Yet did I never breathe its pure sereneTill I heard Chapman speak out loud and bold. (7-8)
In that split-second of awe, it feels like we never truly lived until that precise moment. Keats feels reborn in the moment of his encounter with Chapman's translation.
Then felt I like some watcher of the skies (9)
Not only do we feel alive in a new way in a moment of awe, but we also realize that our lives will never go back to the way they were. In this line, Keats goes from being a traveller of realms on earth to a stargazer. His whole perspective has shifted.
When a new planet swims into his ken (10)
We use metaphors to explain the unexplainable. Awe is like that—in order to describe his emotions, Keats tells us that he feels like an astronomer who stumbles upon a whole new world.