Page:In the high heavens.djvu/78

 which an eye placed at the centre of the earth would be able to see the Pole of the heavens, and if the successive positions of this Pole were marked by pegs driven into the ground, then the several positions in which the Pole would be found must necessarily trace out the circumference of the circle that has been thus described. The period in which each revolution of the Pole around the circle takes place is about 427 days; the result, therefore, of these investigations is to show that the North Pole of the earth is not, as has been so long supposed, a fixed point, but that it revolves around in the earth, accomplishing each revolution in about two months more than the period that the earth requires for the performance of each revolution around the sun.

The detection of the movement of the Pole which I have here described must be regarded as a noteworthy achievement in astronomy, nor is the result to which it leads solely of interest in consequence of the lesson it teaches with regard to the circumstances of the earth's rotation. It has a higher utility, which the practical astronomer will not be slow to appreciate, and of which he has, indeed, already experienced the benefit. There are several astronomical investigations in which the latitude of the observatory enters as a significant element. Latitude is, in fact, at every moment employed as an important factor in many astronomical determinations: to take one of the most simple cases, suppose that we are finding the place of a planet, we deduce its position by measuring its zenith distance, and then to obtain the declination the latitude of the observatory has, of course, to be considered. Now, astronomers have hitherto been in the habit of accepting the determination of their latitude