Page:Popular Astronomy - Airy - 1881.djvu/166

152 of seeing Venus on the sun's face. The most celebrated transits of Venus, as I have said, occurred in 1761 and 1769; and the next will occur in the years 1874 and 1882.

I will now proceed to show you the method by which these transits are made available for measuring the distance of the sun from the earth. I must first point out to you what we know and what we do not know. From observing the distance which Venus goes to the right and left of the sun, we do know the proportion of the distance of Venus from the sun to the distance of the earth from the sun. This must be remembered carefully. We do not know the absolute distance of the earth from the sun, nor the absolute distance of Venus from the sun, but we know their proportion. Now, with respect to the diameter of the sun: suppose that I have an instrument like a pair of compasses, and that I use this so as to be able to observe the apparent angular breadth of the sun. I apply the joint of the compasses to my eye; I direct one of the legs to the lower part of the sun, and the other to the upper part of the sun; I then know the apparent angular breadth of the sun; and from this I determine the proportion of the absolute diameter of the sun to the distance of the sun, and whatever distance I assume for the sun I must take the diameter proportionately to that distance. If the distance of the sun is one hundred millions of miles, the breadth of the sun (roughly speaking) must be one million; if the distance of the sun is fifty millions of miles, the diameter of the sun is half-a-million of miles. Let Figure 43 be a perspective view of the state of things in which the sun's distance is supposed to be one hundred millions of miles; and the breadth of the sun is one million of