welkin vs firmament

welkin and firmament both are nouns.

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

As nouns, welkin and firmament are synonyms defined as:

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

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

welkin (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