Page:In the high heavens.djvu/68

 the obvious consequence it implies with regard to the earth's movement. At this point we find the convenience of the time-honoured illustration in our geography books which likens the earth to an orange. Let us thrust a knitting-needle through the orange along its shortest diameter to represent the axis about which the earth rotates. Not only does the earth perform one revolution about this axis in the space of each sidereal day, but the axis itself has a movement. If the earth's axis always remained fixed, or never had any motion except in a direction parallel to itself, then the point on the sky to which it was directed would never change. We have, however, seen that the Pole in the sky is incessantly altering its position; we are therefore taught that the direction of the earth's axis of rotation is constantly changing. To simulate the movement by the orange and knitting-needle we must imagine the orange to rotate around its axis once in that period of twenty-three hours and fifty-six minutes which is well known as the length of the sidereal day; while at the same time the knitting-needle itself, bearing, of course, the orange with it, performs a conical movement with such extreme slowness that not less than 25,000 years is occupied in making the circuit. The movement, as has often been pointed out, is like that of a peg-top which rotates rapidly on its axis while at the same time the axis itself has a slow revolving motion. Thus the phenomena which are presented in the rotation of the earth demonstrate that the axis about which the earth rotates occupies what is, at all events, approximately a fixed position in the earth, though not a 6xed position in space. We can hardly be surprised at this result; it merely implies that the earth acts like a