Page:Popular Astronomy - Airy - 1881.djvu/197

Rh perpendicular to the plane of the ecliptic, the inclination of the earth's axis to CQ will be the same as the inclination of the earth's equator to the ecliptic; and that, as we have already seen, undergoes no alteration. Consequently the inclination of CP or Cp to CQ is always the same. Therefore, we may represent the motion of the earth's axis by saying that it turns slowly round an axis perpendicular to the ecliptic, but keeping the same general inclination to it, in the direction in which the hands of a watch turn, (as viewed from the outside of a celestial globe,) or in what astronomers call a retrograde direction. Now, the Pole of the heavens is the point in the heavens to which the earth's axis is directed, and therefore that Pole is not absolutely invariable, but turns slowly in a circle in a retrograde direction (or in the same direction as the hands of a watch, as viewed from the outside of a celestial globe) round another point to which the line CQ is directed. The latter point is called the Pole of the ecliptic.

This motion of the earth's axis admits of a most remarkable illustration in the motion of a spinning top; and the more remarkable because the forces which act in that case are of an opposite character to the forces which act on the earth, and the effect which they produce is of an opposite character to the effect produced on the earth. The earth's axis being inclined to the line CQ, in Figures 46, 47, and 48, we have seen that the immediate tendency of the sun's force upon the earth is in all cases to bring the earth's axis CP nearer to CQ, and that if the earth had no motion of rotation, this force would bring the earth's axis CP nearer to CQ; but that in consequence of the earth having a motion of rotation, the effect really produced is, that the earth's axis CP revolves slowly round CQ