celestial sphere

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An imaginary sphere of infinite extent with the earth at its center and on which the stars, planets, and other heavenly bodies appear to be located.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun astronomy An abstract sphere of infinite radius which serves as the imaginary backdrop for celestial objects, and of which the visible sky is one hemisphere.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the apparent surface of the imaginary sphere on which celestial bodies appear to be projected