Riddle me this: i am first in earth, second in heaven, appear two times in aweekyou can only see meonce in a year, althoughi'm in the middle of sea. what am i?

The answer is E. It appears in all the places the riddle exclaims

The gregorian chant was a major cultural milestone in the middle ages. it was one of the first codified forms of music meant to be played as written, where previously it was difficult to mass-produce and disseminate any music. what's more, these chants were also some of the first examples of musical notation: the notes and bars of today's music sheets are a reflection of centuries of evolution beginning with these chants. it became official throughout the entirety of the catholic church, meaning that nearly all of europe followed this musical notation and styling.