Yarn Weights

We had the discussion today about what weight of a yarn Aran is. I thought it was a little finer than worsted. Now, I'm not so sure. I bought a book today that has three methods of knitting socks. In the back, it lists yarns by weights. I know there are alot of books out there who have the same sort of charts, but here's what this one says:

Dk, Light Worsted

Worsted, Afghan, Aran

I guess the only true way to find out is to do a gauge swatch.