declination vs declivity

declination and declivity both are nouns.

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

As nouns, declination and declivity are synonyms defined as:

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

declination (noun)	declivity (noun)
a polite refusal of an invitation	a downward slope or bend
(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