celestial sphere vs welkin

celestial sphere and welkin both are nouns.

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

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

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

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

celestial sphere (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