Page:Popular Astronomy - Airy - 1881.djvu/169

Rh apparent path of Venus over the sun's face, as seen at the point A, near the North Pole of the earth. Let a straight line be drawn from A through V till it meet the sun's face, and the end of that line will describe the path CD on the sun's face, (considering the sun's face as a flat disc, which will suffice for this purpose). Thus from the northern station we see Venus travelling along the line CD. In a similar way we find that, from the point B on the south side of the earth, Venus is seen to travel along the line EF.

Suppose the distance between the points A and B to be 7000 miles, and let us calculate the distance between the two lines CD, EF. The supposition is, that the distance of the earth from the sun is one hundred millions of miles, and the distance of Venus from the sun seventy-two millions of miles, and consequently the distance of Venus from the earth, twenty-eight millions of miles. It follows that the interval between the lines CD, EF (which must have the same proportion to AB that 72 bears to 28), is 18,000 miles.

We will now go on the supposition represented in Figure 44, that the distance of the earth from the sun is only fifty millions of miles, and therefore that the distance of Venus from the sun is thirty-six millions of miles, because, as I have said, the proportion of the distance of Venus from the sun to the distance of the earth from the sun is known beforehand, and the distance of Venus from the earth fourteen millions of miles; and let C'D' be the path of Venus as viewed from A', and E'F' the path of Venus as viewed from B'; and, still supposing the distance between A' and B' to be 7000 miles, let us calculate what is the interval between C'D' and E'F'. This interval is in the same proportion to A'B' as 36 is to