constant vs constancy

constant and constancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
constant	Yes	Yes	No	No
constancy	Yes	No	No	No

constant (noun)	constancy (noun)
a quantity that does not vary	the quality of being enduring and free from change or variation
a number representing a quantity assumed to have a fixed value in a specified mathematical context	faithfulness and dependability in personal attachments (especially sexual fidelity)
	(psychology) the tendency for perceived objects to give rise to very similar perceptual experiences in spite of wide variations in the conditions of observation

constant (adjective)	constancy (adjective)
steadfast in purpose or devotion or affection
uninterrupted in time and indefinitely long continuing
unvarying in nature