The phrase "many a moons" is a colloquial expression used to refer to a long period of time. It can be replaced by various synonyms such as "ages," "eon," "era," or "epoch." These words convey a sense of timelessness and are often used to describe significant historical events or geological eras. Other possible synonyms for "many a moons" include "countless," "numerous," or "unceasing," which can be used to emphasize the abundance or continuous nature of a particular phenomenon. Overall, there are many alternative expressions that can be used in place of "many a moons" to convey a similar meaning in a different way.