zenith vs celestial sphere

zenith and celestial sphere both are nouns.

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

zenith (noun)	celestial sphere (noun)
the point above the observer that is directly opposite the nadir on the imaginary sphere against which celestial bodies appear to be projected	the apparent surface of the imaginary sphere on which celestial bodies appear to be projected