nadir vs vault of heaven

nadir and vault of heaven both are nouns.

Word	Noun	Adjective	Verb	Adverb
nadir	Yes	No	No	No
vault of heaven	Yes	No	No	No

nadir (noun)	vault of heaven (noun)
the point below the observer that is directly opposite the zenith 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
an extreme state of adversity; the lowest point of anything