Page:In the high heavens.djvu/59

 represented on the screen on which the slide was projected.

The interesting circumstance was noted that there appeared to be occasional interruptions to the continuity of the circular arcs. This was due to the fact that clouds had interposed during the intervals represented by the interruptions. A practical application is thus suggested, which has been made to render useful service at Harvard College Observatory. Every night, and all night long, a plate is there exposed to this particular part of the sky, and the degree in which the Pole Star leaves a more or less complete trail affords an indication of the clearness or cloudiness of the sky throughout the course of the night. From the positions of the parts where the trail has been interrupted it is possible not only to learn the amount of cloudiness that has prevailed, but the particular hours during which it has lasted.

This interesting system of concentric polar circles affords us perhaps the most striking visual representation that could possibly be obtained of the existence of that point in the heavens which we know as the Pole. The picture thus exhibited was a graphic illustration of the Copernican doctrine that the diurnal stellar movement was indeed only apparent, being, of course, due to the rotation of the earth on its axis.

Suppose that a photograph, like that which I have been describing, were to be taken at intervals of a century, it would be found that the centre of the system of circles, that is to say, the veritable Pole itself, was gradually changing on the heavens. I do not by this mean that the stars themselves would be found to have shifted their