aphelion vs apoapsis

aphelion and apoapsis both are nouns.

Word	Noun	Adjective	Verb	Adverb
aphelion	Yes	No	No	No
apoapsis	Yes	No	No	No

As nouns, apoapsis is a hypernym of aphelion; that is, apoapsis is a word with a broader meaning than aphelion:

aphelion: apoapsis in solar orbit; the point in the orbit of a planet or comet that is at the greatest distance from the sun
apoapsis: (astronomy) the point in an orbit farthest from the body being orbited

Other hypernyms of aphelion include point of apoapsis.

aphelion (noun)	apoapsis (noun)
apoapsis in solar orbit; the point in the orbit of a planet or comet that is at the greatest distance from the sun	(astronomy) the point in an orbit farthest from the body being orbited