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admirable whole which we call the system of Copernicus, and which is really only the correct arrangement of the planetary system to which we belong." "After long researches" Copernicus him- self said, "I am convinced that if we refer the —motions of — the other planets to the revolution of the earth, calculation will agree well with observation. ... I do not doubt that mathematicians will be of my opinion, if they will take the pains to make themselves

acquainted, — not superficially but profoundly , with the demonstra- 

tion which I shall present in this book."

He reasoned that "every displacement manifest to our view proceeds either from the object perceived or from the subject which perceives, or from the unequal motions of the two, for an equal and simultaneous motion of the object and the subject could

cause no semblance of displacement. The earth is the place 

whence the movement of the sky is presented to our view. Every motion starting from the earth is reflected in the sky, which will appear to move in the opposite direction. Such is the diurnal revolution, which appears to involve the whole universe except the earth. If now we suppose that - the sky has none of this mo- tion, but that the earth turns around itself from west to east (in a contrary direction from the apparent motion of the sky) , we shall find that it is really- so." Among the chief arguments in support of this view, the astronomer insisted especially on the immensity of the sky —as compared with the size of the earth : "The whole mass of the earth," he said, "vanishes before the grandeur of the sky ; the horizon divides the celestial sphere into halves, which could not be if the earth bore any proportion to the extent of the sky, or if its distance from the center— of —the—universe was perceptible ! Compared to the sky, the earth is only a point ; it is as a finite quantity compared with an infinite quantity . It is no more admissible to suppose the earth resting in the center of the universe. What! to believe that immensity turns every twenty-four hours around an insignificancy !" So the inequalities in the movements of the planets their forward and backward — movements and stationary positions were referred to two causes : the movement of translation of the earth and the proper motions of the planets ; correctly, as modern astronomers explain them, only Copernicus was not able to give details and exact figures ?

Ptolemy had argued against the idea of these motions of the earth, because if the earth were translated through space it would leave all the loose things on it behind ; and, if it turned on its ax- is from west to east, it would be impossible for bodies to make any headway to the eastward, for, whatever the rate of their motion, the earth would always reach a given point in that direction first. END Hence the former idea was the most ridiculous of all ( greek text ) and the latter altogether ridiculous ( greek text ). These