Cosmic Order, the Movement of the Earth, and the Scientific Revolution
Chapter VIII. The First Motion of the Earth and the “Optical Argument”
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THE FIRST MOTION OF THE EARTH AND THE “OPTICAL ARGUMENT”
Copernicus presents in Chapter 5 of Book I of De revolutionibus the earth’s possible motion as a logical consequence of its spherical form or shape, and once again underlines the necessity to rethink its position in the universe: “Now that the earth too has been demonstrated to have the form of a globe, we must in my opinion see whether also in this case the form entails the motion, and what place in the universe is occupied by the earth. Without the answers to these questions it is impossible to find certain explanation of the appearances in the heavens.”48 He has shown that the earth is a sphere and that earth and water make up a single sphere with a single centre of gravity and magnitude. He has also asserted that the sphere expresses – and in a way actualizes – its form or shape by moving in a circular motion around its axis. The question that now naturally arises is whether the earth itself should also be assigned circular motion, regardless of the fact that “there is general agreement among the authors that the earth is at rest in the middle of the universe” and that “they hold the contrary view to be inconceivable or downright silly.” Despite the general consensus that the earth is motionless, the preliminary investigations have led Copernicus to conclude that this question “has not yet been solved...
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