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
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,
- ↑ Omnis enim qua videtur secundum locum mutatio, aut est propter locum mutatio, aut est propter spectatæ rei motum, aut videntis, aut certe disparem utriusque mutationem. Nam inter mota cequaliter ad eadem non percipitur motus, inter rem visam dico, et videntcm (De Rev., I. v.).
I have tried to remove some of the crabbedness of the original passage by translating freely.
