Page:Popular Astronomy - Airy - 1881.djvu/184

170 the conjunction will take place, that is to say, the time when Venus will be seen on the sun's face. In eight years Venus goes round the sun thirteen times with very considerable accuracy, but still not with perfect accuracy. Suppose, then that a conjunction of Venus and the earth takes place, at a particular position of the two planets; eight years after that time there will be another conjunction, nearly but not precisely at the same place. In eight years after that there will be another conjunction at a point still more distant than the first, and thus the points of conjunction will recede gradually from V", and it will be a long time before a conjunction occurs again, either at V" or on the opposite side. Venus was seen on the sun's face in 1761 and 1769, at the position opposite to V": she will next be seen on the sun's face in 1874 and 1882, at the position V".

I then proceeded to point out the principle of the method in which these conjunctions are used for determining the linear distance of the sun from the earth. Figures 43 and 44 represent the state of things at a transit of Venus on two assumptions: Figure 43 on the assumption that the distance of the earth from the sun is one hundred millions of miles, and Figure 44 on the assumption that the distance of the earth from the sun is fifty millions of miles. Venus moves in her orbit faster than the earth, and, in consequence, as the earth moves towards a or a’, and Venus moves faster towards v or v’, she will be seen when between the sun and the earth to move across the sun in the direction CD or C'D'. I remarked that, before the observation is made, one linear measure only is known, namely, the size of the earth; but that we do not know the distance of Venus from the sun, or the distance of the earth from