"Oft of one wide expanse had I been told/ That deep-brow'd Homer ruled as his demesne;"
The speaker, by only reading certain translations, had only been "told" one version of Homer's epics. Because of this, he felt he'd seen the extent of Homer's domain. This is significant because of the challenge Chapman's Homer poses to the speaker's previous encounters with Homer's poetry. Homer's universe, already wide, will suddenly become broader and deeper, filled with new discoveries waiting for the speaker.
"Yet did I never breathe its pure serene/ Till I heard Chapman speak out loud and bold:/ Then I felt like some watcher of the skies/ When a new planet swims into his ken;"
In these lines, the poem's volta, or turn of thought, occurs. In lines 7-8, the speaker pivots the sonnet towards its turn through words like "Yet" and "Till," which hint at the change about to take place. Finally, in line 9, "Then" signals this change: the poem shifts from a description of past encounters, impressions, and assumptions of Homer's work to the profound impact that Chapman's Homer makes upon the speaker. The phrase "like some watcher of the skies" echoes his earlier description of the "wide expanse" of Homer's domain: as the speaker approaches Homer's world through new eyes, he feels like an astronomer who discovers a new planet against the vast night sky. Because the places of stars and planets probably don't shift too often, the planet the astronomer sees was most likely present to begin with—he is only noticing it for the first time. Likewise, the speaker has read Homer's epics before, but Chapman's approach as a translator allows him to see elements about the work he may have missed before.
