The Evening Star WEDNESDAY, FEBRUARY 25, 1874

Evening Star, Issue 3436, 25 February 1874, Page 2

The Evening Star WEDNESDAY, FEBRUARY 25, 1874

We concluded our last Monday night’s article on the approaching Transit .of Venus by saying that on the 9 th of December next astronomers would be able to use the Sun’s disc as a dialplate on which to effect the measurements they require for the determination of the Sun’s distance. If two persons, the one a few yards behind the other, are walking in Princes street on the side opposite to the Provincial Buildings, and going from the Caversham end to the Water of Leith end of the town, that is from south to north, any. object on their right hand side, a cab for instance, that they might pass would not appear to both of them to be opposite exactly the same part, say of the Provincial Buildings, at the same time. If the person in front could see the axle of the cab-wheels in a line with the south part of the Post Office door-way, then a person behind might see the same axle in a line with the north side of the door-way. That is to say, then, that the person who was further north would see the axle projected south on the Provincial Buildings, and the one further south would see it projected north. Now, when Venus passes over the Sun’s face, exactly the same thing will take place ; to a person far north of the Earth’s equator the passage of the planet will appear to take place over a more southerly part of the Spn than it will to a person at the equator, and it will appear to a person far south of the equator, say at Dunedin, much further towards the north of the Sun than it will to a person in the Sandwich Islands. Moreover, the further the places are apart, the greater will be the apparent distance between the two positions of Venus on the Sun’s disc. Now, fortunately there is a method by which this apparent distance can be very accurately measured. Venus takes a certain time to pass over the Sun. If it passed over the centre of the Sun it would occupy a comparatively long time in performing this passage; and the line that it would traverse \yould be, roughly, a diameter of the Sun’s disc. If it passed through a point above or below the centre, moving in a direction parallel to the former one, it would take less time, because the line described would obviously be less than a diameter, and if it passed ovep a line parallel to the two former ones, but, still speaking roughly, near the top or the bottom of the Sun, the time occupied in the passage would be comparatively short. But as we showed above, Venus will actually appear to describe two different lines, one further so to speak, from the centre of the Sun than the other. Now if we can find what time it takes Venus to pass over each of these lines, we know the comparative length of these lines. We know also by observation the apparent diameter of the Sun, and by a simple mathematical process we can compute the ratio which the distance between the two lines over which Venus appears to pass at the two stations, has to the whole diameter. Thus we know what is the angular distauee between the two chords apparently described by Venus during its transit, as seen from •two different stations. We have now two similar triangles with Venus at the apex of each ; we know also the real length of the base-line on the Earth, and the angular value of the solar base line, the distance between the two chords on the Sun ; and lastly, we know the relative heights of the two triangles. With these elements it is quite easy to get the real distance between the two chords, from this to get the real diameter of the Sun, and we may now use this as a base line to enable us to arrive at last at the garth’s distance from the Sun. In the above explanation of “ Hal-

ley’s Method,” . we have made no attempt to be mathematically exact; in fact we have neglected altogether several very important particulars, for instance, the Earth’s motion, the sensible diameter of Venus, the fact that the planet does not describe quite a straight line in its passage across the sun : all that we proposed to ourselves was to give nonmathematical readers a general idea of the whole thing. And now we may add a few words as to the result which is likely to be attained. Those of our readers who are unacquainted with such matters will be surprised to learn what a very small quantity all this fuss is about. Everybody knows what degrees, minutes, and seconds are. “60 seconds make one minute, 60 minutes one degree ” —and so forth. Have we not all learnt our tables at school 1 Well, if we take a circle, say the surface of a crown piece, and divide its circumference into 1,296,000 parts, each of these parts will be a second of arc. So that- a second is not a large quantity as compared with a circumference. The quantity which astronomers want to find is technically termed the horizontal parallax ; if a line be drawn from the centre of the Earth to that of the Sun, and ario’ther line from a point on the Earth’s surface to the Sun’s centre (a line from the point on the Ear th’s surface to the Earth’s centrebeing perpendicular to the line from the Earth’s centre to the Sun), then the angle formed at the Sun’s centre is this horizontal parallax. Now it has already been found that the value of this quantity is about eight seconds and nine-tenths : what is to be determined is by how many hundreths of a second this is to be increased, how near, in short the quantity is to nine seconds of arc—“ls that all that is to be done ?” Yes, that is all; but on that depends the accuracy of every astronomical measurement except those which relate to the Moon, and so the quantity, though extremely minute, is of very great importance. But the approaching transit has its chief interest from the fact that it will probably enable astronomers to at last reach a thoroughly satisfactory solution of a problem which has exercised the minds of the noblest members of our race for more than two thousand years, and because its solution, from the enormous difficulties it has presented, will complete the greatest scientific victory which man has ever yet gained.