According to the other theory, it is the _sun_ which occupies the centre of the system, and it is the _earth_ which, accompanied by the remainder of the planets, revolves around _it_. This theory is likewise of considerable antiquity, though generally known as the _Copernican system_. But four-and-twenty centuries prior to the epoch of Nicholas Copernicus, it was taught by the "Samian sage," Pythagoras, and his disciples. The system then in acceptance, however, imposed upon them the necessity of silence. Ptolemæus was acquainted with it, but endeavoured to turn it into ridicule. "There are people," he says, "who pretend that heaven is immovable, and that it is earth which revolves on its own axis; evidently these individuals are unaware how supremely absurd is their opinion (πάνμ γελοιότατον)." And it was in the name of logic and mathematics that Ptolemæus thus treated the Pythagoreans! In the system of Copernicus,--the diurnal movement of the right sphere,--it is the earth's rotation upon its own axis which, being prolonged into the heavens, marks there, by its extremities, what are called the _Poles of the world_, just as the _Equator of the world_ is simply the prolongation of the terrestrial Equator. As for the Equator of the oblique sphere (the Ecliptic), in which the sun _apparently_ moves, it is, in reality, the identical plane in which the earth moves during its annual revolution round the sun. Now, in this movement of translation, the axis of the earth does not remain constantly parallel to itself; it deviates,--very slightly, it is true,--and so as to be scarcely perceptible to several generations of men. It is then quite natural that our successors should see, for a long time to come, the northern pole of the starry sphere near the extremity of the tail of Ursa Minor. But, two thousand years hence, this slow deviation will have become very perceptible; astronomers will then see the pole of the world in another constellation, and, as this displacement is continuous, the prolonged axis of the earth will have traced on the firmament, in 25,000 to 26,000 years, a circle parallel to the plane of the Ecliptic, and having for its centre the pole of that plane. This circle is the base of a cone whose summit rests upon the earth. (Fig. 27, _a_.) [Illustration: FIG. 27.]