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does not arise from anything going on in the heavens, but only from a slow change in the direction of the earth's axis from year to year as it moves around the sun.

If we should suppose the platform in Figure 6 to last for six or seven thousand years, and the earth to make its six or seven thousand revolutions around it, we should find that, at the end of this time, the north end of the axis of the earth, instead of being tipped toward our right hand, as shown in the figure, would be tipped directly toward us. At the end of another six or seven thousand years it would be tipped toward our left; at the end of a third such period it would be tipped away from us, and at the end of a fourth, or about twenty-six thousand years in all, it would have gotten back to its original direction. Since the celestial poles are determined by the direction of the earth's axis, this change in the direction of the axis makes them slowly go around a circle in the heavens, having a radius of about twenty-three and a half degrees. At the present time the pole star is a little more than a degree from the pole. But the pole is gradually approaching it and will pass by it in about two hundred years. In twelve thousand years from now the pole will be in the constellation Lyra, about five degrees from the bright star Vega of that constellation. In the time of the ancient Greeks their navigators did not recognize any pole star at all, because what is now such was then ten or twelve degrees from the pole, the latter having been between it and the constellation of the Great Bear. It was the latter which they steered by, and which they called the Cynosure.