Page:In the high heavens.djvu/176

 by measurements made with the meridian circle, and when, after the lapse of a number of years the place of the same star is again determined by observation, it not unfrequently happens that the two places disagree. The explanation is, of course, that the star has moved in the interval. Thus the constellations are becoming gradually transformed by the movements of the several stars which form them. It is true that the movements are so slow that even in thousands of years the changes do not amount to much when regarded as a disturbance of the configuration. Thus, to take an example, we know the movements of the stars forming the Great Bear sufficiently well to be able to sketch the position of the stars as they were ten thousands years ago, or as they will be in ten thousand years to come, and though, no doubt, some distortion from the present lineaments of the Great Bear is shown in each of these pictures, yet the identity of the group is in each case well preserved. In Fig. 27 we have shown the amount of distortion in this constellation which would be produced in the course of 36,000 years.

It is, however, obvious that if a star should happen to be darting directly towards the observer or directly from him, the telescopic method of determining its movement becomes wholly inapplicable. No change in its position could be noticed. It is, no doubt, conceivable that if the distance of a star from the earth were determined, and if the investigation were repeated after a sufficient lapse of time, then the differences between the two distances would give an indication of the star's movement along the line of sight during the interval. But we may say at once that such a method of research is wholly impracticable. Our knowledge of the star-distances is far too imperfect