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PTOLEMY versus COPERNICUS .

We do not forget that in the ancient system of the Egyp-tians, notwithstanding that the Sun and the other planetswere supposed to circle round the Earth , an exception wasmade in the case of Mercury and Venus , to which a specialrotation round the Sun as a centre was assigned. We havealso read that Aristarchus of Samos, and Cleanthes of Assos ,about 270 b.c., were (as it was then doubtless considered) so im-pious as to suggest that the Sun, rather than the Earth , was thecentre of the celestial movements. But these opinions gained

Fig. XXX.—Cycles and Epicycles of Mars (M), Jupiter (J) f and Saturn (S).

j--

m(

—• E

of tKj.

little or no credit, and were no doubt looked upon as wildvagaries. The system of cycles and epicycles was thereforeextended so as to embrace Mars, Jupiter, and Saturn . Itsapplication to those planets is illustrated in our next diagram,Fig . XXX.

In order to represent their apparent motions it was foundnecessary to suppose that the centres of the epicycles respec-tively described their cycles in rather less than 2 years forMars, in rather less than 12 for Jupiter , in rather less than