welkin vs celestial sphere

welkin and celestial sphere both are nouns.

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

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

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

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

welkin (noun)	celestial sphere (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