Similar Triangles?

It is a book about geometry.

Page 8/9 and it is all triangles with equal areas and somehow it all relates to conservation of angular momentum.

We are busy proving Kepler’s laws, that a line segment joining a planet to the Sun sweeps out equal areas in equal times.

Angular momentum is a frequent visitor in this story.

When I hear angular momentum, there is an implied rotation, but about what axis?