beautiful astronomical discovery of antiquity, is that of the
precession of the equinoxes. Hipparchus, to whom the honour of it is
due, gave a complete and precise statement of all the consequences which
flow from this movement. Two of these have more especially attracted
attention.

By reason of the precession of the equinoxes, it is not always the same
groups of stars, the same constellations, which are perceived in the
heavens at the same season of the year. In the lapse of ages the
constellations of winter will become those of summer and reciprocally.

By reason of the precession of the equinoxes, the pole does not always
occupy the same place in the starry vault. The moderately bright star
which is very justly named in the present day, the pole star, was far
removed from the pole in the time of Hipparchus; in the course of a few
centuries it will again appear removed from it. The designation of pole
star has been, and will be, applied to stars very distant from each
other.

When the inquirer in attempting to explain natural phenomena has the
misfortune to enter upon a wrong path, each precise observation throws
him into new complications. Seven spheres of crystal did not suffice for
representing the phenomena as soon as the illustrious astronomer of
Rhodes discovered precession. An eighth sphere was then wanted to
account for a movement in which all the stars participated at the same
time.

Copernicus having deprived the earth of its alleged immobility, gave a
very simple explanation of the most minute circumstances of precession.
He supposed that the axis of rotation does not remain exactly parallel
to itself; that in the course of each complete revolution of the earth
around the sun, the axis deviates from its position by a small quantity;
in a word, instead of supposing the circumpolar stars to advance in a
certain way towards the pole, he makes the pole advance towards the
stars. This hypothesis divested the mechanism of the universe of the
greatest complication which the love of theorizing had introduced into
it. A new Alphonse would have then wanted a pretext to address to his
astronomical synod the profound remark, so erroneously interpreted,
which history ascribes to the king of Castile.

If the conception of Copernicus improved by Kepler had, as we have just
seen, introduced a striking improvement into the mechanism of the
heavens, it still remained to discover the motive force which, by
altering th