empyrean vs firmament

empyrean and firmament both are nouns.

Word	Noun	Adjective	Verb	Adverb
empyrean	Yes	Yes	No	No
firmament	Yes	No	No	No

As nouns, empyrean and firmament are synonyms defined as:

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

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

empyrean (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

empyrean (adjective)	firmament (adjective)
of or relating to the sky or heavens
inspiring awe