27. _The Precession of the Equinoxes._--The equinoctial points have a slow westward motion along the ecliptic. This motion is at the rate of about 50'' a year, and would cause the equinoxes to make a complete circuit of the heavens in a period of about twenty-six thousand years. It is called the _precession of the equinoxes_. This westward motion of the equinoxes is due to the fact that the axis of the earth has a slow gyratory motion, like the handle of a spinning-top which has begun to wabble a little. This gyratory motion causes the axis of the heavens to describe a cone in about twenty-six thousand years, and the pole of the heavens to describe a circle about the pole of the ecliptic in the same time. The radius of this circle is 23-1/2°. [Illustration: Fig. 36.] 28. _Illustration of Precession._--The precession of the equinoxes may be illustrated by means of the apparatus shown in Fig. 36. The horizontal and stationary ring _EC_ represents the ecliptic; the oblique ring _E'Q_ represents the equator; _V_ and _A_ represent the equinoctial point, and _E_ and _C_ the solstitial points; _B_ represents the pole of the ecliptic, _P_ the pole of the equator, and _PO_ the celestial axis. The ring _E'Q_ is supported on a pivot at _O_; and the rod _BP_, which connects _B_ and _P_, is jointed at each end so as to admit of the movement of _P_ and _B_. On carrying _P_ around _B_, we shall see that _E'Q_ will always preserve the same obliquity to _EC_, and that the points _V_ and _A_ will move around the circle _EC_. The same will also be true of the points _E_ and _C_. 29. _Effects of Precession._--One effect of precession, as has already been stated, is the revolution of the pole of the heavens around the pole of the ecliptic in a period of about twenty-six thousand years. The circle described by the pole of the heavens, and the position of the pole at various dates, are shown in Fig. 37, where o indicates the position of the pole at the birth of Christ. The numbers round the circle to the left of o are dates A.D., and those to the right of o are dates B.C. It will be seen that the star at the end of the Little Bear's tail, which is now near the north pole, will be exactly at the pole about the year 2000. It will then recede farther and farther from the pole till the year 15000 A.D., when it will be about forty-seven degrees away from the pole. It will be noticed that one of the stars of the Dragon was the pole star about 2800 years B.C. There are reasons to suppose that this was about the time of the building of the Great Pyramid.