Earh and moon rotate around their common center of gravity with orbit radiuses b and a. The units are relative to R, the radius of this center's orbit around the Sun.
For actual Earth and Moon, a=1, b=0, m=13, R=400. But then it's hard to tell them apart.
To see a clear example of a convex Moon orbit, set a=10, b=0.1, R=400, m=6.