celestial sphere vs firmament

celestial sphere and firmament both are nouns.

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

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

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

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

celestial sphere (noun)	firmament (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