Page:A short history of astronomy(1898).djvu/148

102 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 and an object at, then whether the object move from  to , the observer remaining at rest, or the observer move an equal distance in the opposite direction, from  to , 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 and in the second by  are the same.

Thus if in the course of a year either the sun passes successively through the positions (fig. 38), the earth remaining at rest at, or if the sun is at rest and the earth passes successively through the positions a, b, c, d,