There seems to be a contradiction in the assumptions that Copernicus makes when attempting to explain the motion of the planets:


Assumptions 1 and 3 both refer to 'celestial spheres', but 1 seems to say that these spheres are not concentric, while 3 says that they are (with their centre at the Sun).

Is he referring to the same spheres in each assumption?

  • 3
    $\begingroup$ His spheres, like Hipparchus's and Ptolemy's, were slightly eccentric (i.e. the center of rotation is shifted away from the center of the Sun, and he also had epicycles, like them. Since this did not preclude them from making the Earth "the center of the universe" it did not preclude him from making the Sun that. $\endgroup$
    – Conifold
    Apr 20, 2019 at 0:56

1 Answer 1


Assumption 1 seems nothing more than a vague distinction of the centers of the lunar and planetary orbits, clarified in the two following postulates.

Assumption 2 : the center of the lunar sphere (and of gravity) is the center of the earth.

Assumption 3 : all the (planetary ones, included that of the earth) spheres revolve around the sun.

The core doctrine expressed by the above postulates is that the "centre of the universe" (i.e. of the celestial bodies) does not coincide with the "centre of gravity" of terrestial bodies.

  • $\begingroup$ That doesn't really answer the question. $\endgroup$
    – 1123581321
    Apr 20, 2019 at 0:38
  • $\begingroup$ And why does it not answer it? Proposition 1 is about all spheres, so that includes the Moon’s sphere, which is centred on the Earth. Mind you, I found a French translation (I only have the book, no computer file) that says the spheres surround the Sun, which thus is near the center of all. $\endgroup$ Aug 16, 2023 at 5:09

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