Page:The Solar System - Six Lectures - Lowell.djvu/107



rotation and pull entirely alters the problem. From being a statical, it becomes a kinematical one, and the outcome is utterly different from what we might expect. Instead of bringing the plane of the equator into the plane of the ecliptic, it swings the pole of the equator round the pole of the ecliptic in a direction at right angles to the pull, and opposite to the rotation, but without changing the inclination of the two planes permanently at all. If the axis be in such position that the pull is perpendicular to the rotation, no change of inclination, even temporarily, occurs. If the axis be so circumstanced that the pull is at any other angle to it, then the change of axis being always perpendicular to the pull, one component of the change rotates the axis as before, the other alters its inclination.

Now if, as is the case with the attracting bodies of the solar system, the body which exerts the pull revolve about the other, either really or virtually, the axis will be presented to the force under varying angles. The axis will then alternately approach and recede from the pole of its small circle while going round it. But at the end of its orbital revolution it will come out again at the point on the celestial sphere from which it started. And this will happen whether the orbit be a