apoapsis vs aphelion

apoapsis and aphelion both are nouns.

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

As nouns, aphelion is a hyponym of apoapsis; that is, aphelion is a word with a more specific, narrower meaning than apoapsis:

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

Other hyponyms of apoapsis include apogee, apojove, apolune, aposelene.

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