io2 X Short 'Hhtvry.of Astronomy [CH. iv. 



no motion is perceived, as between the object seen and the 

 observer." * 



Coppernicus gives no proof of this principle, regarding 

 it probably as sufficiently obvious, when once stated, to 

 the mathematicians and astronomers for whom he was 

 writing. It is, however, so fundamental that it may be 

 worth while to discuss it a little more fully. 



Let, for example, the observer be at A and an object at 

 B, then whether the object move from B to B', the observer 

 remaining at rest, or the observer move an equal distance 

 in the opposite direction, from A to A', the object remaining 

 at rest, the effect is to the eye exactly the same, since in 



FIG. 37. Relative motion. 



either case the distance between the observer and object 

 and the direction in which the object is seen, represented 

 in the first case by A B' and in the second by A' B, are the 

 same. 



Thus if in the course of a year either the sun passes 

 successively through the positions A, B, c, D (fig. 38), the 

 earth remaining at rest at E, or if the sun is at rest and 

 the earth passes successively through the positions a, b, c, d, 



* Omnts enim quce videtur secundum locum mutatio, aut est propter 

 locum mutatio, aut est propter spedatce rei motum, aut videntis, aut 

 certe disparem utriusque mutationem. Nam inter mota cequaliter 

 ad eadem non percipitur motus, inter rent visam dico, et mdentem (De 

 Rev., I. v.). 



I have tried to remove some of the crabbedness of the original 

 passage by translating freely. 



