firmament vs celestial sphere

firmament and celestial sphere both are nouns.

Word	Noun	Adjective	Verb	Adverb
firmament	Yes	No	No	No
celestial sphere	Yes	No	No	No

As nouns, firmament and celestial sphere are synonyms defined as:

firmament and celestial sphere: the apparent surface of the imaginary sphere on which celestial bodies appear to be projected

Other synonyms of firmament include empyrean, heavens, sphere, vault of heaven, welkin.

firmament (noun)	celestial sphere (noun)
the apparent surface of the imaginary sphere on which celestial bodies appear to be projected	the apparent surface of the imaginary sphere on which celestial bodies appear to be projected