We have already said that the signs of the zodiac are not now the same as the zodiacal constellations, and this is now easily understood. It is not worth while to say that the sun enters such and such a part of the Fishes at the equinox, and changes every year. So the part of the heavens it _does_ then enter--be it Fishes, or Aquarius, or the Ram--is called by the same name--and is called a _sign_; the name chosen is the Ram or Aries, which coincided with the constellation of that name when the matter was arranged. There is another equally important and instructive result of this precession of the equinox. For the earth's axis is always perpendicular to the plane of the equator, and if the latter moves, the former must too, and change its position with respect to the axis of the ecliptic, which remains immovable. And the ends of these axes, or the points they occupy among the stars, called their poles, will change in the same way; the pole of the equator, round which the heavens appear to move, describing a curve about the pole of the ecliptic; and since the ecliptic and equator are always _nearly_ at the same angle, this curve will be very nearly a circle, as represented on preceding page. [Illustration: FIG. 12.] Now the pole of the equator is a very marked point in the heavens, because the star nearest to it appears to have no motion. If then we draw such a figure as above, so as to see where this pole would be at any given date, and then read in any old record that such and such a star had no motion, we know at once at what date such a statement must have been made. This means of estimating dates is less certain than the other, because any star that is nearer to the pole than any other will appear to have no motion _relatively_ to the rest, unless accurate measurements were made. Nevertheless, when we have any reason to believe that observations were carefully made, and there is any evidence that some particular star was considered the Pole Star, we have some confidence in concluding the date, examples of which will appear in the sequel; and we may give one illustration now, though not a very satisfactory one. Hipparchus cites a passage from the sphere of Eudoxus, in which he says, _Est vero stella quaedam in eodem consistens loco, quae quidem polus est mundi._ (There is a certain immovable star, which is the pole of the world.)