declivity vs declination

declivity and declination both are nouns.

Word	Noun	Adjective	Verb	Adverb
declivity	Yes	No	No	No
declination	Yes	No	No	No

As nouns, declivity and declination are synonyms defined as:

Other synonyms of declivity include declension, decline, descent, downslope, fall.

declivity (noun)	declination (noun)
a downward slope or bend	a polite refusal of an invitation
	(astronomy) the angular distance of a celestial body north or to the south of the celestial equator; expressed in degrees; used with right ascension to specify positions on the celestial sphere
	a downward slope or bend
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline