solar day vs morrow

solar day and morrow both are nouns.

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

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

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

solar day (noun)	morrow (noun)
time for Earth to make a complete rotation on its axis	the next day