Page:Popular Astronomy - Airy - 1881.djvu/139

Rh own orbits round their respective centres; the planets Mars, Jupiter, and Saturn, being supposed to have a retrograde motion with respect to the bar. I also said that it was found necessary, in order to account for the motion a little more accurately, to suppose that the planets did not revolve strictly in circles, but that the radial bar as nMa, Figure 26, carried another bar as MaN, jointed on it, and moving on the joint, and that this second bar carried the planet. Supposing a similar construction of each of the planets, we get a terrible complexity of motions, and all independent of the sun.

I then remarked, it was a strange thing that persons did not think of connecting these notions more closely with the sun. It would have answered their purpose quite as well to take one centre as another centre for the orbits of the planets, provided it were in the same direction, and provided the proper dimension were given to the orbit: that for instance, in Figure 27, the apparent motion of Venus, as seen from the earth E, would be the same, whether it moved in the small orbit V, whose centre is v, or in the large orbit V', whose centre is S. Having, then, the power of choosing the centre of the orbit as we please, we might as well take the sun for the place of the centre; and it is wonderful to me that such a simplification was not sooner adopted by the ancient astronomers.

The system Copernicus fixed upon in his successive steps was, first to bring all the centres of the orbits to the sun—still retaining the notion that this sun, together with the various orbits connected with it, were carried round the fixed earth—and then to suppose that the earth was in motion round the sun, (which would explain the appearances just as well,)