celestial sphere

celestial sphere (plural celestial spheres)

1. (astronomy) The apparent outer limit of space; an abstract sphere of infinite radius which serves as the imaginary backdrop for celestial objects, and of which the visible sky is one hemisphere.
2. One of the nested concentric transparent spheres around the Earth in which celestial bodies such as stars and planets were said to be embedded in ancient and medieval Western cosmology, and whose rotation was said to produce the motion of such bodies.

• empyrean
• firmament
• heavens
• vault of heaven
• welkin