Page:Popular Science Monthly Volume 6.djvu/235

Rh being constantly turning and changing the observers' positions whether they will or no—that such things as these, and many more, need not occupy us here, except as they suggest how excessively intricate the actual details are with which the astronomer deals.

Quite another method might be used by our imaginary observer, if we suppose him to incline his head so that one eye is higher than the other, and to be able to see over the passing car. In this case, if the lower eye had the view of the lower part of the window hidden, the other, seeing more over the car, would see somewhat farther down—how much farther down would be easily calculated if the proportionate distances of the car from the eyes and the window were known. This suggests a very important method for actual use in the transit; for, if we now have two stations, one in the north or upper side of the earth (upper to us, that is), the other in the south or lower side, it is clear that the upper observer, seeing more over Venus, so to speak, will see it as it crosses the sun at V2, nearer the centre than the observer who is in the south, and who sees it at V1. (Fig. 3).

If the northern station is 6,000 miles higher than the other, since Venus is two and a half times as far from the sun as from us, it will appear to cross nearer the centre by two and a half times 6,000, or 15,000 miles. Knowing how large an angle this 15,000 miles on the sun's face fills, we have, as it will readily be seen, the knowledge of how large an angle a line any given part of its length (such as the earth's radius) would fill as seen from this distance.



But it is immaterial whether we see such a length as the earth's radius from here, when it is supposed to be laid down on the sun, or from the sun when it is here. In either case we have got the same parallax and hence the same distance.

This apparent displacement of Venus will give us two chords of a