vault of heaven vs nadir

vault of heaven and nadir both are nouns.

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

vault of heaven (noun)	nadir (noun)
the apparent surface of the imaginary sphere on which celestial bodies appear to be projected	the point below the observer that is directly opposite the zenith on the imaginary sphere against which celestial bodies appear to be projected
	an extreme state of adversity; the lowest point of anything