Skeptical and Natural Philosophy

The Scientific Revolution

How did Copernicus change the Ptolemaic system?

The system introduced by Nicolaus Copernicus (1473–1543) was that Earth and all of the planets revolved around the sun in concentric circles. Copernicus was further able to reduce the number of postulated epicycles to 34, still saving the appearances, or not contradicting what was observed. This shifted the fundamental frame of astronomical reference from Earth to the fixed stars. As he wrote:

First and above all lies the sphere of the fixed stars, containing itself and all things, for that reason immovable; in truth the frame of the Universe, to which the motion and position of all other stars are referred. Though some men think it to move in some way, we assign another reason why it appears to do so in our theory of the movement of the Earth. Of the moving bodies first comes Saturn, who completes his circuit in xxx years. After him, Jupiter, moving in a twelve year revolution. Then Mars, who revolves biennially. Fourth in order an annual cycle takes place, in which we have said is continued the Earth, with the lunar orbit as an epicycle. In the fifth place Venus is carried round in nine months. Then Mercury holds the sixth place, circulating in the space of 80 days.

Copernicus’ conclusions were based mainly on mathematics, drawing on the perennial value of simplicity and the doctrine that nature always behaves in the most “commodious” (simple) way. To the objection that objects would fly off a moving earth, he responded that a moving sky, because it was larger, would move even faster and do more damage.


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