"Do you know where north is?"
Translation:Ξέρεις πού είναι ο βορράς;
Why is it ο βορράς and not τον βορράς; the north is not the object of the sentence
Exactly; it's not the object of the sentence. That is why it is not in the accusative case, τον βορρά, but in the nominative case, ο βορράς.
Oh, and τον βορράς is simply wrong (accusative article, nominative noun ending).