Well, you are thinking correctly!! The species of moss that grow on trees do indeed tend to grow on the north side of the tree, if the tree is at least somewhat exposed and not heavily sheltered by other trees. There are a couple of reasons for this as you stated, "because it is shady and cool". And due to those facts more water moisture is retained by the moss growing in this location which is a major requirement for it to survive. If the moss is on a tree that is heavily sheltered by other trees in can grow on any side because the moss is protected from the wind and sun which would remove the water moisture from the moss.

I don't think moss always grows on the north side of trees and moss can grow on other parts of trees.It may be true that moss grows more often on the north side of trees and I think you have a very good reason for why this might be. Why is it usually shady and cool on the north side of a tree? Would there be a difference if you were in South America?

Your hypothesis that moss grows best in shady, cool places is right. Mosses usually need a lot of water because they don't transport water very well. Mosses also need a layer of water to reproduce. Your hypothesis that the north side of trees tends to be shadier and cooler than the other sides is also right. However, moss *doesn't* always grow (only) on the north side of trees. In some forests, trees are cool and moist enough on all sides to allow the moss to grow. Consider a tree that is crowded with other trees. Is much light filtering in to any side of the trunk? Sometimes one side of a tree will be moister or cooler than the north side due to local shading or moisture.

To find directions in the wild, a compass is best. The position of the sun and the north star will give you a rough idea of the direction you're facing. If you're ever lost in the woods, though, your best bet is to stay where you are so that you're easier to find.

Your ideas about this question are very good. It is true that moss likes to grow in places where it's moist, shady, and cool. The question then becomes, what would make the north side of a tree moist, shady, and/or cool and thus good for the growth of mosses?

Fortunately for us, moist, shady, and cool conditions all tend to occur together where the sun doesn't shine as much. Shady places are, by definition, places where the sun isn't directly shining. These places are also cool because there is no direct sunlight to warm them up and they are more moist because there is no direct sunlight to evaporate water.

So we now want to find a place where the sun shines less. Because we live in the northern hemisphere (latitude about 34.5 north), the path of the sun is always to the south of us (another discussion that you can ask about if you're curious). With the sun always to the south, the south side of trees get more light throughout the day, leaving the north side in shade. As I mentioned above, with shade you also get coolness and
moisture, providing good conditions for moss!

This doesn't mean that moss only grows on the north side of trees. Moss will grow other places that you get coolness, shade, and moisture and in some forests (more likely in northern California where it's more wet), you can get moss covering the entire tree. Still, the moss will prefer the north side as a place to grow, because of the characteristics you mentioned. Good ideas!