seems astonishing that it did not occur to people before. When we suppose the earth fixed as at E, eFigure 27, and take Venus (for instance) revolving
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Fig. 27.
round a centre, we may alter the place of that centre and its distance from the earth, as much as we please, and we shall then get the same appearances, provided we alter the dimensions of the orbit of Venus in the same proportion. As, for instance, in Figure 27, suppose E to be the earth, and suppose the small circle in which V is to be the orbit of Venus, the sun being at S; then, in revolving in her orbit, Venus appears to go to a certain extent to the right and to the left of the sun. But we might take any other point on the bar, even the point S itself, for the centre of the orbit of Venus, provided we give Venus a larger circle to revolve in. In the large orbit in which V is seen, Venus will appear (as viewed from the earth) to move to the right or left of the sun; and if we do but make the orbit large enough, it will, as viewed from E, appear to move just as much to the right or left of the sun as if it moved in the small orbit. We may then fix the centre of the orbit of Venus where we please. When we have got thus far, we may easily make another step. Suppose w
