The advance of photography : its history and modern applications (1911)

ASTRONOMICAL PHOTOGRAPHY 313 added the distance of the earth from Venus, we obtain the distance of the earth from the sun, which was required. Thus the determination of our distance from the sun depends on that of our distance from Venus, which must be taken at the moment when Venus is between the earth and the sun. But Venus is only visible at the moment when it is between the sun's disc and the earth. This is only exceptionally the case — twice in every century — and then it appears as a small black point on the sun's disc, which, however, continually changes place, on account of the earth's movement and its own. This circumstance renders simultaneous measurements at two different and remote points of the earth very difficult, and therefore the idea has been entertained of using photography as an auxiliary. If by its help, and in the manner described above, a sun picture is taken during the transit of Venus, the distance of Venus from the sun's centre can be easily measured upon it. The centre of the sun is a fixed point which can be assumed to be stationary. If the earth is supposed to be at E (fig. 131), Venus at V, and the sun at S, the observer at a will see Venus below the centre of the rig. 101. sun, while an observer at b will see Venus above it. Accordingly, Venus will present a different position to the sun's centre on photographs taken at various points of the earth. Now, the position of the centre of the sun is very accurately known. The sun's diameter subtends an angle of about 30 minutes, so that if divided into 30 parts, each part would represent an angle of one minute. It is only necessary therefore to measure how many of such parts Venus is distant from the sun's centre, to find at