Page:Astronomy for Everybody.djvu/345

The principles on which distances in the heavens are determined was set forth in our chapter explaining how the heavens are measured. For distances of the moon and nearer planets, we use, as a base line for measurement, the radius of the earth, or the line joining two points of observation on its surface. But this is entirely too short to serve for measuring a distance so great as that even of the nearest star. For this purpose we take as a base line the whole diameter of the earth's orbit. As the earth moves from one side of the orbit to the other, the stars must seem to have a slight motion in the opposite direction. But this motion is found to be almost immeasurably small. It can be made out with sufficient precision only by comparing the stars among themselves in the following way:

Let the little circle on the left of the following figure represent the orbit of the earth. Let S be the star, supposed to be near us, of which we wish to measure the distance. Let the dotted lines almost parallel to each other show the direction of a star T many times farther away. When the earth is at one side of its orbit, say at P, we measure the small angle SPT, which seems to us to separate these two stars. When the earth goes to the opposite side, it is readily seen that the corresponding angle SQT will be greater. We again measure it. The difference