Page:Popular Astronomy - Airy - 1881.djvu/196

182 centre, and in pushing the distant parts of the earth from the earth's centre, is about treble the effect of the sun. And as the precession depends entirely on this differential effect, the precession produced by the moon is about treble that produced by the sun.

It will be well for you here to consider the consequences of this precession on the position of the earth's axis. In Figure 48, after a certain number



of days or years, the position of the earth's equator has changed from e f h to g k l its inclination to the circle a b in the plane of the ecliptic remaining the same as before. The earth's axis of revolution must be always perpendicular to the plane of the earth's equator. From this it will be seen that the earth's axis has changed its position from such a direction as CP to such a direction as Cp. If we draw a line CQ