where Dk is the difference in longitude between the minimum moment of inertia, A, and the x-axis.
For planets which rotate "sufficiently rapidly" (typically those which have not been tidally despun), the B and A moments of inertia are much smaller than the maximum moment of inertia, C. Thus,
|B - A| < |C - A| and |B - A| < |C - B|: (3:12)
Equation (3.9) can then be rewritten as
Thus, by determining J2 from the gravity analysis, one can determine the differences of the principal moments of inertia. To obtain absolute values of the principal moments of inertia, one uses the precession constant (H), which can be derived from the rate of precession of the planet's rotation axis:
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