What does the "distant northern sea" refer to in "Dover Beach"?

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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.