As a person who took Trigonometry as an adult (at 29!), I was amazed by how much more fascinating the relationship between π and radii became. I even spent some time building squares of golden sections with a compass, just like the ancients used to. In this post below, Vi Hart makes a compelling argument that we should move on from π and move to τ. It’s a fascinating concept, but what I like most about it is that it still preserves some of the aesthetic mysteries that fundamental trigonometry provides.