Eternal City vs national capital

Eternal City and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
Eternal City	Yes	No	No	No
national capital	Yes	No	No	No

Eternal City (noun)	national capital (noun)
capital and largest city of Italy; on the Tiber; seat of the Roman Catholic Church; formerly the capital of the Roman Republic and the Roman Empire	the capital city of a nation