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moment of inertia factor

(characterization of mass distribution within a planet)

A body's moment of inertia factor is a measure that characterizes the mass distribution within the body, of use in working out the dynamics of bodies' rotation, useful for objects such as stars, planets, and moons. It is essentially that factor of the object's polar moment of inertia that is independent of the object's mass and radius:


Such a value would be of interest regarding the rotation history of objects, such as the timescale necessary for tidal forces to produce tidal locking. Example values:

sphere of uniform density .4
object with higher density toward the surface > .4
object with higher density toward the center < .4
Sun .070
Mercury .346
Earth .3307
Moon .3929
Mars .3662
Jupiter .254
Saturn .210
Uranus .23
Neptune .23

A small number indicates a lot of mass toward the center, i.e., a dense "core", and both a higher total mass and lower rigidity would contribute to this.

The moment of inertia of an object is a tensor indicating its resistance to rotation, i.e., what force it takes to change its rotation (much like the way mass determines what linear acceleration results from a given force). It is more than a scalar so as to include sufficient information to characterize such resistance regarding rotation around any axis through its center of mass. A polar moment of inertia of an object is a scalar characterizing the object's moment of inertia's implied resistance around a specific axis. For astronomical bodies, the body's axis of rotation is what is of generally of interest.