Mean radius (astronomy)

The mean radius in astronomy is a measure for the size of planets and small Solar System bodies. Alternatively, the closely related mean diameter (${\displaystyle D}$), which is twice the mean radius, is also used. For a non-spherical object, the mean radius (denoted ${\displaystyle R}$ or ${\displaystyle r}$) is defined as the radius of the sphere that would enclose the same volume as the object. In the case of a sphere, the mean radius is equal to the radius.

For any irregularly shaped rigid body, there is a unique ellipsoid with the same volume and moments of inertia. In astronomy, the dimensions of an object are defined as the principal axes of that special ellipsoid.