= Inclination to the invariable plane of two orbital planes
For two groups carrying <angular momenta> $L_a,L_b$ with <mutual inclination> $\Delta I$, resolving total <angular momentum> parallel and perpendicular to the second group's normal gives its <orbital inclination> relative to the <invariable plane>:
$$
I_b=\operatorname{atan2}(L_a\sin\Delta I,L_b+L_a\cos\Delta I).
$$
For a positive denominator, $\tan I_b=\sin\Delta I/(L_b/L_a+\cos\Delta I)$. The larger-angular-momentum group lies closer to the <invariable plane>.
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