The Japanese Maple in Spring

I think I was bringing the garbage pail up from the street when I noticed that part of the Japanese maple had buds that had opened.  But this is interesting - only the buds on the south side of the tree had been warmed enough by the sun to open.  The buds on this side of the tree, which is the north side, were in the shade and hadn't opened yet.  Thats why you can see foliage on the tree and still see the bare branches on the north side.  Such an interesting phenomena which I have never noticed before.