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The "distant northern sea" refers to the sea where Arnold is writing, which is the sea of the United Kingdom. Note that in the stanza another sea is referred to which is the Aegean, where Sophocles heard the same "tremulous cadence slow." Thus, compared to the Aegean, the sea surrounding the British Isles is the "distant northen sea" as it is in the North compared to the Aegean and also distant from the Aegean sea. It is important to realise though how Arnold uses the sea as a metaphor for the Sea of Faith that he feels is withdrawing rapidly from the shores of the Britain, leaving it "naked" and exposed.