I know the poem is a huge conceit, using a compass to stand for the speaker and his beloved and how his beloved's love make's him stable and whole: "Thy firmness makes my circle just,/And makes me end where I begun." There love is also compared to Gold, and malleability (when they are separated, their love does not break, it only "expands" or "stretches").
I don't know if that helps any, but good luck.