firmament vs welkin

firmament and welkin both are nouns.

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

As nouns, firmament and welkin are synonyms defined as:

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

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

firmament (noun)	welkin (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