6. Besides its annual motion around the sun, which produces the seasons; its rotary movement upon itself in twenty-four hours, which produces day and night, - the Earth has a third movement, which is completed in about twenty-five thousand years (or, more exactly, 25,868 years), which produces the phenomenon designated in astronomy “the precession of the equinoxes” (Chap. V, n° 11).
This movement, which it would be impossible to explain in a few words without figures and without a geometrical demonstration, is described by a waved curve very nearly circular, which has been compared to the movement of a dying spinning-top, in consequence of which the axis of the Earth, changing in inclination, described a double cone, of which the summit is at the center of the Earth, and the bases embrace the surface circumscribed by the polar circles; that is to say, an amplitude of twenty-three and a half degrees of radius.
