Page:Popular Astronomy - Airy - 1881.djvu/165

Rh Venus. The orbit of each planet is in a plane passing through the sun, but the orbits of all the planets are not in the same plane; the orbit of Venus is not in the same plane as the orbit of the earth. In Figure 42 let S be the sun, E, the earth in its orbit,



and V, Venus in her orbit. It is plain that Venus in her motion crosses the plane of the earth's orbit in two positions, and no more. Suppose, now, that E and V represent the relative positions of the earth and Venus, at the time when they are, generally speaking, in the same direction from the sun. If a person on the earth looks at Venus now, he will see her above the sun. If it happen that the two are, generally speaking, in the same direction from the sun at E' and V', then when the conjunction takes place she will be seen below the sun. But there is one state of things to be considered, viz.: when the conjunction occurs at V" and E", Venus being very near that part of her orbit where it crosses the plane of the earth's orbit. At this time, if an observer on the earth were to look at Venus, he would see her upon the sun's face. The same would happen if the conjunction took place on the side of the orbit opposite to V" and E". It is a matter of great importance to ascertain at what times this will occur; at what time the conjunction takes place when Venus is near that point (called the node) of her orbit. Now, whenever the conjunction occurs at V", or on the side exactly opposite, then there is an opportunity