COPPERNICUS 167 



himself free, at last, from the bondage of appearances. He 

 offers a simple demonstration of the infinite range of the 

 heavens, which has already been sketched on a former page. 1 

 He very briefly outlines the ideas of the ancients, then sets 

 himself to disprove them. You turn the leaf, and with no flare 

 of trumpets, hardly an air of novelty, you find a chapter inquiring 

 whether or no the earth has not one motion, but several. 



You reflect that this is a work which turns the world upside 

 down. You have somehow the idea that the proof should be as 

 long and detailed as Darwin's demonstration of natural selec- 

 tion ; instead, the subject is swiftly compassed in a few pages. 

 The next chapter sets out the order of the planets. Then comes 

 the final section of the first book, wherein Coppernicus sets 

 forth what he calls the third of the earth's motions. This last 

 alone would have entitled him to a high rank among astrono- 

 mical discoverers, yet he gives it simply as a part of his system, 

 without any ado, making no note that it is wholly and en- 

 tirely his own. You perceive how carefully and patiently he 

 has thought his subject out and woven it into a firm and 

 connected piece. 



In the expression of his ideas regarding this third motion, 

 Coppernicus was unfortunate. To account for the changing 

 seasons, the varying length of the days, he speaks of a " motion 

 of declination," as though the tilt of the earth's axis to the plane 

 of its orbit were variable throughout the year. Hence he 

 assumes a motion of the axis. We know now, of course, that 

 the assumption was needless. There is no annual motion as 

 he thought ; the tilt remains very nearly invariable. But, in 

 following out this idea of a changing inclination, he got sight 

 of the real sway of the axis, and works it out clearly ; though, 

 of course, he could not at that day divine the cause, nor, 

 geology unborn, could he surmise the effect. 



Every school student of astronomy knows what this third 

 motion was. Hipparchus and others had made it clear that 

 what we call a year falls a little short of making a complete 

 circle of the heavens, that the sun seems to move a little in 

 its position, say, from one spring to another. This gives rise 

 to the so-called precession of the equinoxes. That there was 

 some such a shift there could be no question ; it was an in- 

 contestable fact ; but there was no theory to account for it. 

 1 " The Ideas of Aristarchus," p. 113. 



