulness. If it could only be observed at two stations, little apart from each other north and south, the value of the phenomenon would thereby be lessened in a proportionate degree.

And now, what really are the benefits astronomy will derive from the transit being observed at stations either close or apart from each other, and what are the principles involved in the process?

The first part of this question may be very briefly answered by stating that one important result will be the verification or correction of the sun's present estimated size and distance, and the latter part explained by referring to a very simple mode of illustration. Let any of the readers of the 'Evangelist' hold up his finger at arm's-length between his eye and the opposite wall of the room in which he sits, and observe the particular point on the wall which his finger eclipses; then let him move his head to the right or left, and he will find that his page 51 finger has apparently changed place in relation to the background against which he views it. Now this change of place in astronomical language would be called parallax, and in this lies the whole rationale of the question; for if he could measure very exactly the distance that his head has moved from one side to the other it would form a ground of computation whereby he could measure just as exactly the distance between the two points on the wall from the one to the other of which his finger has apparently moved, without rising from his seat to do so by mere mechanical means; and further could the principles involved in the process be rendered available when we come to try and ascertain the size of the sun during Venus' next transit. In 1769, when Capt. Cook was sent to these southern latitudes to observe the last that took place, another competent observer was also sent to the extreme north of Europe to observe it there. Their several observations were made simultaneously and successfully, and from a computation based on the results the sun was found, or supposed to be found, to be eight hundred and eighty-two thousand miles across, and to be at a mean distance from the earth of ninety-five millions of miles. Since that time, however, from several data upon which is based a more correct knowledge of the speed at which light travels, strong suspicions—amounting almost to a certainty—have arisen that the sun is not so distant by nearly three millions of miles as was supposed; and if this be correct, the sun's size heretofore has been misapprehended by us to a corresponding extent. Now, when the next transit takes place, Venus' parallax will be determined by a number of observations taken from different parts of the earth, some of the observers being stationed at as extreme limits north and south of each other as possible: the mean of the results will then, so far as accuracy is concerned, leave nothing to be desired for. Now the application of the illustration will be perceived—the finger is the planet, the wall is the sun, and the different points of sight from which the finger has been looked at represent the different positions the observers will occupy when determining the planet's parallax. Let the reader suppose that small circle to represent the sun, and the distance between the two transverse lines as representing the parallactic displacement of the planet, arising from its being viewed from different parts of the earth (the effect being exaggerated, however, so as to render it more apparent). Here, then, is something determinate—something that can be measured in consequence of a trigonometrical relation existing between it and the distance of the observers from each other, and the angles subtended from the different points of view. But this is only half of what is sought page 52 to be discovered; the distance between the two lines has been got; but their several lengths must also be ascertained, and the proportion they bear to each other. How shall this be arrived at? Easily! By watching and noting the difference of time the planet takes to traverse the one and the other. Here, then, the whole question is solved, because if that portion of the sun's disc enclosed between the two lines of transit can be measured, it follows that the sun is just as many times larger than that portion as it forms part of the integral whole. Here, then, is a mode—a rarely recurring mode of testing former calculations, and when it occurs the size of the sun, its distance from us and the distance of Venus from the sun and from us will all be determined by such accurate modes of observation as cannot fail to ensure the correctest results.

Such are the problems; and these only form a part of the problems Man is continually endeavoring to solve. There are unsatisfied longings within him that ever prompt him to the task of unravelling the higher mysteries of the wondrous scheme of the universe—a scheme that though continually unfolding still leaves him so much to search after and comprehend. The more he explores it, still the more are opened up to view new beauties of design, and greatness and fitness of purpose—purposes that, though general in their scope, are still particular in their tendencies, ever operating and ever fulfilling the highest ends.

Before closing these remarks, it will only be in place here to say that the study of the great truths of astronomy forms a consistent and even congenial part of the Christian's duty. Let him depend upon it that so many worlds—some of them far exceeding our own in size—have been created for other and higher purposes than merely to shine upon us as insignificant points of light. If a single drop of water be a universe in itself—peopled with vital organisms whose number manifests an unstinted liberality on the part of the Creator in the bestowal of life, shall we in the face of that, deem all space beyond our own sphere to be a desert without an oasis—a wilderness without one fertile spot on which may be manifested instances of the Creator's power and associated with proofs of His benignity? From the physical aspect of some at least of the planets, it is apparent that they are in all respects fitted to he the habitation of beings like ourselves, and when once the admission is made that they are so, where are our conclusions to end? Who shall place a limit to the number of animated groups that may people immensity? If we are surrounded by numberless proofs of wisdom and power, as developed in terrestrial creation, and which. page 53 impress us with a sense of their almost infinite variety as well as number, how much higher ought our conceptions to be of the great I Am when the thought steals over us that such indications of His vivifying presence may extend throughout worlds whose number is only surpassed by the infinitude of His deity?