Page:Popular Astronomy - Airy - 1881.djvu/111

Rh assume the centre of the orbit of Venus to be the same point as the centre of the sun: we shall not have so much complexity. Suppose now we assume also that the centres of the orbits of the other planets are in the centre of the sun: we have seen that we can thus account for the motion of Venus, by giving proper dimensions to her orbit; and we can do the same thing for Mercury, and for Mars, and for everyone of the planets. Just observe the state of things we have got to, as in Figure 28; instead of having



different centres of motion for different planets, we have got them all in the centre of the sun; and the sun turns round the earth, carrying the orbits of the planets with it. That is a considerable simplification. In this state, I believe, the theory was received by the great Danish Astronomer, Tycho Brahé.

But now, instead of supposing the sun to be travelling, being itself the centre of the other orbits, and by some imaginary power causing the planets to revolve round itself as their travelling centre, suppose 4em