392 HISTORY OF FORMAL ASTRONOMY. 



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 direction when the cup is carried 

 round a room. And this was noticed by Coper- 

 nicus'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 errour of Copernicus, 

 if it be looked upon as an errour, 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, 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 parallelism. A similar confusion 

 of geometrical conception, produced by a double 

 reference to absolute space and to the center 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 no. 



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 



3 Tycho. Epist. i. p. 184, A. D. 1590. 



