great year vs period of time

great year and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
great year	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of great year; that is, period of time is a word with a broader meaning than great year:

great year: time required for one complete cycle of the precession of the equinoxes, about 25,800 years
period of time: an amount of time

Other hypernyms of great year include period, time period.

great year (noun)	period of time (noun)
time required for one complete cycle of the precession of the equinoxes, about 25,800 years	an amount of time