CELESTIAL SPHERE

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 affixed. 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

the Celestial Sphere

The Celestial Sphere was (and remains) an useful way in which to represent the Universe and, in fact, it is still the way astronomers choose to model the observable sky. [The Celestial Sphere also gives us a natural way in which to picture the daily motions (diurnal motions) of the objects that we observe in the heavens. However, in order to explain the complex behavior of the planets, contrived scenarios needed to be postulated. This state-of-affairs contributed to the assigment of god-like status to things like the planets].

Hmmm, but wait a minute. Physically the model does not make sense as we know that the Earth is not stationary, it is not in the center of the Universe, and the stars (and other celestial objects) are not attached to the surface of a large sphere (i.e., stellar positions) and they do not orbit around the Earth. So,

How is the Celestial Sphere model able to provide such an accurate representation of the Universe (and sky)?

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 -- relative motion ( feel rotation [ fact,: consequence 1, consequence 2, ...]?)

  • Revolution about the Sun -- trigonometric parallax?

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

  Since: (1) it is hard to tell that the Earth is moving and effects of the relative motion can be seen from either viewpoint; and (2) 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 it is irrelevant whether the Earth is moving or the Celestial bodies move (at this levele). Because of this, 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.