Page:Popular Astronomy - Airy - 1881.djvu/137

Rh them. I remarked that for the inferior planets, this was made easier by referring their apparent motion to the sun. I pointed out that Venus moved (apparently) sometimes to the left of the sun, and sometimes to the right of the sun; never exceeding a certain angular limit to the right or left of the sun. I told you that her motion was sometimes backward and sometimes forward; and when referred to the sun, was pretty nearly similar, first on one side of the sun and then on the other side of the sun. All these conclusions are obtained by determining the places of Venus and the sun, with respect to the stars, by means of the transit instrument and mural circle, in the way described in my first lecture, and then laying them down on a globe, or treating them by other methods of calculation. There is no difficulty at all in figuring to ourselves, as the ancients supposed, that this apparent motion of Venus may be generally represented by supposing that she describes a circle round some imaginary centre, which centre is always in the line joining the earth and sun; and that the earth is at no very great distance outside that circle: as, for instance, in Figure 26, if we suppose Venus to revolve in tha circle V, whose centre v is in the line ES. They had similar notions in regard to Mercury, the centre of whose orbit might be supposed to be in some other place, as m.

With regard to Mars, Jupiter, and Saturn it was supposed by the ancients that they also moved in circles, and that the centres were somewhere in the same line ESmn, but that the circles were so large that they completely surrounded the earth. If we consider the apparent motion of Jupiter at the present time, when he is seen nearly opposite to the sun,