It's simple, really. Up is the direction away from the sphere if that sphere is the primary gravitational influence. This up gets normalized to the center pole of the celestial sphere, the pin about which the celestial sphere rotates. In the northern hemisphere this is the Pole Star, in the southern hemisphere there is no star quite that close to the preferred spot, but the celestial sphere does rotate about a point to the south that is the definitive up. If you get close enough to the equator you'll find that cultural definitions of up, as shown on maps, will be in other directions, usually based on some important city.

(with apologies to ron o.)