Page:Astronomy for Everybody.djvu/346

between these two angles will furnish a basis for computing, by trigonometric methods, the distance of the nearest star when that of the farthest is known. Practically we have to assume that the star T is at an infinite distance, so that the dotted lines are parallel. Then the measured difference between the angles will enable us to calculate the angle subtended by the radius of the earth's orbit, as seen from the star S. This angle is what astrono-

mers habitually use in their computations, not the distance of the star. It is called the Parallax of the star. If we wish to obtain the distance of the star, we have to divide the number 206,265 by the parallax of the star expressed as a fraction of a second. This will give its distance in terms of the radius of the earth's orbit as a unit of measure. One second is the angle subtended by an object one inch in diameter at a distance of 206,265 inches, or more than three miles. It is, of course, completely invisible to the naked eye.

It will be seen that this method of measurement implies that we know which of the two stars is the nearer; in fact, that we know the farther star to be at an almost infinite distance. The question may be asked how this knowledge is obtained, and how a star is selected as being near to us. The most careful measures that can be made with the finest instruments show that the great mass of small