The idea that moss grows on the north side of trees is an old one, says Dan Johnson, Assistant Professor of Forest Biology in the College of Natural Resources at the University of Idaho, "and it makes a lot of sense. Since the north side of a tree gets less sunlight than the other parts of a tree it should be cooler, more damp and have more shade—perfect conditions for mosses."
Still, whether or not moss grows predominantly on the north side of trees depends on where you live. "There is a tendency for moss to grow on the northern side of a tree—in the northern hemisphere," he says. "In the southern hemisphere mosses would have a tendency to grow on the south, shady, side of trees."
But it's still not that simple, Johnson points out, because lots of things can create the shady conditions that moss prefers, including other trees, or the slopes that trees grow on. "Shady and damp is best for most mosses, so anything that can create those conditions could result in moss growing there," he says. "There has also been some work showing that certain moss species preferentially grow on certain tree species."
As for whether or not you should use moss to navigate in a pinch, Johnson doesn't recommend it: "Compasses are cheap insurance in case you get lost."