Page:Popular Astronomy - Airy - 1881.djvu/107

Rh supposing E to be the fixed earth; E v S m n a bar turning in a circle, having one end fixed as at E ; S

the sun carried by it; v , the centre of the orbit in which Venus revolves; V being the planet Venus, connected with v by a bar (real or imaginary), and thus describing a circle round v, while v itself is carried on the bar round the earth.

They supposed that Mercury (see Me in Figure 26) revolves in another circle, and that its centre is on the same bar, but perhaps beyond the sun, as at m. They did not, however, pretend to judge exactly where these centres are; all that they were certain of was this: that the centre of the motion of each planet is on the same bar that supports the sun. Now, you may easily see that, on these suppositions, the planets being viewed from the earth, Venus is at one time to the right, and at another time to the left of the sun, and the sun is carried round the earth in one year. The same is the case with regard to Mercury.

With regard to Mars, they found out that its motion can be represented extremely well, by supposing that this same bar carries another centre as n, around