If you go out at night (where the sky is dark) and look up at the sky, it will appear as if you are standing at the center (the stationary Earth) of a large hemispherical dome onto which the Celestial bodies are fixed. This conceptualization serves as a very nice representation of the positions of the objects on the sky. This all encompassing sphere was first postulated by the ancients and was called the Celestial Sphere. Some markers on the Celestial Sphere: North Celestial Pole (NCP) --the point above the North Pole where the continuation of the Earth's rotation axis strikes the Celestial Sphere. South Celestial Pole (SCP) --the point above the South Pole where the continuation of the Earth's rotation axis strikes the Celestial Sphere. Celestial Equator --The circle where the continuation of the Earth's equatorial plane strikes the Celesital Sphere.

The Celestial Sphere was (and remains) a useful way in which to represent the sky and, in fact, it is still the way astronomers represent the observable sky; the Celestial Sphere gives us a natural way in which to understand the daily motions (diurnal motions) of the objects that we observe in the heavens. However, it fares less well when it is applied to the long-term motions of the planets. (The daily motion of a planet is generally similar to that of a star, planets also roughly circle about the NCP or SCP.) The long-term motion is, however, more complex which led to contrived scenarios as explanations for planetary motion. More on this later.

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, ...]?)

  • 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? [ annual trigonometric parallax?])

• Stars:
  • Distances?, Proper Motions?, ... .

  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.

  Later, we will return to the issue of how you can convince someone that the Earth moves and is not stationary.