https://en.wikipedia.org/wiki/Hyperbolic_function

For a hyperbola, angle theta is not used, instead u is used, and is an area.
(For a circle, angle = 2*Area, so either can be used.)

- Brian

http://www.clayton.k12.mo.us/cms/lib/MO01000419/Centricity/Domain/241/ArticleHandout.pdf