什么是 Moment-of-inertia factor？

The moment-of-inertia factor of a planet is the number

where $I$ is its moment of inertia about its rotation axis, $M$ its mass and $R$ its mean radius. A uniform sphere has $C = \tfrac25 = 0.4$; a planet whose density increases toward the centre has $C < 0.4$, the smaller the more the mass is concentrated.