The Evening Star MONDAY, FEBRUARY 23, 1874

Evening Star, Issue 3434, 23 February 1874, Page 2

The Evening Star MONDAY, FEBRUARY 23, 1874

Very early to-morrow morning the planet Venus will be in superior conjunction with the Sun ; that is to say, that Venus, the Earth, and the Sun will be almost in a straight line, the Sun being in the middle. This is of course not a very uncommon state of things, as it takes place once in every period of nineteen months and a-half. This superior conjunction, however, has a sort of interest of its own, for Venus may now be considered as fairly on its way to the inferior conjunction of 18/4. In other words, when the Earth, Venus, and the Sun are again iu a line, Venus will be visible even to the naked eye (a piece of smoked glass being used as a protection) as a round black disc passing slowly over the face of the Sun. The phenomenon in question is of course the “Transit of Venus.” In many respects this may be considered as a more important astronomical event than any "that has happened during the present century. An attempt, therefore, the nature and causes of tbis'^phenomenon, and the results which will probably be obtained by the careful observation of it, can hardly fail to be interesting to the general reader. It would of course be futile to try to give more than a very general idea of the problem which astronomers have set themselves to solve, or of the methods by which they mean to attack it; but if such a general notion can be attained without any very severe mental effort, it is surely worth while to try to get it. The problem then is briefly this:—To find the distance of the centre of the Earth from the centre of the Sun. Now, if the Earth were pretty near the Sun, say at four times the Moon’s distance—or, roughly, 1,000,000 miles—the problem would be, with modem appliances, a comparatively easy one. It is not at all difficult to find the distance of a tree or other object on the opposite side of a river, by first pacing off a base line of, say, twenty yards ; then taking a large pair of compasses and making one of the legs lie along the base line, and the other point to the distant object; next, having performed the same operation at the other end of the base line, laying down on a sheet ol paper a figure corresponding to the observations made ; then, ascertaining bv measuring with a foot-rule the ratio existing between the base line on the paper and either of the lines running from its extremities; and, lastly, finding by a simple rule-of-three sum the ratio which one of the lines across the river has to the corresponding line on the paper. Just in the same way, by observing at two places lying on the same meridian, and at the same time, the direction of the Sun’s centre, the distance between the two places being known, it would be comparatively easy to lay down on paper, and still easier to calculate by trigonometrical methods, the length of the line joining the centre of the Suntothecentre of the Earth. By using this method the ancients were able to find, with surprising accuracy, the distance of the Moon from the Earth ; indeed they succeeded in computing this distance correctly, to within one-fiftieth part of its value. But this plan is almost useless for determining the distance of the Sun; because, instead of being distant from the Earth 1,000,000 miles, it is at least 90 times further off than this, and, consequently, instead of having in the triangle two sides which bear a reasonable sort of relation to the base line, we have a triangle wiiose sides are each 15,000 times as long as the base line. Now, if our readers will construct or imagine a triangle whose base is lin. long, and whose sides are each somewhat less than a quarter of a mile long, they will be able to see how very difficult it must be to deal with such a figure, when it is impossible to measure more than the base line and the two angles at the base, the two long sides having to be f computed. It will be at once seen bow every error in measuring the base line and in determining the angles made by it with the lines running from its extremities will produce errors enormously greater than those which would result from similar errors if the object to be measured were near at hand. Astronomers, however, would be able to get over even these difficulties by means of the appliances which they now have at their disposal, if they could get ‘perfectly satisfactory views of the Sun. But this is not the case. There is always more or less rippling or unsteadiness about the limb (or border) of the Sun, so that it is impossible to measure directly the angles required, with sufficient accuracy. This method, then, though it might give approximate results, would not satisfy the exactions of modem science. Venus and the Earth both revolve round the Sun—.Venus in about seven '

months and a-half and the Earth in about twelve months, the orbit or course of the Earth always lying outside that of Venus. Venus, therefore, passes between the Sun and the Earth, and sweeps round the Sun, gradually getting, further ahead of the Earth till it reaches the point on the other side of the Sun, when it begins, so to speak, to chase the Earth. After a time it overtakes it, and again passes between the Sun and the Earth. Now, as Venus is an opaque body, we might think that every time it passes between the Sun and the Earth, its black disc ought to be seen projected on the face of the Sun, and it would be so seen if the three bodies were in the same plane, like three marbles lying in a line on a level table; but the Earth and Venus revolve in different planes. The oval which Venus describes in space lies in such a way, that she is sometimes above and sometimes below the oval described by the Earth, and it is only when the Earth and Venus are together in, or very near to that part of the orbit where the plane of the Earth's orbit cuts the plane of the orbit of Venus, that a transit can take place. This happens very seldom, as may be learnt from the fact that the last two transits took place in 1761 and 1769 respectively. Now, Venus at its inferior conjunction with the Sun is distant from the Earth only somewhere about one-fourth of the distance of the Sun, and one might say prpnia facie that it would be much easier to measure the smaller distance than the larger one; this is really the case, and, if we can succeed in measuring the distance of Venus frbih the Earth, we virtually know the Sun’s distance too, for astronomers have been able to determine with very great accuracy the relative, though not the absolute distance of Venus from the Sun, and of the Earth from the Sun. Now, the distance of Venus, and consequently that of the Sun, can and will be very accurately obtained from the observations that will be made on the planet when it is passing over the face of the Sun on the 9th of December next, for astronomers will be able to use the disc of the Sun as a dial-plate on which to effect the required measurements. How this is to be done will be explained in a future article.