The Big Dipper and the Southern Cross are the North and South celestial guideposts. They are at the same Right Ascension and reach their highest point at the same time at opposite ends of the sky. From mid-Northern latitudes, the Big Dipper is circumpolar (never sets) and Crux is not visible. From mid-Southern latitudes, Crux is circumpolar and the Big Dipper is not visible. From the Tropics, there is a 2 month period (Aug-Oct) when neither can be seen in the night sky. See how that works?