btw, if the orbits are ellipses, is the radius of orbit an average value?

[color=blue]yes, it is always changes, it is observable in the simulation as well.[/color]

If not, do you know how is this radius value derived for an elliptical shape?

Kepler's Third Law:

III. The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semi-major axes.[img]http://www.astro-tom.com/images/axes.gif[/img]

[color=blue]In our syllabus, it is a slightly inaccurate as we assume all orbits to be circular thus our teaching of Kepler's 3rd Law is adapted to circular motion.[/color]