For a solid sphere Eric Weisstein's World of Math with radius R, mass M, and density,
the moment of inertia tensor is
which is diagonal, and so it is in principal axis form. Furthermore, because of the symmetry of the sphere, each principal moment is the
same, so the moment of inertia of the sphere taken about any diameter is
Moment of Inertia, Moment of Inertia--Spherical Shell
