You will say that the earth must certainly get into the right position for the eclipse next June (1927); so it cannot be free to go anywhere it pleases. I can put that right. I hold to it that the earth goes anywhere it pleases. The next thing is that we must find out where it has been pleased to go. The important question for us is not where the earth has got to in the inscrutable absolute behind the phenomena, but where we shall locate it in our conventional background of space and time. Fig. 6 We must take measurements of its position, for example, measurements of its distance from the sun. In Fig. 6, shows the ridge in the world which we recognise as the sun; I have drawn the earth’s ridge in duplicate because I imagine it as still undecided which track it will take. If it takes we lay our measuring rods end to end down the ridges and across the valley from to , count up the number, and report the result as the earth’s distance from the sun. The measuring rods, you will remember, adjust their lengths proportionately to the radius of curvature of the world. The curvature along this contour is rather large and the radius of [Pg 150] curvature small. The rods therefore are small, and there will be more of them in than the picture would lead you to expect. If the earth chooses to go to the curvature is less sharp; the greater radius of curvature implies greater length of the rods. The number needed to stretch from to will not be so great as the diagram at first suggests; it will not be increased in anything like the proportion of to in the figure. We should not be surprised if the number turned out to be the same in both cases. If so, the surveyor will report the same distance of the earth from the sun whether the track is or . And the Superintendent of the Nautical Almanac who published this same distance some years in advance will claim that he correctly predicted where the earth would go. And so you see that the earth can play truant to any extent but our measurements will still report it in the place assigned to it by the Nautical Almanac. The predictions of that authority pay no attention to the vagaries of the god-like earth; they are based on what will happen when we come to measure up the path that it has chosen. We shall measure it with rods that adjust themselves to the curvature of the world. The mathematical expression of this fact is the law of gravitation used in the predictions.