solar day vs eve

solar day and eve both are nouns.

Word	Noun	Adjective	Verb	Adverb
solar day	Yes	No	No	No
eve	Yes	No	No	No

As nouns, eve is a hyponym of solar day; that is, eve is a word with a more specific, narrower meaning than solar day:

Other hyponyms of solar day include tomorrow, today, yesterday, morrow, date, day of the month, date, snow day.

solar day (noun)	eve (noun)
time for Earth to make a complete rotation on its axis	the latter part of the day (the period of decreasing daylight from late afternoon until nightfall)
	the day before
	the period immediately before something