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In chapter 3 of "The Revolutions of the Heavenly Spheres" where Copernicus defines "how the Earth forms a single sphere with water", the Edward Rosen translation of the book states this.

Moreover, there is no difference between the earth's centers of gravity and magnitude. This can be established by the fact that from the ocean inward the curvature of the land does not mount steadily in a continuous rise.

Based on this. I'm believe that the center of magnitude is referring to the center of mass. And that the figure that he is referring to with a difference between the center of gravity and mass, would be a hollow Earth. Assuming that this is correct, how would a hollow Earth have land that would continuously rise with no changes in elevation from that.

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I think here "center of magnitude" simply means the geometric center (center of the sphere). If it did not coincide with the center of gravity, the ocean surface would be different from from the surface of the solid part.

The argument seems to be closely related to Archimedes, On floating bodies, Section II, where Archimedes proves that the surface of the ocean must be spherical. As the surface of the land only slightly differs from the ocean surface, this indicates sphericity of Earth.

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