"Did you know that the earth revolves around the sun?"
He asked me if I had known that the earth revolved around the sun.
I know this follows all the rules of reported speech, but "had known" sounds kind of fishy to me. Is it correct?
This "Did you know" does not normally have the full sense of a past tense, and it would normally be unnecessary or odd to backshift it to "had known" in reported speech. One would say "He asked me if/whether I knew that the Earth revolved/revolves around the Sun". It is possible, though less likely in your particular example, that "Did you know" could be meant as a true past tense, asking about past knowledge, in which case backshifting would be appropriate.
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This "Did you know" does not normally have the full sense of a past tense, and it would normally be unnecessary or odd to backshift it to "had known" in reported speech. One would say "He asked me if/whether I knew that the Earth revolved/revolves around the Sun". It is possible, though less likely in your particular example, that "Did you know" could be meant as a true past tense, asking abou