Page:Popular Astronomy - Airy - 1881.djvu/172

158 be taken of this circumstance to increase the difference of times of passage of Venus over the sun's disc as seen at the two places, and thus to render the result more accurate. (For if, on the supposition that the sun's distance is one hundred millions of miles, we can by proper choice of stations increase the difference of times from 16 minutes to 24 minutes, and if the uncertainty in observation from chance errors is five seconds, then that uncertainty is $1⁄192$ of the whole in the former case, and $1⁄288$ of the whole in the latter case; and the proportionate uncertainty in the sun's distance will be the same.) The transit of Venus in 1769, occurred on June 3, a day very near to the summer solstice. The North Pole of the earth was turned partly towards the sun. Venus appeared to pass across the sun's disc from left to right, or in the retrograde direction. The earth was revolving from right to left, or in the way which we call direct. Now let Figure 45 represent the view which would be had by a person (for instance, an inhabitant of the planet Mars,

if it had happened to be in the proper position) looking over the earth, and seeing the sun beyond it. To avoid confusion, Venus is omitted from this figure. The South Pole, P', of the earth would be seen by him, but the North Pole, being nearer the sun, would be invisible. A station near the South Pole would, in the course of its diurnal revolution, describe a circle, of which the greater part (represented by the dark arc of a circle) would be towards his eye, and the smaller part would be on the