Doubtless many of our readers may think it premature to say anything about an event six years before it will transpire, but there are good reasons in this case for such an apparently ill-timed proceeding. The transits of Venus to take place in 1874 and 1882 are justly looked forward to by astronomers as the greatest astronomical events of the cen* tury in which they will occur. Why they are so considered, and the necessity for anticipating them by extensive preparations, it is the object of this article to show. The phenomena called transits occur only with the inferior planets, that is, those whose paths of revolution around the sun lie wholly within that of the earth. A transit is nothing less than an eclipse of the sun by an inferior planet, that is, the passage of either Venus or Mercury directly between the earth and the sun, so that their disks partially obscure its face, and appear as round, dark spots upon it. Conventional usage has limited the term eclipse of the sun to the obscuration of its disk by the moon, and transit to the same effect produced by the passage of Venus and Mercury between the earth and the sun, although there is no essential difference in the nature of the phenomena. The transits of Venus occur very seldom. The first one, we believe, of which there is any record, was observed in 1639, by the gifted young astronomer, Horrox, whose brilliant career was so suddenly terminated by death at an age when few have even begun to achieve immortality. The celebrated Dr, Halley communicated a paper to the Royal Society in 1691, with a view of calling attention to a proposed method for determining the parallax of the sun, and thereby its real distance from the earth. Since his time only two transits of Venus have occurred viz., in 1761 and 1769. Dr. Halley ex. pressed in his paper the belief that, in the way proposed, the sun's distance from the earth would be determined with great accuracy. The feasibility of the method at once attracted the attention of astronomers, and, upon the occurrence of the transits in 1761 and 1769, the sun's distance was computed to be 95,173,000 English miles. The parallax of heavenly bodies is the difference in their apparent relative position, when viewed from different stations. It is usually expressed in degrees, minutes, and seconds, of angular measurement. This may be illustrated by the following simple method. Take a station at any point where a tree, or lamp-post, or stake can I e brought into range with a corner of a house or any other fixed object, representing the sun. The intervening object may be considered to represent the planet Venus, and the station at which the two observed objects are in line, may represent a portion of the earth's surface. If, now, the observer take a station to the right or left of the first station, the objects will no longer appear superimposed, but separated to a distance, depending upon the distance between the two stations, the distance of the stations from the remotest object, and the distance of the stations from the intervening object. The angular difference between the apparent positions of the bodies observed, and the distance between the stations, are sufficient data for determining all the other distances, provided the angle formed by a line joining the two stations, and a line joining either station with the intervening object is also known. The problem is then reduced to the finding of one side of a triangle, another side and two angles being given, a very simple operation in plane trigonometry. In astronomical observation there are always some determinate errors, arising from refraction and other causes, which may, however, be readily corrected, and do not affect the general principle of the method as above illustrated. In calculating the distance of the sun from the earth, the stations, from which the observations are made, can be so placed that the semidiameter of the earth becomes one side of a triangle. The parallax of the sun was thus calculated from the transits of 1761 and 1769, and found to be 8*65 seconds angu- j lar measurement, and the distance of the sun was hence de-1 termined to be 95,173,000 English miles, as given above, j Subsequent calculation by Encke made the parallax to be i 8*5776 seconds. j It will be seen that the correctness of these results depends i upon the accuracy of the observations upon which the mathematical calculations were based. That these were not accurate, seems probable from the fact that there is every reason to believe, from the sun's parallax, as more recently determined, that the distance as originally computed is wrong by at least 4,000,000 miles. Many hypotheses have been made as to the origin of such a grave mistake some attributing the error to confounding a part of the planet with its penumbra, and others to mistakes j i n the computation, but these are of little importance. The time is approaching when the problem can be reworked, and, with the improved apparatus now possessed by astronomers, and the wonderful advances made in methods of observation, it may well be hoped that this time a reliable result will be obtained. The Standard (London) says of the extensive preparations now initiating for the observation of the coming transits, that " the Astronomer Royal is doing good service in preparing betimes lor the great event. Though it may seem a long time to look forward to, to those who are unacquainted with the amount of preparation required for such observations, those who know the difficulty of procuring a large number of j first-rate instruments, unless plenty of time is allowed, will j know that there is really no time to be lost, especially if, as ! we should hope would be the case, all the expeditions sent out are provided with precisely similar instruments and appa- j ratus. It is imperative upon the government to put no obstacle in the way of carrying out these observations in the most perfect manner. England must not be behind the Continent, at any rate. If any amount of failure takes place, it will not bo from want of preparation on Mr. Airy's part. At the late meeting of the Royal Astronomical Society he showed that there was nothing indefinite about his ideas; he had already prepared careful maps both for observing the ingress and egress of the planet. He showed the importance of sending j expeditions to several places, because, among other considera-' tions, a thousand obstacles might interfere with the observations in any particular place. There are places which, if weather, etc., are favorable, will be admirable for all purposes, but, as in the case of Kerguelin Island, the chances are very much against a clear atmosphere. Captain Toynbee said that this island is seldom to be found on account of the fog. If practicable, no expedition will be of the importance of one sent to the South Pole, that is, as near to it as possible. At the South Pole the effect of parallax will be the greatest that is to say, the position of Venus will vary to the greatest extent on the sun's disk. The Astronomer Royal in his maps suggests two points, one in Enderby's Land, but here the sun would be too low for it to be a certainly advantageous position he greatly preferred a point in the Antarctic Continent, where Sir James Ross landed. As a place for observation nothing could be better. The only point is, Will the seHferity of the climate admit of the expedition ? Captain Richards, the hydrographer to the Admiralty, spoke well upon it. He showed that if properly fitted out and provided with good huts, clothing, and food, there would be no further objection to the place than must stand in the way of any Arctic expedition. Those, however, who joined in it would have to make up their minds to one thing, namely, that they would have to spend a year upon the spot; for that it was unapproachable at anything near the time when the transit will take place. To show, however, that he did not consider this in any way fatal to the position as a station for observation, he said that he should much like to be one of the party himself. In this he was fully borne out by Captain Davis, who landed there with Sir James Ross. So that we may hope that this, at least, will be one station, and that the government will not postpone till too late the preparations to make it as favorable for the comfort of the spirited observers who will join in the expedition ns for the objects of the enterprise. It may possibly be advisable to send out an exploring party previously, though Captain Davis did not seem to think that it would be necessary. The first great difficulty in all places will be to get the absolute longitude. No ordinary nautical longitude will be of the slightest value. Observations necessary can be made at many places easily accessible, as far :a England is concerned, as at Alexandria, where the telegraph will be of great use ; at many places, too, in the United States, where we can safely leave the work to Americans. We may especially do the same in the case of the Russians, where the exact longitude of Orsk, the extremity of the great arc of longitude extending from that place to Valencia, is known to a millionth part of a second, or in other words, to absolute certainty. The other places which are reepmmended to the English government are Mauritius for one reason, and Madagascar for another. If, however, it should be thought unnecessary to fix both of these'spots, then an intermediate station viz., on the Island of Bourbon, would be preferable. If the Astronomer Royal can show that the two stations would be of considerable advantage, we hope that no financial reasons will prevent his wishes being carried out. Above all things we would urge upon the authorities the importance of making up their minds as to the instruments to be used, and in losing no time in hav-1 ing them put in hand. There is one more point worth notic- j ing. How far photography can be depended on as to accuracy in helping to discover the sun's distance is not easy to answer off-hand ; but certainly it is not to be doubted that much useful and interesting information may be secured by its means; and it is highly desirable that at none of the stations its use should be neglected. This part of the question is not, how- ever, of the same pressing importance as the fixing of the stations suitable for observing the ingress and egress of the planet, and of the preparation in good time of the instruments and apparatus required." Our readers will now be prepared to appreciate the importance of this subject, and to understand why its discussion is j likely to occupy, to a large extent, the attention of the scientific press for a considerable time to come.
This article was originally published with the title "The Transits of Venus in 1874 and 1882" in Scientific American 20, 18, 281-282 (May 1869)