Page:Popular Astronomy - Airy - 1881.djvu/130

116 is what we mean by parallax, that it is the difference of direction of an object as seen in two different places. The two different places in the experiment which I have illustrated, are the two eyes in the head. This is the way in which, as will he seen, the distance of the moon from the earth is to be found. The two eyes in the head will be two observatories; and will be supposed to be placed at a considerable distance from each other on the earth. Without any exception at all, the principle is precisely the same. You will thus see how, by the observation of the difference of directions, the distance of the moon from the earth may be obtained. I shall give a more detailed explanation of this in the course of my next lecture.