264 HISTORY OF FORMAL ASTRONOMY. 



great and small Epicycle ; or else by means of an Eccentric and Epicy 

 cle, modified from Ptolemy's, for reasons which we shall shortly men- 

 tion. This mode of representing the motions of the planets continued 

 in use, until it was expelled by the discoveries of Kepler. 



Besides the daily rotation of the earth on its axis, and its annual cir- 

 cuit about the sun, Copernicus attributed to the axis a " motion of dec- 

 lination," by which, during the whole annual revolution, the pole was 

 constantly directed towards the same jart of the heavens. This con- 

 stancy in the absolute direction of the axis, or its moving parallel to 

 itself, may be more correctly viewed as not indicating any separate 

 motion. The axis continues in the same direction, because there is 

 nothing to make it change its direction ; just as a straw, lying on the 

 surface of a cup of water, continues to point nearly in the same direc- 

 tion when the cup is carried round a room. And this was noticed by 

 Copernicus's adherent, Rothman, 3 a few years after the publication of 

 the work De Revolutionibus. "There is no occasion," he says, in a 

 letter to Tycho Brahe, " for the triple motion of the earth : the annual 

 and diurnal motions suffice." This error of Copernicus, if it be 

 looked upon as an error, arose from his referring the position of the 

 axis to a limited space, which he conceived to be carried round the 

 sun along with the earth, instead of referring it to fixed or absolute 

 space. When, in a Planetarium (a machine in which the motions of 

 the planets are imitated), the earth is carried round the sun by being 

 fastened to a material radius, it is requisite to give a motion to the 

 axis by additional machinery, in order to enable it to preserve its par- 

 allelism. A similar confusion of geometrical conception, produced by 

 a double reference to absolute space and to the centre of revolution, 

 often leads persons to dispute whether the moon, which revolves about 

 the earth, always turning to it the same face, revolves about her axis 

 or not. 



It is also to be noticed that the precession of the equinoxes made it 

 necessary to suppose the axis of the earth to be not exactly parallel to 

 itself, but to deviate from that position by a slight annual difference. 

 Copernicus erroneously supposes the precession to be unequable ; and 

 his method of explaining this change, which is simpler than that of the 

 ancients, becomes more simple still, when applied to the true state of 

 Ihe facts. 



The tendencies of our speculative nature, which carry us onwards in 



3 Tycho. Epist. i. p. 1S4, A. D. 1590. 



