Page:Popular Astronomy - Airy - 1881.djvu/185

Rh the sun, but we know that proportion to be as 72 to 100 nearly; and, similarly that we do not know the absolute breadth of the sun, but we know that, whatever the distance of the sun may be, the breadth of the sun bears a certain proportion to that distance, namely, that it is nearly the hundredth part of the distance.

Then I pointed out that if the transit of Venus be observed at two points of the earth, A and B, or A' and B', which are 7000 miles apart, Venus will appear to describe on the sun's face the line CD or C'D' as seen from A or A', and EF or E'F as seen from B or B', and that the distance between the lines EF and CD, or between E'F and C'D', will be the same quantity in linear measure, namely, 18,000 miles, whatever be the supposition as to the distance of the sun; and therefore, as the general position of the lines on the sun's face must be the same on any supposition, and therefore they will cut the edge of the sun's disc at nearly the same angle, the difference of the lengths of the two paths CD and EF, or C'D' and E'F must be the same number of miles; but, as the linear breadth of the sun is not the same on the two suppositions, and therefore the linear length of the lines CD or EF is about double the length of C'D' or E'F, it follows that the difference of their lengths bears a smaller proportion to their whole length in Figure 43 than in Figure 44; and therefore, the difference of the times occupied by the apparent passage of Venus, as seen from A and from B, bears a smaller proportion to the whole time on the assumption of Figure 43 than on that of Figure 44. It is plain that here we have something which will guide us immediately to a decision on the distance of the earth from the sun, if we can but make observations at two stations, as A and B. For, observing with telescopes and clocks the