Page:Popular Astronomy - Airy - 1881.djvu/90

76 that intersection is, as if it were marked by a conspicuous point in the heavens.

Now, the statement that the sun appears to move in a great circle on the globe amounts to this: that the sun appears to move in a plane which passes through the earth. For all globes represent the stars as they would appear if the observer were at the centre of the globe; and a great circle is one whose plane passes through the centre of the globe. If we found that the sun was describing one of the smaller circles on the globe, then we should say the sun was not moving in a plane round the earth. But, as it appears to describe a great circle, then we can assert that the sun appears to be moving in a plane round the earth. Now we come to the question. Cannot we explain this differently in another way? Is it certain that the sun is moving in a plane round the earth, or is it certain that the earth is moving in a plane round the sun? Either supposition will do. At the present moment we have no evidence to guide us; we have nothing to tell us whether the sun is moving round the earth in a plane, or whether the earth is moving round the sun in a plane. But we shall shortly have evidence on the point.

In the meantime I will mention this: that there is no inconsistency in supposing that the earth does move round the sun. As regards the position of the earth's axis there are two suppositions: either that the earth remains with the place of its centre unmoved and with its axis in a certain position, and that the sun goes round at a certain inclination to that axis, thereby causing the change of seasons; or else, if the sun is fixed and the earth goes round the sun, the position of the earth's centre is changed with regard to the sun, but the position of the axis must