firmament vs empyrean

firmament and empyrean both are nouns.

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

As nouns, firmament and empyrean are synonyms defined as:

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

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

firmament (noun)	empyrean (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

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