Is the Celestial Sphere a Physical Model?

Physically, the model does not make sense because we know that: (i) the Earth is not stationary; it is not the center of the Universe; (ii) the stars (and other celestial objects) are not attached to the surface of a large sphere; and (iii) the stars do not orbit around the Earth. So,

Why did the Greeks consider the Celestial Sphere model to be a viable model of the Universe?

To answer this question we must think about the kinds of effects produced by the motions of the Earth, the Sun, and the stars and how the motions manifest themselves.

Consequences of the Motions of Celestial Bodies

• Earth:
  • Rotation of Earth
    In terms of why the Celestial Sphere allows us to understand how objects move, it doesn't matter as relative motion is the important thing. However, can we feel the rotation? [ fact,: consequence 1, consequence 2, ...]?) Foucault Pendulum
    

  • Revolution about the Sun
    Again, for the utility of the Celestial Sphere, it doesn't matter. Can we tell we orbit the Sun in a simple way?

    The Greeks did not see annual trigonometric parallax, because it is so small. The closet star, proxima Centauri has a parallax angle of 0.0001 degrees, too small to be resolved with the unaided eye. It wasn't until seen until 1838 by Bessel.

• Stars:
  • Distances?, ...,

  Since: (a) it is hard to tell that the Earth is moving and effects of the relative motion can be seen from either viewpoint; and (b) that over the course of a night (and over the short term) it is difficult to measure the predicted effects of the motions of the Celestial bodies and the Earth, the Celestial Sphere turns out to be an useful representation of the sky.