THE TRANSIT OF VENUS. SECTION I. FIRST of all, let me explain what the transit of Venus is. It is, in fact, the passing of the planet between the Earth and the Sun in such a way as that it will be actually seen from the Earth travelling across the Sun's face. Were Venus nearer the Earth, so as to appear as large as the Moon, the planet being in fact nearly three times as large in diameter as the Moon is, her transit across the Sun would cause an eclipse of the Sun. Being very much more distant from us, however, than the Moon is, and consequently appearing very much smaller, the eclipse' is reduced to a 'transit,' and, instead of covering the face of the Sun, the planet appears during the transit like a small round black spot. We all know that an eclipse of the Sun is caused by the Moon passing between the Sun and the Earth, they being all three in a line. In like manner, to cause a transit, the Sun, the planet, and the Earth must be all in a line, the planet, of course, being between the Sun and the Earth. A transit of Venus, as my readers have doubtless inferred from the fact of its causing so much stir, is a comparatively rare event. It is none the less certain and regular, however, in its periods. The transit occurs twice in about every 116 years, that is to say, it occurred last in the year 1769, having previously occurred in the year 1761. It occurs now in 1874, and it will occur again in 1882, and it will not occur again for another 108 years, and eight years afterwards it will appear again on the Sun's disc. This long interval arises from the fact, that the orbit of Venus round the Sun is inclined to the orbit of the Earth round the Sun, that is to say, they do not both make their circuit round the Sun in the same plane. Were they to do so, transits would be very frequent. A reference to diagram No. I will show what I mean by the orbit of Venus being inclined to that of the Earth. It will be seen that it is impossible for the Sun, Venus, and Earth to be in a line except when, assuming the plane of the Earth's orbit to be horizontal, Venus is ascending or descending in her orbit and so cutting through the plane of the Earth's orbit, and then, of course, only when the Earth happens to be in one of two particular positions when the plane is cut through by Venus. By calculation it is ascertained as a fact that the Earth would be in one of those positions on the 9th December, 1874, at a time when Venus would be cutting through the plane of the Earth's orbit; in other words, it is found by calculation that the Sun, Venus, and the Earth should all be in a line at a particular moment. Unfortunately for Europeans, the transit takes place before the sun rises in Europe. A 4 SECTION II. THE object astronomers have in view in observing the transit is that of ascertaining the distance of the Sun from the Earth. There is supposed to be an error of some three millions of miles in the present estimate of the distance, and the transit of Venus is looked forward to as a means of determining it with greater accuracy. There will probably always be some error in the calculation of the distance, and all that is hoped for is that it may be reduced to inappreciable dimensions. The Sun is an inaccessible object, and as such of course its distance cannot be measured in the same way as an accessible object. Let us see first of all, therefore, how the distance of inaccessible objects on the Earth are measured. This will assist us considerably in comprehending the measurement of the distance of the Sun by means of the transit of Venus. Suppose a man standing on the bank of a river wants to ascertain the distance of a particular object on the opposite bank. We will assume that he has no possible means of measuring it with a tape or chain. We will, however, suppose him in possession of an instrument which every surveyor is familiar with, viz., a theodolite.1 He first of all measures as accurately as he can, by means of a tape or something of that kind, and parallel to the river, a base line, the length of which he fixes according as the object is far off or near. If far off a large base line is chosen, if near he requires only a small one. At one end of the base line he then plants his theodolite, and looks through the telescope at some object at the end or exactly in a line with the base line, and to make the measurement as simple as he can, moves the graduated disc so that the point where it is marked 360 lies exactly on the base line; he then clamps or fixes the disc. He then turns the telescope to the object, and obtaining a view of it he refers to the disc to see how many degrees he has had to turn his telescope, and having ascertained them and noted them down, he then takes his theodolite to the other end of the base line and goes through a precisely similar process, only on this occasion he looks first towards the end of the base line where he began. In this case also he notes the number of 1 A theodolite is an instrument used to determine the size of angles. It consists, in fact, of a small telescope fixed on a pivot, round which it can move horizontally. The pivot is fixed in the centre of a circular disc, which disc is also placed horizontally. The rim of the disc is marked in degrees, that is to say, it is divided into 360 equal parts, and a point or needle corresponding with the telescope travels with the telescope over this rim like the hand of a watch. The whole apparatus when in use stands upon three moveable legs. |