Page:Popular Astronomy - Airy - 1881.djvu/106

92 sun, and after that she will be seen as an evening star; she will then be going away from the sun; she will go away for a certain distance, but more and more slowly, and the sun will be approaching towards her; she will become stationary; then she will turn backwards and seem to meet the sun.

Can we make a reasonable theory to account for this? We can do it more easily if we refer the apparent motion of Yenus to the sun and not to the stars. Venus sometimes passes the sun in going from left to right (relatively to the sun), and sometimes going from right to left (relatively to the sun); and her extreme angular distance from the sun towards the right is almost exactly the same as her extreme angular distance towards the left. The Greek astronomers began with a good assumption; they laid down at once the notion which they conceived must be the most natural and most proper, which was this: that every planet revolved in a circle. They then supposed that the earth is fixed, and that the sun moves. They supposed that a bar, or something equivalent, is connected at one end with the earth, and that on some part it carries the sun; and as they saw that the planet Venus was apparently sometimes on one side of the sun, and sometimes on the other side, they said that the planet Venus moves in a circle, whose centre is on the same bar. Whether they have expressed themselves distinctly concerning this bar I cannot say, but all their notions of the position of the centre of the orbit of Venus come to the same thing. Then, suppose that Venus is revolving round the centre at the same time that the bar is moving, we then get a perfect representation of the apparent motion of Venus and the sun, as seen from the earth. These suppositions will be represented on Figure 26 by